tek / splain   0.2.6

MIT License GitHub

better implicit errors for scala

Scala versions: 2.12 2.11 2.10

A scala compiler plugin for more concise errors

This plugin removes some of the redundancy of the compiler output and prints additional info for implicit resolution errors.



                 │What's your Scala version?│                  
                           │       │                           
                           │       └───────────────────┐       
                           v                           │       
    ┌─────────────────────────────────────────────┐    │       
    │                ( >= 2.13.6 )                │    │       
    │                ─────────────                │    │       
    │Do you want experimental features & bugfixes?│    │       
    └──┬───────────────┬──────────────────────────┘    │       
       │               │                               │       
       v               v                               v       
 ┌──────────┐ ┌─────────────────┐ ┌───────────────────────────┐
 │ ( yes )  │ │     ( no )      │ │( 2.12 / 2.13.0 .. 2.13.5 )│
 │ ───────  │ │     ──────      │ ├───────────────────────────┤
 │Splain 1.x│ │Compiler built-in│ │       Splain 0.5.x        │
 └──────────┘ └─────────────────┘ └───────────────────────────┘

Compiler built-in

(main article: https://docs.scala-lang.org/overviews/compiler-options/index.html#Verbose_Settings)

The basic Splain features has been integrated into Scala compiler (since 2.13.6, through contributions like this and this), they can be enabled immediately by using the right compiler options (see Option section for detail).

v1.x (master branch)

(Only available for Scala 2.13.6+)

Splain 1.x is a simplified rewrite that aims to incrementally introduce enhancement to the already integrated Splain features. Additional features and bugfixes will first be released and refined here, then be periodically contributed back into Scala compiler.

Effectively, Splain 1.x is now a feature preview patch of Scala compiler, if a relevant compiler built-in option (see Option section for detail) malfunctions, it may work with Splain v1.x enabled.

It is also the only branch under active development.

v0.5.x (maintenance branch)

(Only available for Scala 2.12 and Scala 2.13.0 .. 2.13.5)

The latest v0.x will continue to be maintained and published regularly to stay compatible with the latest Scala 2.12.x release (until it's end-of-life), but no newer version will be published for Scala 2.13, splain 0.5.x will be the last release for Scala 2.13.

We strongly recommend you to upgrade to Scala 2.13.6+ to benefit from active support and up-to-date features.

Build Matrix

Version Status Compatibility
(current) - latest
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v1.x, v0.x

Include this line in your build.sbt (not project/plugins.sbt!!):

addCompilerPlugin("io.tryp" % "splain" % "0.5.8" cross CrossVersion.patch)

If you want to support scala versions both newer and older than 2.12.5, use:

libraryDependencies += {
  val v =
    if (scalaVersion.value.replaceFirst(raw"\.(\d)$$",".0$1") <= "2.12.04") "0.4.1"
    else "0.5.8"
  ("io.tryp" %% "splain" % v cross CrossVersion.patch).withConfigurations(Some("plugin->default(compile)"))

If you are using gradle with scala plugin, include this line under the dependency section of your build.gradle:

scalaCompilerPlugins group: 'io.tryp', name: 'splain_${scalaVersion}', version: '0.5.8'

or build.gradle.kts:


compiler built-in, no plugin declaration required

Its effects however still have to be enabled in your compiler options, in minimal case, by the following 2 options (see Options for details):

-Vimplicits -Vtype-diffs


The plugin can be configured via compiler Options with the format:

v0.x built-in, v1.x
-P:splain:<param>[:<value>] -<param>[:<value>]

param can be one of the following:

v0.x built-in, v1.x default value
all enabled true
infix (dropped)
foundreq Vtype-diffs false
implicits Vimplicits false
bounds (dropped) false
color (dropped)
breakinfix (dropped) 0
tree Vimplicits-verbose-tree
compact (dropped) false
boundsimplicits (dropped)
truncrefined Vimplicits-max-refined 0
rewrite (dropped) (do not rewrite)
keepmodules (dropped) 0
(N/A) P:splain:Vimplicits-diverging false
(N/A) P:splain:Vimplicits-diverging-max-depth 100
(N/A) P:splain:Vtype-detail 1
(N/A) P:splain:Vtype-diffs-detail 1

value can either be true or false. If omitted, the default is true for both value and parameter.

The parameter all can be used to deactivate all features.

The parameters can be applied like this:

(in sbt)

scalacOptions += "-P:splain:implicits:false"

(in gradle with scala plugin)

withType<ScalaCompile> {
    scalaCompileOptions.apply {
        additionalParameters = listOf("-P:splain:implicits:false")

infix types

Instead of shapeless.::[A, HNil], prints A :: HNil.

found/required types

Rather than printing up to four types, only the dealiased types are shown as a colored diff:


special consideration for shapeless.Record:


In the case of refined types in the form of Client with Database with Publisher, the types will be matched with each other and a missing or surplus type will be indicated by a <none> label.

implicit resolution chains

When an implicit is not found, only the outermost error at the invocation point is printed. This can be expanded with the compiler flag -Xlog-implicits, but that also shows all invalid implicits for parameters that have been resolved successfully. This feature prints a compact list of all involved implicits: implicits

Here, !I stands for could not find implicit value, the name of the implicit parameter is in yellow, and its type in green.

If the parameter tree is set, the candidates will be indented according to their nesting level:


If the parameter compact is set, only the first and last implicit in a chain will be printed.

If the parameter boundsimplicits is set to false, any nonconformant bounds errors will be suppressed.

For comparison, this is the regular compiler output for this case (with formatted types):

[info] unit/src/basic.scala:35: f is not a valid implicit value for
splain.ImplicitChain.T2 because:
[info] hasMatchingSymbol reported error: could not find implicit value for
parameter impPar2: (D *** (C *** String)) >:< ((C,D,C) *** D)
[info]   implicitly[T1]
[info]             ^
[info] unit/src/basic.scala:35: g is not a valid implicit value for
splain.ImplicitChain.T1 because:
[info] hasMatchingSymbol reported error: could not find implicit value for
parameter impPar1: D *** ((C >:< C) *** (D => Unit))
[info]   implicitly[T1]
[info]             ^
[error] unit/src/basic.scala:35: could not find implicit value for
parameter e: (C *** D) >:< C with D {type A = D; type B = C}
[error]   implicitly[T1]

infix type and type argument line breaking

If the parameter breakinfix is given and greater than 0, types longer than that number will be split into multiple lines:

implicit error;
!I e: String
f invalid because
!I impPar4: List[
    VeryLongTypeName ::::
    VeryLongTypeName ::::
    VeryLongTypeName ::::
  (Short :::: Short) ::::
    VeryLongTypeName ::::
    VeryLongTypeName ::::
    VeryLongTypeName ::::
  VeryLongTypeName ::::
  VeryLongTypeName ::::
  VeryLongTypeName ::::

truncating refined types

A type of the shape T { type A = X; type B = Y } will be displayed as T {...} if the parameter truncrefined is set to a value /= 0 and the refinement's length is greater than the value.

truncating module paths

Default behaviour when printing type names is to omit the whole module path and only print the last segment. Two options modify this behaviour:

regex rewrite

The option rewrite takes a string that is parsed as a ;-delimited list of regexes and optional replacements.

For example:


This parses as two rewrite items:

  • transform cats.data into cd
  • delete all occurences of .Type

If a slash is present, the string following it will be used as a replacement for the matched text. If it is absent, the empty string is substituted.

dropping module segments by count

The option keepmodules determines how many segments of the module path before the type name will be displayed, but only if the rewrite mechanism hasn't changed anything.

So with -P:splain:keepmodules:2, the qualified type cats.free.FreeT.Suspend will be displayed as free.FreeT.Suspend, keeping the two segments free.FreeT before the type name. The default is 0, so only the type name itself will be displayed

expanding diverging implicit errors (experimental)

A diverging implicit error is thrown by compiler if it cannot decide if an implicit search can terminate in polynomial time (e.g. if the search algorithm encounter a loop or infinite expansion). In most cases, such error will cause the entire search to fail immediately, but there are few exceptions to this rule, for which the search can backtrack and try an alternative path to fulfil the implicit argument. Either way, the Scala compiler error is only capable of showing the entry point of such loop or infinite expansion:

diverging implicit expansion for type splain.DivergingImplicits.C
starting with method f in object Circular

If the parameter -P:splain:Vimplicits-diverging is enabled, it will instruct the compiler to continue its implicit search process until an implicit resolution chain can be correlated with such error(s):

implicit error;
!I e: C
f invalid because
!I c: C
diverging implicit expansion for type C
starting with method f in object Endo
――f invalid because
  !I c: C
  diverging implicit expansion for type C
  starting with method f in object Endo

EXPERIMENTAL! sometimes this feature may cause failed implicit resolution to succeed, due to the delay in throwing the diverging implicit error. It may also increase compilation time slightly. If your build has been broken by this feature, please consider simplifying your code base to create a minimal reproducible test case, and submit it with a pull request.

type detail (experimental)

The option -P:splain:Vtype-detail:X can take an integer from 1 to 6 to attach different kinds of details to type information in any error message.

  • 1 (DEFAULT) : type info in short form, by using toString (same as pre-1.1.0)
  • 2 = long : type info in long form, by using toLongString
  • 3 = 2 + (existential : existential context)
  • 4 = 3 + (reduction : explain type reduction process)
  • 5 = 4 + (position : type definition position in code)
  • 6 = 5 + (alias : explain type aliases, this generally contains duplicate information with 3, it is only included for completeness)

For example:


XXX.scala:15: error: type mismatch;


In addition, multiple names of the detail kind (denoted by bold text in the above list) can be appended to the option value to enable it, e.g. -P:splain:Vtype-detail:1,reduction,position can attach type reduction process & type definition position while bypassing long and existential.

type diffs detail (experimental)

The option -P:splain:Vtype-diffs-detail:X can take an integer from 1 to 4 to augment type diff errors with different kinds of details.

  • 1 (DEFAULT) : no augmentation (same as pre-1.1.0)
  • 2 = disambiguation : augment type info with disambiguation for both sides in <found>|<required> and infix types (e.g. A =:= B, A <:< B) in error message
  • 3 = 2 + (builtIn : attach built-in found/required errors emitted by Scala compiler IF AND ONLY IF both sides of the error message are identical)
  • 4 = 3 + (builtInAlways : ALWAYS attach original found/required error info, even if both sides of the error message are different)

In addition, multiple names of the detail kind (denoted by bold text in the above list) can be appended to the option value to enable it, e.g. -P:splain:Vtype-diffs-detail:1,builtIn can attach built-in errors while bypassing disambiguation.

For example:


XXX.scala:16: error: implicit error;
!I ev: Long =:= Long
  Cannot prove that Long =:= Long.




Due to the nature of the hack that allows splain to hook into the implicit search algorithm, other plugins using the same trick may not work or cause splain to be inactive.

Another victim of splain is scaladoc – doc comments might disappear when running the task with splain active, so make sure it is disabled before doing so.

Users are encouraged to submit issues and test cases directly through pull requests, by forking the project and adding new test cases under:

v0.x v1.x
<project root>/src/test/scala/splain <project root>/core/src/test/scala/splain/plugin

The bug can thus be identified by the team quickly on our continuous integration environment. Submission on our GitHub issue tracker is also welcomed, but it generally takes much longer for the team to respond.

How to compile

v1.x (from git branch master)

Built with the latest Gradle, to compile and publish locally:

./gradlew clean testClasses publishToMavenLocal

to run all tests:

./gradlew test

v0.x (from git branch Maintenance/master)

Built with the latest stable SBT. to compile and publish locally:

sbt clean publishM2

to run all tests:

sbt test

How to edit

Most project contributors uses neovim, IntelliJ IDEA or visual studio code.

The team strive for a strong discipline in software engineering. All commits (including SNAPSHOTs and PRs) will be compliant with scalalfmt standard.


  • @tek - reviewer for built-in/v0.x bugfix, new features
  • @tribbloid - reviewer for v1.x bugfix
  • @dwijnand - reviewer for scala compiler integration