serhiyshamshetdinov / sugar-tms   0.2.4

Apache License 2.0 GitHub

Transparent Monads syntax and Monadic Flow Control interpretation in Scala

Scala versions: 2.13 2.12 2.11

Transparent Monads syntax and Monadic Flow Control interpretation

If you are here for the first time, please, read the tms and mfc article (or in the root of repo or artifact) which explains ideas and aims, maps intuition to implementation, shows some examples and limitations of macros usage.

sugar-tms is the library of macros with built-in set of implicits that enables additional Scala syntax and code interpretation where:

  • by transparentMonads macro: each expression of the monadic type (or of the monadic stack type) may be used in any place where value of its any inner type is expected. In other words, treat the value of the Monad[T] type as the value of type T wherever type T is expected for any depth of inner Monad[T] types recursively
  • by monadicFlowControl macro (along with above transparentMonads feature): sequence of statements is treated as joined by flatMaps-map of detected Monadic Flow Type. In other words, the statements (possibly grouped to subsequences) of the passed block are executed not just unconditionally one by one (as usual flow control) but in terms of Monadic Flow Control (being joined like in the for-comprehension header: statement (or group) execution is controlled by the previous monad)

The aim is the easier defined and more clear business code where required obvious monadic manipulations (for-yield, flatMap, map) are derived and built by macros. So, compile time implicits satisfy typechecker and designate access to inners dropping the top of types stack, macros - map input code to the for-comprehensions stack that extracts the inners of a monadic stack values.

The practical difference between these two macros is that monadicFlowControl enables the use of the local stable vals as tms-values while limiting position (actually: the scope of usage) of definitions of the other types (lazy vals, var, types, classes, objects, defs) to the resulting yield (after the last stable val definition or Monadic Flow Type expression).

Code translation difference:

  • transparentMonads macro does pointwise replacement of tms-values and tms-expressions to ones inner values obtained in the embracing fors stack. After replacements, the entire passed input code becomes the yield part of the built fors stack.
  • monadicFlowControl macro, along with transparentMonads transformations, maps the init of passed block statements to one for of the built stack that corresponds to Monadic Flow Type of the first statement (val or expression). Only tailing statements (after the last stable val definition and last expression of the Monadic Flow Type) become the yield part of the built fors stack.

Thus, transparentMonads is good for expressions (even complex) or blocks with any definitions that are not used as (or in) the Transparent Monads, monadicFlowControl - for blocks of statements with stable val definitions that are used as (or in) the Transparent Monads of the passed code.

Table of contents


Artifact currently is being built for Scala 2.11, 2.12 & 2.13. Since starting Scala 2.12.13 Predef implicits are changed, to acknowledge the changes the macros code defers from compiled for previous 2.12 versions. That is why there are two different artifacts for Scala 2.12. Incorrect usage of 2.12 artifact with wrong Scala version will be reported as an error at compile time.

Required dependencies

To use the macros add the following to library dependency:

  • for Scala 2.11, 2.12.1-2.12.12 & 2.13 use classic Scala dependencies:

    "" %% "sugar-tms" % "0.2.4"
  • for Scala 2.12 starting 2.12.13 use:

    "" % "sugar-tms_2.12at13+" % "0.2.4"

The artifact only depends on the "scala-reflect" and both are used exclusively at compile time (or toolbox code compile time during the toolbox tests - tests runtime) and do not require runtime dependency for your project. So, to exclude "sugar-tms" and "scala-reflect" dependency from publishing with you project add "compile-internal, test-internal" configuration to the dependency:

"" %% "sugar-tms" % "0.2.4" % "compile-internal, test-internal"

Please, be careful with IDEA, and drop this configuration if you have any troubles. Plugin still works strange with one while importing the project (sometimes it skips such dependency at all, but after the first import works stably with it).

To use dependency for any Scala version you may add the following to project's setting:

libraryDependencies += (if ("""^2\.12\.(\d+).*""".r.findFirstMatchIn(scalaVersion.value).exists( >= 13))
  "" % "sugar-tms_2.12at13+"
  "" %% "sugar-tms"
) % "0.2.4" % "compile-internal, test-internal",

This sample of usage may be found in the root project of the code. Root is made for the purpose of testing.

Compatibility and versioning

Project does not require the runtime dependency on its artefact. Everything is done at compile time.

While the project is in research & growing state, it will follow the full backward versions compatibility for all 0.x.x versions.

For all new versions above means:

  • semantic behaviour of old macros will not be changed (for the same input code the code produced be the macro will not change semantically: monadic manipulation order, order of side effects, etc.), but may to overcome past limitations extending the functionality that was previously errors
  • any defaults will not be changed
  • new functionality will be realized by new macro or by new option of existing macro
  • new option for existing macro will have the default disabled state to repeat the old behaviour without macro usage changes

Such strategy simplifies lots of things for users, requires only one primary branch for developing & support and lets to focus on new ideas and implementations rather than maintaining old versions code. It allows different idea realizations to exist simultaneously.

So, upgrading to new version will not require additional efforts on the old usages, will bring new semantic functionality with new macros reflected in the minor version up and bugs fixes or new helper functionality reflected by the patch version up.

Macros call variants

Macros are represented by full and sort names with and without options variants: Transparent Monads macro have the following signatures that call the same implementation:

def transparentMonads[O](tmsCode: Any): O
def tms[O](tmsCode: Any): O
def transparentMonadsFor[O](tmsLiteralOptions: String*)(tmsCode: Any): O
def tmsFor[O](tmsLiteralOptions: String*)(tmsCode: Any): O

Monadic Flow Control macro implementation is based on Transparent Monads and includes its full functionality while adding Monadic Flow Control interpretation of the passed block of statements:

def monadicFlowControl[O](tmsCode: Any): O
def mfc[O](tmsCode: Any): O
def monadicFlowControlFor[O](tmsLiteralOptions: String*)(tmsCode: Any): O
def mfcFor[O](tmsLiteralOptions: String*)(tmsCode: Any): O

O is the result type defined by required Transparent Monads extractions of the input tmsCode. tmsCode may represent any Scala expression or the block of statements containing Transparent Monads.

Output O type and passed tmsCode should match by Transparent Monads' extraction possibility. Not every O type may correspond to passed tmsCode but some variants of tmsCode may have several valid matching O. This mapping is defined by Transparent Monads order and types in tmsCode.

Tms options may additionally be specified by @tmsOptions annotation to apply options for all macros enclosed by annotated target (class, object, method). Default and overriding options for all macro calls may be specified by environment variables and/or system properties.


Usage of both macros in the code requires the only import:

import sands.sugar.tms.TransparentMonads._ 

that imports all required implicits, macros and optional @tmsOptions annotation.

For example, the function that summaries the passed Try[Int] list:

import sands.sugar.tms.TransparentMonads._

def sumTries(intTries: Try[Int]*): Try[Int] = intTries.foldLeft(Try(0))((t1, t2) => tms[Try[Int]](t1 + t2))

// to see the compile time debug info call tmsFor with the debug option passed:
def sumTries(intTries: Try[Int]*): Try[Int] = intTries.foldLeft(Try(0))((t1, t2) => tmsFor[Try[Int]]("Debug")(t1 + t2))

macro will replace the input t1 + t2 expression buy the following for

  for {
    valueOfTry$macro$1 <- t1
    valueOfTry$macro$2 <- t2
  } yield {

monadicFlowControl macro enables the usage of local stable vals as Transparent Monads values. The sample is:

import sands.sugar.tms.TransparentMonads._

// assuming we have
case class User(personalData: Option[PersonalData])
def getUser(userId: String): Future[User]

// then we may get the Future[String] status of user by
def getUserEmails(userId: String) =
  mfc[Future[String]] {
    val user = getUser(userId) // Future[User]
    if (user.personalData.isEmpty) // direct access to User properties on Future[User] value
      "should provide personal data"
      "may operate"

// macro generated block is
  for {
    userValue$macro$1 <- getUser(userId)
  } yield {
    if (userValue$macro$1.personalData.isEmpty)
      "should provide personal data"
      "may operate"

How to test

To run internal tests you should import the project, select required Scala version by changing build.sbt (default is 2.13) or by sbt command ++2.12.12, for example.

To run tests for selected Scala version execute sbt command sugar-tms/test.

Header comments of the build.sbt describes additions options to run the tests. For example, to run any test in IDEA with debug & trace output you may add system property -DtmsTestDebug=true to VM Options (by default tmsTestDebug=false and tmsTestTrace=tmsTestDebug) of the test run configuration (or scalatest configuration pattern for all test suites). Or add environment variable tmsTestDebug to always see the macros debug output (for the sbt and IDEA runs).

To run tests for all supported scala versions (with the latest hardcoded patch versions) just execute sbt command +sugar-tms/test.

For Scala 2.13 project passes 4500 testcases (the part of tests run with different tms option combinations).

Project is compiled for target jvm-1.8 and is tested with JDK8, JDK11 & JDK16. Project's root folder contains tools directory with batch files to run tests under different JDK versions.

Sbt test command will only execute simple root project tests to test the public symbols accessibility of the artifact or source sugar-tms code: all macro calls in the native and toolbox tests. Version of the tested artifact is passed by system property or environment variable rootTmsArtifactDependencyVersion. When not specified then the source project will be used as test dependency. For all supported Scala versions these root tests may be run by +test sbt command.

To test macros in your project inside the toolbox you may copy and adopt the sources of tms-test-base subproject or call macros passing Debug and/or Trace options to macro call directly, with tmsOptions annotation, environment variables and/or system properties.

Additionally, it is possible to enable "Embedded Fors Code View" tms option to add the code generated by macro
to macro output as a local string value. This option allows you to view the fors stack representation of the macro output while using standard scalac option -Ymacro-debug-lite or to analise by test code the macro output run in the toolbox.

Shortly how it works

The implementation is fully immutable (not counting Scala 2 reflection).

The most general explanation is: the set of compile time implicits makes possible Transparent Monads (tms) syntax giving access to any inner value of the monad (or of the stack of monads) designating the tms access and dropping top stack types. Macros replace ttsNxxx implicit calls of the input code to the inner values of monads which are extracted in the built fors stack of the passed type O.

Transparent Monads syntax by implicits

Built-in implicits satisfy typechecker and designate tms-values and tms-expressions with dropped types and access depth levels of the required inner values.

The general ttsN implicits set is the following:

@compileTimeOnly(ctom) implicit def tts1[T1[_] <: AnyRef, T](t: T1[T]): T = ???
@compileTimeOnly(ctom) implicit def tts2[T1[_] <: AnyRef, T2[_] <: AnyRef, T](t: T1[T2[T]]): T = ???
@compileTimeOnly(ctom) implicit def tts3[T1[_] <: AnyRef, T2[_] <: AnyRef, T3[_] <: AnyRef, T](t: T1[T2[T3[T]]]): T = ???
@compileTimeOnly(ctom) implicit def tts4[T1[_] <: AnyRef, T2[_] <: AnyRef, T3[_] <: AnyRef, T4[_] <: AnyRef, T](t: T1[T2[T3[T4[T]]]]): T = ???
@compileTimeOnly(ctom) implicit def tts5[T1[_] <: AnyRef, T2[_] <: AnyRef, T3[_] <: AnyRef, T4[_] <: AnyRef, T5[_] <: AnyRef, T](t: T1[T2[T3[T4[T5[T]]]]]): T = ???

AnyRef upper bound is required to make the implicits priority order in this set. tt5 has higher priority and is applied first during resolving implicits overloads, thus if stacked types have the same accessor overload then this accessor is applied to the value of the most inner type of the stack which has such overload.

Other bundled ttsNxxx implicits make support for Scala features realised by Predef and primitives conversion implicits, includes the set of identity implicits for different types to have proper order of implicits applying.

In the tms source code the stack sets of tts1xxx-tts5xxx implicits are built by macro annotations like in the following example:

@replaceByTtsImplicits @compileTimeOnly(ctom) def wrapString[T <: String](s: T): WrappedString


@compileTimeOnly(ctom) implicit def tts1wrapString[T1[_] <: AnyRef, T <: String](s: T1[T]): WrappedString = ???
@compileTimeOnly(ctom) implicit def tts2wrapString[T1[_] <: AnyRef, T2[_] <: AnyRef, T <: String](s: T1[T2[T]]): WrappedString = ???
@compileTimeOnly(ctom) implicit def tts3wrapString[T1[_] <: AnyRef, T2[_] <: AnyRef, T3[_] <: AnyRef, T <: String](s: T1[T2[T3[T]]]): WrappedString = ???
@compileTimeOnly(ctom) implicit def tts4wrapString[T1[_] <: AnyRef, T2[_] <: AnyRef, T3[_] <: AnyRef, T4[_] <: AnyRef, T <: String](s: T1[T2[T3[T4[T]]]]): WrappedString = ???
@compileTimeOnly(ctom) implicit def tts5wrapString[T1[_] <: AnyRef, T2[_] <: AnyRef, T3[_] <: AnyRef, T4[_] <: AnyRef, T5[_] <: AnyRef, T <: String](s: T1[T2[T3[T4[T5[T]]]]]): WrappedString = ???

All implicits may be used explicitly (even omitting types) to change default depth of the inner value to be reached.
Transparent TypeS: "types" because ones are still not a monads at this level and any one-parameter type may be caught by these implicits.

Transparent Monads macro

Let's use simple example. Imagine we want to add long value to each option integer of the passed list. The result will be of List[Option[Long]] type:

import sands.sugar.tms.TransparentMonads._

def addValue(list: List[Option[Int]], value: Long): List[Option[Long]] =
  // if you want to see debug output of the macro while compilation use tmsFor[List[Option[Long]]]("Debug") call of add @tmsOptions("D") annotation to method
  tms[List[Option[Long]]] {  
    value + list // for `list + value` the result will be the same
  1. tts implicits will be applied to the input code value + list. Typechecked code at the macro input will look like
    value.+(sands.sugar.tms.TransparentMonads.tts2intIdentity[List, Option, Int](list)) // of type Long = yield type
  1. Scala tree of the passed expression or block is parsed to the internal TmsTree AST. It accepts full Scala syntax available in the Block.
  2. Preprocessing is done to drop types of some implicit operations on String which conflicts with native string methods. During preprocessing the nested ttsNxxx calls are flattened to one. If "Predef Compliance" option is enabled then the ttsNxxx implicit calls with .+(s: String) accessor are replaced by Predef any2stringadd calls
  3. Extraction of ttsNxxx implicits and lifting TmsTree nodes to partially untypechecked Scala tree transforms TmsTree to TmsExtracted AST which is the skeleton of the fors stack. Each ttsNxxx implicit has exactly 1 parameter (where Transparent Monads expression or value is passed) and list of dropped types that represent the part of types stack of the Transparent Monads. This list of dropped types (in our case it is List, Option) determines the fors in which Transparent Monads expression should be extracted. Macro extracts the expression or value passed to ttsNxxx call in fors stack and replaces the ttsNxxx call by the value obtained in the last for.
  4. Postprocessing optionally flattens result and optimizes the output fors stacks (inlines not used directly Monadic Flow Type vals, drops degenerate fors, etc.).
  5. Since the output code built with for-comprehensions will be desugared to flatMap/map while the final typecheck, macro builds fors-code view representation of the output code for easy reading and understanding when it is requested by options "Debug" or "EFCV".
  6. Lifting of the fors stack Scala tree on TmsExtracted AST and untypechecking definitions.

In out case we will have the following fors-code view at the macro output:

  for {
    valueOfList$macro$1 <- list
  } yield {
    for {
      valueOfOption$macro$2 <- valueOfList$macro$1
    } yield {

So, Transparent Monads macro takes the expression or the Block of statements and pointwise replaces monadic values by the inner ones obtained in fors stack. After replacing ttsNxxx calls by the inner values the entire passed code becomes yield part of the built fors stack.

Monadic Flow Control macro

In addition to Transparent Monads macro transformations, the Monadic Flow Control macro at the above "extraction" step treats the passed Block of code (single passed expression will be wrapped to the Block) as having Monadic Flow Control relations between statements.

Actually it translates the passed sequence of statements to one (of the stacked) for-comprehension of Monadic Flow Type which is detected by the first statement.

Macro detects stable val definitions, groups expressions of non-Monadic Flow Type, separates tailing yield statements, does definitions scope validation and usage possibility, and then maps that parts to for <- or = enums and yield.

Each separate part of input statements is added to the fors stack after tms-extraction of its expressions. This means that Monadic Flow statement may also contain tms-values inside its expressions, but these tms-values (or tms-expressions) may only be extracted in the top of fors stack that precedes Monadic Flow Type (inclusively). Tms-expressions of the yield part of the mfc-block may be extracted in the entire types stack.

For example, we need to duplicate the string pattern n/m integer times with logging. We have the pattern as Try[String]:

import sands.sugar.tms.TransparentMonads._

def log(v: Any): Unit = {}

def patternTimes(n: Int, m: Int, pattern: Try[String]): Try[String] = mfc[Try[String]] {
  val times = Try(n / m) // Monadic Flow Type is Try
  log(s"$n/$m=")           // the group of 2 non Monadic Flow Type expressions:
  val len = pattern.length // non Monadic Flow Type stable val & the last val in the block
  // yield starts here
  pattern * times

The typechecked code of macro input after implicits applying is:

  val times = scala.util.Try.apply[Int](n./(m));
  log((("".+(n).+("/").+(m).+("=")): String));
  val len = sands.sugar.tms.TransparentMonads.tts1stringIdentity[scala.util.Try, String](pattern).length();
  sands.sugar.tms.TransparentMonads.tts1augmentString[scala.util.Try, String](pattern).*(sands.sugar.tms.TransparentMonads.tts1intIdentity[scala.util.Try, Int](times))

The macro output code with Monadic Flow Control block mapped to the for is:

  val times = scala.util.Try.apply[Int](n./(m))
  for {
    timesValue$macro$1 <- times       // `name <-` for all Monadic Flow Type stable vals 
    wcNonMf$macro$2 = {               // `_ =` for groups of statements of non Monadic Flow Type 
      log(("".+(n).+("/").+(m).+("="): String));
    valueOfTry$macro$3 <- pattern     // this is a tms extraction of String value of Try[String] pattern  
    len = valueOfTry$macro$3.length() // `name =` for all non Monadic Flow Type stable vals 
  } yield {

Monadic Flow Type may be any type of resulting type O of the monadic stack: its for may be any one of the built fors stack. When types stack contains several types for which Monadic Flow Type conforms to then the first outer one becomes Monadic Flow Control for.

Flattening the result

Both macros may flatten the result of built fors stack.

When the resulting yield block (or expression) has the same type as the most inner type of the built fors stack then the result may be flattened. Flattening is controlled by the passed to macro result type O. If the yield block type conforms to the last used in fors stack type then result will be flattened when the yield type does not conform to the next not used inner type closest to that used type.

In other words, if we define inside the stack type O additional inner type that is (confirmed to) the yield type then it will not be flattened but will be filled by the yielding block (or expression).

For example, to calculate the following tms-expression:

Some( Some(1) + 2 )

we may specify two variants of the resulting type in the macro call:

tms[Option[Int]](Some( Some(1) + 2 ))
// that is built to 
  for {
    valueOfSome$macro$1 <- scala.Some.apply[Int](1)
    flatMappedValueOfSome$macro$2 <- scala.Some.apply[Int](valueOfSome$macro$1.$plus(2))
  } yield {


tms[Option[Option[Int]]](Some( Some(1) + 2 ))
// that is built to 
  for {
    valueOfSome$macro$1 <- scala.Some.apply[Int](1)
  } yield {

Limitations and notes

Type variables (type classes) and tts implicits

Due to the inner implicits stack priority (any tts5xxx has higher priority than any tts1xxx implicit has to reach overloaded accessor of the inner type first) implicits have AnyRef upper bound for transparent type parameters that forms types stack. That is why to be applied to type variables latter should be defined with AnyRef upper bound. Like in

  def sample[M <: AnyRef : Typeclass]() =
    /* implicit */ tts1[M, Int](...)

This is required only when any ttsNxxx implicit should be applied to the stacked type that contains type variable M (when M should be a type parameter of ttsNxxx[.., M, ...](...) call). In other words, when type parameter is the part of the "opened" types stack of the tts-value. When type variables (thus type classes) are not accessed as tms-values then upper bound may be omitted. Please, keep in mind that missed implicit applying may not be controlled or verified and thus may result in skipping tms interpretation of the value and lead to non-clear errors.

Array collections overload precedence

Most accessors of Array[T] collections are implemented by Predef implicit extensions. Implementing ones tts1generic...-tts5generic... stack companions of genericArrayOps and genericWrapArray leads to Array[T] collections have more priority on implicit overloads resolving over other collections. In practice this leads to the following: if Array type is outer than other collection type (let the Seq type) in the types stack then the Array overloaded accessor of both types will be used first (contrarily to Seq accessor "should" be used by general priority rule). So Option[Array[Seq[Int]]].length will result Option[Int] getting the optional length of an Array contrarily to expected (by general rule) optional array of sequences length Option[Array[Int]].

Call by name caution

The way used to extract inner values in the embracing fors stack breaks call-by-name feature when tms-value (or tms-expression) is passed to call-by-name parameter. By the current implementation such tms- or tms-dependent expression should be evaluated (or partially evaluated) in the embracing for thus not inside the function it is passed to. This limitation looks irresistible. For example, in:

  tms[Option[Boolean]] {
    true || Option(false)
  // built as
    for {
      valueOfOption$macro$1 <- scala.Option.apply[Boolean](false)
    } yield {

Option(false) will be evaluated first in the embracing fors stack and will be passed to .|| call-by-name parameter as a value unlike usually expected behavior when Option(false) is not evaluated at all (without tms).

When expression passed to call-by-name parameter partially depends on the tms-expression (or tms-value) then such tms-part will be evaluated first, resulting the expression passed to call-by-name parameter is partially evaluated even when not used.

When tms option for building inner fors stacks for apply parameter are used then expressions that partially depend on tms will be evaluated completely before passing to call-by-name parameter.

Evaluation order

Resulting evaluation flow is determined by the order of tms-expressions extraction. Tms-expressions are added to fors stack for inners extraction in the order ones appear in the usual evaluation flow of the input block or expression.

For composite expressions like if, match, try, etc. with several flow branches, the branches are processed in the order ones appear in definition. I.e. for the if statement (actually: composite expression that consists of expressions):

  • predicate expression
  • true-branch expression
  • false-branch expression

For transparentMonads the yield (actually: entire passed block) becomes the function of all tms-expressions evaluated in the embracing fors. Thus, all inner values of all tms expressions are evaluated in fors stack first then yield block is evaluated as usual having that inner values.

monadicFlowControl macro has mixed evaluation order since the initial statements of mfc-block are mapped to the one for-enums inside fors stack. The order looks like:

  • fors that correspond to the types outer than Monadic Flow Type (if any)
  • Monadic Flow Type for with mfc-expressions mapped to its enums
  • fors that correspond to the types inner than Monadic Flow Type (if any)

This order also limits tms-expressions used in the init part of mfc-block (not yield part) to be extracted only for types that are outer than Monadic Flow Type (inclusively). In other words, to "open" such tms-expression for getting inner value only types of the stack above (or including) Monadic Flow Type may be dropped. fors inner than mfc-for may not be used for ones extraction.

It is more or less simple when all used tms-expressions only have monads of the same one type.

If monads stack is used then we have the following picture. Tms-expressions are evaluated partially in the order of types in the stack (in the order of fors in the built stack): first, outer types of the stack are evaluated, then inner types. So tms expression are evaluated not one-by-one completely but in the "across" "by type" mode depending on dropped types of ones.

For example, we have 2 tms expressions Try(Some(0) + 1) and Some(2) + Try(3) appeared in the specified order as parts of the input code for resulting types stack Option[Try[Int]]. The order in which ones are evaluated is:

  • s1 <- Some(0) outer for of Option
  • s2 <- Some(2)
  • t1 <- Try(s1 + 1) inner for of Try
  • t2 <- Try(3)
  • st = s2 + t2 in the place of the last expression usage (yield for both macros or value expression for monadicFlowControl macro if Monadic Flow Type is Try)

So, all Options of all tms-expressions first then all Trys.

To change this "across" tms-expressions evaluation order there are tms options to built subexpressions that depend on tms each in separate inner fors stacks.

Macro output type and nested tms-expressions

Resulting macros output type O depends on type of the input code and tms-expressions used in it. Above tms-expressions extraction order determines the types and order in which these types should exist in the output types stack O.

Special attention is required for tms-expressions or tms-values nested into tms-expression.

Let's take two examples of separate tms-expressions and the nested ones. Separate is Try(Some(1)) + Try(Some(2)) and the nested is Try( Try(Some(1)) + 2 ) + 3.

Try(Some(1)) + Try(Some(2)) has two independent Try(Some(*)) tms-expressions. In the Try(Some(1)) + Try(Some(2)) each Try(Some(*)) will consume Try and Option fors of the stack extracting corresponding inner Int value and then ints are summarized. The result type is Try[Option[Int]]. Each tms-expression uses fors stack for extraction separately starting its top.

Nested one Try( Try(Some(1)) + 2 ) + 3 has two tms expressions: inner Try(Some(1)) + 2 that consumes Try and Option fors like the first sample does but its extracted int value (let intValue) should be used inside the second tms-expression Try( intValue ) + 3. Now, to extract Try( intValue ) we should use another additional Try for that is inside already used fors stack of Try-Option. The first Try for may not be used since it extracts intValue. That is why the result type of Try( Try(Some(1)) + 2 ) + 3 is Try[Option[Try[Int]]].

Macros control the usage of requested types stack (requested by the type O) during extraction, and reports an error when required type is absent in a non-consumed rest of the stack at the point of extraction.

Local definitions limitation

  • for transparentMonads macro: any type of local definition may not be used as tms or inside tms-expression
  • for monadicFlowControl macro: local stable values defined in the root block of passed code may be used as tms or inside tms-expressions but definitions other than stable vals may occur only after the last stable val definition or Monadic Flow Type expression (only be ones that go to the resulting yield part)

Those are embracing fors stack implementation limitations.

The reason of stable vals limitation of transparentMonads is: to reach the inner value of tms it should be used in embracing fors stack that apriori is out of the val definition scope.

For other types of definition of monadicFlowControl: ones do not have for-comprehension equivalents like stable vals have.

Implicits scope workaround

When you encounter the "compile time only" compiler error on tts implicits call then this primarily mean that ttsNxxx implicit is applied outside the macros tmsCode passed. The workaround is to move import of sands.sugar.tms.TransparentMonads._ closer to the macro call to shrink implicits scope up to the call only.

The found reasons are:

  • some scalatest (or other packages) implicits behaviour may be overridden by tts implicits
  • predef any2stringadd feature syntax interfere with tms. Inside passed tmsCode it is corrected by macro (standard Scala behaviour is recovered or ignored depending on "Predef Compliance" option) but outside the macro call nothing does this.

2.11 lazy vals scope is not verified

Support of lazy val in 2.11 reflection generates tree that makes hard to validate and work with the ones symbols. The incorrect scope usage check are not implemented for 2.11 lazy val but compiler will emit corresponding error.

IDEA plugin syntax checks

Unfortunately, implicits highlighting and based errors check by the IDEA plugin continue to surprise with ignorance of the complete Scala implicits knowledge. Looks like there is something insurmountable in complex expressions with implicits. This project is very sensitive to that knowledge by instruments thus some highlighted errors are wrong in the IDEA.

Please, try to compile code first before treating highlighted syntax "error" as an actual error. To prove some controversial point the explicit ttsN calls may also be used.

Tms Options

Options passed to environment variable or system property should be separated by ';'. Options passed to @tmsOptions() annotation or directly to *For()() call variants should be a list of literal strings and may be empty. Each literal string may contain separate option or its abbreviation (both ignoring case), or a list of ones separated by ';'. All following option lists have the same effect:

    "debug", "No trace", "No Predef Compliance", "embedded fors code view"
    "D;NT", "NPC;Embedded Fors Code View"

Incorrect option will be reported as an error. When directly specified pre-evaluation type is not found in the O types stack of the macro call then an error is raised too.

Options summary

Options Abbreviations Description
"Debug" / "No Debug" "D" / "ND" enables / disables macros debug output
"Trace" / "No Trace" "T" / "NT" enables / disables macros trace output
"Predef Compliance" / "No Predef Compliance" "PC" / "NPC" enables / disables any2stringadd predef compliance. When enabled Some(1) + "2" is "Some(1)2", when disabled - Some("12")
"Fors Stack For Apply Source" / "No Fors Stack For Apply Source" "FSFAS" / "NFSFAS" enables / disables building apply source containing ttsN in a separate inner Fors Stack
"Fors Stack For Apply Parameter" / "No Fors Stack For Apply Parameter" "FSFAP" / "NFSFAP" enables / disables building apply parameter containing ttsN in a separate inner Fors Stack
"Embedded Fors Code View" / "No Embedded Fors Code View" "EFCV" / "NEFCV" enables / disables embedding of the string containing fors-view of the macro output to macro result as first local val in the output block
"Single Fors Stack" "SFS" equivalent to applying "No Fors Stack For Apply Source" and "No Fors Stack For Apply Parameter" options at the same time
"All Fors Stacks" "AFS" equivalent to applying "Fors Stack For Apply Source" and "Fors Stack For Apply Parameter" options at the same time
"Pre Evaluate No Types" "PENT" disables pre evaluation for all Fors Stack types
"Pre Evaluate All Types" "PEAT" enables pre evaluation for all Fors Stack types
"Pre Evaluate Monadic Flow Type" "PEMFT" for mfc()/mfcFor()() macro calls it enables pre evaluation of the detected Monadic Flow Type
"Pre Evaluate class_name/FQN, ..." "PE cn/FQN, ..." adds the list of class names or fully qualified names to the set of pre evaluated types. For example, "PE Future, scala.util.Try"

Order of applying

  • default hardcoded options

      "No Debug"
      "No Trace"
      "Predef Compliance"
      "No Embedded Fors Code View"
      "Single Fors Stack"
      "Pre Evaluate No Types"
  • options from environment variable defaultTmsOptions (if present) separated by ';', for example

  • options from system property defaultTmsOptions (if present) separated by ';', for example, passed to sbt as

      -DdefaultTmsOptions="Debug;Trace" // quated when string contains ';'
  • options from @tmsOptions() annotations (if present) starting outer enclosing symbol of macro call to inner ones, for example

      @tmsOptions("No Debug", "NT", "Pre Evaluate All Types") 
      class Test // containig tms/mfc macro calls
  • options passed to tmsFor()() or mfcFor()() macro calls as the first group of parameters, for example

      tmsFor[Option[Int]]("Debug; PE Option") {
        Some(1) + 2
  • options from environment variable overrideTmsOptions (if present) separated by ';', for example

  • options from system property overrideTmsOptions (if present) separated by ';', for example, passed to sbt as

      -DoverrideTmsOptions="Debug;NT;PENT" // quated when string contains ';'

Any option may be changed at any level of the above "tree" overriding option value collected at previous steps. This makes possible to specify options for the groups of macro calls enclosed by, for example, a class, specify default or override options for all macro calls in the project by environment variable or system property, or set preferred options for each macro call separately.


This set of options enables building of pre-evaluation vals of the specified (or of all) types before the for corresponding to that type(s). Each expression of the for arrow enums is pre-evaluated before for until it depends on previous for-vals. We often use this technique to start several Futures in parallel before using them in for.

Fo example, enabling of the pre-evaluation in the following:

tmsFor[Future[Int]]("PE Future") {
  Future(1) + Future(2)

results the macro output:

  val valFuture$macro$1 = scala.concurrent.Future.apply[Int](1)(ec)
  val valFuture$macro$3 = scala.concurrent.Future.apply[Int](2)(ec)
  for {
    valueOfFuture$macro$2 <- valFuture$macro$1
    valueOfFuture$macro$4 <- valFuture$macro$3
  } yield {

If directly specified type of this option is absent in passed types stack then macro reports an error to prevent misprints. Short name or FQN (or both) may be passed to this option.

The group of options to control pre-evaluation is:

"Pre Evaluate No Types"
"Pre Evaluate All Types"
"Pre Evaluate Monadic Flow Type"
"Pre Evaluate <class_name>|<FQN>[, <class_name>|<FQN>]+"

Directly specified types in the stack of options (external, of enclosing annotations and directly passed to macro call) are accumulated. By default, pre-evaluation is off: "Pre Evaluate No Types".

Inner Fors Stacks to control evaluation Order

As noted above, by default the order of evaluation of tms-expressions that have stacked type of 2 or more monads becomes the "across" like "by-type" order: first, the parts of outer type are evaluated in the outer for, next - of the next inner monad type in the next for and so on.

Two options "Fors Stack For Apply Source" and "Fors Stack For Apply Parameter" make possible the complete evaluation of one tms-expression before evaluation of the next one with the same stacked type.

These options influence the order of evaluation of tms-expressions that are represented by .apply tree.

Using terminology apply_source.apply(apply_parameters) the following expression:

Some(1) + Some(2) + 3

has "apply sources":

  • Some
  • Some(1)
  • Some
  • Some(1) + Some(2)

and "apply parameters":

  • 1
  • 2
  • Some(2)
  • 3

When such option for apply-source or apply-parameter is enabled then expressions being the apply-source or passed to the apply-function as parameters are evaluated in the inner for stacks of the O type when the following condition is met: tms-expression is not a solid tms but depends on tms-subexpression.

When expression does not contain tms inside or is just solid tms value (wrapped to ttsNxxx call without inner tms) then building it in separate fors stack has no sense - fors stack will be degenerate.

In the above example only Some(1) + Some(2) apply source meets this condition:

tmsFor[Option[Int]]("FSFAS") {
  Some(1) + Some(2) + 3


  for {
    valueOfOption$macro$3 <- {
      for {
        valueOfSome$macro$1 <- scala.Some.apply[Int](1)
        valueOfSome$macro$2 <- scala.Some.apply[Int](2)
      } yield {
  } yield {

Example of "apply parameter"

tmsFor[Option[Int]]("FSFAP") {
  Some(1) + (Some(2) + 3)


  for {
    valueOfSome$macro$1 <- scala.Some.apply[Int](1)
    valueOfOption$macro$3 <- {
      for {
        valueOfSome$macro$2 <- scala.Some.apply[Int](2)
      } yield {
  } yield {

These options guarantee that in the case first evaluated tms-expression fails (or empty, etc.) then the second tms-expression will not be evaluated even partially (by its types stack).

More complex example with two types in the stack & inner fors stacks building for both apply-sources & apply-parameter:

tmsFor[Try[Option[Int]]]("All Fors Stacks") {
  (Try(Some(1)) + 2)*(Try(Some(3)) + 4)


  for {
    valueOfTry$macro$3 <- {
      for {
        valueOfTry$macro$1 <- scala.util.Try.apply[Some[Int]](scala.Some.apply[Int](1))
      } yield {
        for {
          valueOfSome$macro$2 <- valueOfTry$macro$1
        } yield {
    valueOfTry$macro$7 <- {
      for {
        valueOfTry$macro$5 <- scala.util.Try.apply[Some[Int]](scala.Some.apply[Int](3))
      } yield {
        for {
          valueOfSome$macro$6 <- valueOfTry$macro$5
        } yield {
  } yield {
    for {
      valueOfOption$macro$4 <- valueOfTry$macro$3
      valueOfOption$macro$8 <- valueOfTry$macro$7
    } yield {

Built degenerate fors are optimized while postprocessing.

These options are applicable only to the solid expressions based on the tree of .applys. Thus, for instance, if composite expression does not follow this options as a whole, only its predicate & branch expressions may do but independently and when ones are apply tree based.

These options also additionally influence the ability to use local definitions as a part of tms-expressions. If any local definition identifier (val, lazy val, var, type, class, object, def) is the part of tms-expression (is not a tms itself), for example:

val localVal = 2
Try[Int](Some(1) + localVal) // here `Some(1) + localVal` is a tms-expression due to `Some(1)` is used as Transparent Monads to add the Int   

then it will be compiled correctly with "Fors Stack For Apply Parameter" option disabled, but will fail when this option is enabled.

When "Fors Stack For Apply Parameter" is enabled then the whole expression Some(1) + localVal passed as parameter to Try[Int].apply should be extracted in the inner fors stack outside localVal definition and transparenMonads macro emits an error.

When "Fors Stack For Apply Parameter" is disabled then only Some(1) tms expression should be extracted in the embracing fors stack and no error occurs.

"Fors Stack For Apply Parameter" option may be enabled when this sample code with val localVal = Some(2) is passed to monadicFlowControl macro but this rule works only for stable val definitions and when the stack types outer than the Monadic Flow Type are not used in that tms-expression.

Using definitions of other types (not a stable vals) as part of tms-expression in source or parameter position with "Fors Stack For Apply Source" or "Fors Stack For Apply Parameter" option enabled results in an error for both macros.

The group of options that controls inner fors stacks building is:

"Single Fors Stack"
"Fors Stack For Apply Source" / "No Fors Stack For Apply Source"
"Fors Stack For Apply Parameter" / "No Fors Stack For Apply Parameter"    
"All Fors Stacks"

By default, building of the inner fors stacks is disabled: "Single Fors Stack".

Predef compliance (any2stringadd)

ttsNxxx implicits reacts to syntax implemented by any2stringadd Predef implicit and interprets it as tms syntax.

To prevent this behaviour macros replace such ttsNxxx wrapping of Any values to Predef any2stringadd method keeping predef compliance.

The option "Predef Compliance" controls this macro's behaviour.

In the example:

tmsFor[Try[String]]() {
  Try("1") + "2"

the output will be (resulting no tms at all):


When option is disabled:

tmsFor[Try[String]]("No Predef Compliance") {
  Try("1") + "2"

the input will be treated as tms expression:

  for {
    valueOfTry$macro$1 <- scala.util.Try.apply[String]("1")
  } yield {

By default, any2stringadd Predef compliance is enabled: "Predef Compliance".

Helper options

"Debug" and "Trace" options enable the tracing of the macro workflow, printing the input & output trees and code representations, tracing of the inner AST parsing & tms extraction, tracing of definition symbols, etc.

The full debug & trace output of the last example is the following:

[info] * tms >>> debug/trace of macro processing
tmsFor[Try[String]]("No Predef Compliance", "D", "T") {
 ^ .../
[debug] * tms TmsOptions: debug=true, trace=true, monadicFlowControl=false, forsStackForApplySource=false, forsStackForApplyParameter=false, preEvaluateTypes=Set(), predefCompliance=false, embeddedForsCodeView=false
[debug] * tms INPUT code.tree: sands.sugar.tms.TransparentMonads.tts1stringIdentity[scala.util.Try, String](scala.util.Try.apply[String]("1")).+("2")
[debug] * tms INPUT showCode(code.tree): sands.sugar.tms.TransparentMonads.tts1stringIdentity[scala.util.Try, String](scala.util.Try.apply[String]("1")).+("2")
[debug] * tms INPUT raw Expr: Expr(Apply(Select(Apply(TypeApply(Select(Select(Select(Select(Ident(sands), sands.sugar), sands.sugar.tms), sands.sugar.tms.TransparentMonads), TermName("tts1stringIdentity")), List(TypeTree(), TypeTree())), List(Apply(TypeApply(Select(Select(Select(Ident(scala), scala.util), scala.util.Try), TermName("apply")), List(TypeTree())), List(Literal(Constant("1")))))), TermName("$plus")), List(Literal(Constant("2")))))
[trace] * tms syntax tree: Unlifted TmsExpression = TmsOtherTree(sands,true)
[trace] * tms syntax tree: Unlifted TmsExpression = TmsOtherTree(sands.sugar,true)
[trace] * tms syntax tree: Unlifted TmsExpression = TmsOtherTree(sands.sugar.tms,true)
[trace] * tms syntax tree: Unlifted TmsExpression = TmsOtherTree(sands.sugar.tms.TransparentMonads,true)
[trace] * tms syntax tree: Unlifted TmsExpression = TmsOtherTree(scala,true)
[trace] * tms syntax tree: Unlifted TmsExpression = TmsOtherTree(scala.util,true)
[trace] * tms syntax tree: Unlifted TmsExpression = TmsOtherTree(scala.util.Try,true)
[trace] * tms syntax tree: Unlifted TmsExpression = TmsOtherTree("1",true)
[trace] * tms syntax tree: Unlifted TmsExpression = TmsOtherTree("1",true)
[trace] * tms syntax tree: Unlifted TmsExpression = TmsOtherTree(scala.util.Try.apply[String]("1"),true)
[trace] * tms syntax tree: Unlifted TmsExpression = TmsApply(RangePosition(<console>, 58, 61, 66),String @@ sands.sugar.tms.TransparentMonads.TmsStringImplicitExtensions,Some(TmsOtherTree(sands.sugar.tms.TransparentMonads,true)),tts1stringIdentity,None,List(scala.util.Try, String),List(List(TmsOtherTree(scala.util.Try.apply[String]("1"),true))))
[trace] * tms syntax tree: Unlifted TmsExpression = TmsOtherTree("2",true)
[trace] * tms syntax tree: Unlifted TmsExpression = TmsApply(RangePosition(<console>, 58, 67, 72),String,Some(TmsApply(RangePosition(<console>, 58, 61, 66),String @@ sands.sugar.tms.TransparentMonads.TmsStringImplicitExtensions,Some(TmsOtherTree(sands.sugar.tms.TransparentMonads,true)),tts1stringIdentity,None,List(scala.util.Try, String),List(List(TmsOtherTree(scala.util.Try.apply[String]("1"),true))))),$plus,None,List(),List(List(TmsOtherTree("2",true))))
[trace] * tms input code definition Symbols: none
[trace] * tms fors stack extraction: Created new TmsForsBuilder = TmsForsBuilder(0,List(TmsForCollector(scala.util.Try[String],List(),List())))
[trace] * tms fors stack extraction: Building Fors Stack monadic value extraction for tts types [Try] of expression = scala.util.Try.apply[String]("1")
[trace] * tms Definition Symbols of TmsExtractedTree(scala.util.Try.apply[String]("1")): none
[trace] * tms fors stack extraction: extractMonadicValue: Added For Arrow Enum = 'valueOfTry$macro$1 <- TmsExtractedTree(scala.util.Try.apply[String]("1"))'
[debug] * tms extracted fors code view:
  for {
    valueOfTry$macro$1 <- scala.util.Try.apply[String]("1")
  } yield {
[debug] * tms postprocessed fors are unchanged
[trace] * tms syntax tree: Setting NoSymbol to definition tree = ValDef(Modifiers(PARAM), TermName("valueOfTry$macro$1"), TypeTree(), EmptyTree)
[debug] * tms OUTPUT code.tree: scala.util.Try.apply[String]("1").map(((valueOfTry$macro$1) => valueOfTry$macro$1.$plus("2")))
[debug] * tms OUTPUT showCode(code.tree): scala.util.Try.apply[String]("1").map(((valueOfTry$macro$1) => valueOfTry$macro$1.+("2")))
[debug] * tms OUTPUT raw Expr: Expr(Apply(Select(Apply(TypeApply(Select(Select(Select(Ident(scala), scala.util), scala.util.Try), TermName("apply")), List(TypeTree())), List(Literal(Constant("1")))), TermName("map")), List(Function(List(ValDef(Modifiers(PARAM), TermName("valueOfTry$macro$1"), TypeTree(), EmptyTree)), Apply(Select(Ident(TermName("valueOfTry$macro$1")), TermName("$plus")), List(Literal(Constant("2"))))))))

Just sometimes helps to answer the question to yourself: "what are you doing, dude?" 😄 and saves time as usual.

The output is written to scalac stdout by println.

"Embedded Fors Code View" is another helpful option that embeds the output code represented by fors stack as a local string of the output code. It maybe used (but still not used) in tests withing toolbox for parsing and comparing results with expected or for usage with standard scalac debug options.

When this option is enabled and scalac has "-Ymacro-debug-lite" option passed then the scalac output of the last example will look like (run in worksheet):

performing macro expansion sands.sugar.tms.TransparentMonads.tmsFor[scala.util.Try[String]]("NPC", "EFCV")(sands.sugar.tms.TransparentMonads.tts1stringIdentity[scala.util.Try, String](scala.util.Try.apply[String]("1")).+("2")) at RangePosition(<console>, 0, 35, 55)
  val forsCodeView$macro$2 = "\n{\n  for {\n    valueOfTry$macro$1 <- scala.util.Try.apply[String](\"1\")\n  } yield {\n    valueOfTry$macro$1.$plus(\"2\")\n  }\n}\n";
  scala.util.Try.apply[String]("1").map(((valueOfTry$macro$1) => valueOfTry$macro$1.$plus("2")))
Block(List(ValDef(Modifiers(), TermName("forsCodeView$macro$2"), TypeTree(), Literal(Constant("
  for {
    valueOfTry$macro$1 <- scala.util.Try.apply[String]("1")
  } yield {
")))), Apply(Select(Apply(TypeApply(Select(Select(Select(Ident(scala), scala.util), scala.util.Try), TermName("apply")), List(TypeTree())), List(Literal(Constant("1")))), TermName("map")), List(Function(List(ValDef(Modifiers(PARAM), TermName("valueOfTry$macro$1"), TypeTree(), EmptyTree)), Apply(Select(Ident(TermName("valueOfTry$macro$1")), TermName("$plus")), List(Literal(Constant("2"))))))))
val res1: scala.util.Try[String] = Success(12)

The group of helper options is:

"Debug" / "No Debug"
"Trace" / "No Trace"
"Embedded Fors Code View" / "No Embedded Fors Code View"

By default, helper options are: "No Debug", "No Trace", "No Embedded Fors Code View".


Please, feel yourself free (in terms of LICENSE 😄 ) to use, polish & continue project, report & fix bugs, research & build theories, ask the questions, discuss and get the pleasure of its further development & thinking up.

Implement your own ideas based on the ready code base or add the new one here independently using the tests base to verify. Later (if it will grow and find the followers & users) we may make the project the part of some virtual organization on GitHub.

All repo related moments (bugs, questionable functionality, code suggestions, doc corrections, etc.: anything that may lead to the clear task to be done) are in Issues.

Questions and discussions of current & new ideas, ways to implement ones, restrictions and possible solutions, theory, etc. are in Discussions.

See for more details (or in the root of repo or artifact).

Hope these ideas with the opportunity to play realizations will give some good evenings and a pleasant thinking.

Spacial thanks to

  • authors and implementers of the beautiful Scala language
  • contributors & future one-thinkers this project will meet
  • 20 min. video that frees 20% of the brain (of monadic intuition) for dummies like me who started with assembler/C 😄 How the flow control monad was born


Number Released Changes
0.2.4 July, 2021 initial release