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Contextual makes it simple to write typesafe, statically-checked interpolated strings.

Contextual is a Scala library which allows you to define your own string interpolators—prefixes for interpolated string literals like url""—which specify how they should be checked at compiletime and interpreted at runtime, writing very ordinary user code with no user-defined macros.


  • user-defined string interpolators
  • introduce compiletime failures on invalid values, such as url"htpt://"
  • compiletime behavior can be defined on literal parts of a string
  • runtime behavior can be defined on literal and interpolated parts of a string
  • types of interpolated values can be context-dependent

Getting Started

About Interpolators

An interpolated string is any string literal prefixed with an alphanumeric string, such as s"Hello World" or date"15 April, 2016". Unlike ordinary string literals, interpolated strings may also include variable substitutions: expressions written inline, prefixed with a $ symbol, and—if the expression is anything more complicated than an alphanumeric identifier—requiring braces ({, }) around it. For example,

s"Hello, $name"


s"Tomorrow will be Day ${day + 1}."

Anyone can write an interpolated string using an extension method on StringContext, and it will be called, like an ordinary method, at runtime.

But it's also possible to write an interpolator which is called at compiletime, and which can identify coding errors before runtime.

Contextual makes it easy to write such interpolators.

Contextual's Interpolator type

An interpolated string may have no substitutions, or it may include many substitutions, with a string of zero or more characters between at the start, end, and between each adjacent pair.

So in general, any interpolated string can be represented as n string literals, whose values are known at compiletime, and n - 1 variables (of various types), whose values are not known until runtime.

Contextual's Interpolator interface provides a set of five abstract methods—initial, parse, insert, skip and complete—which are invoked, in a particular order, once at compiletime, without the substituted values (since they are not known!), and again at runtime, with the substituted values (when they are known).

The method skip is used at compiletime, and insert at runtime.

The methods are always invoked in the same order: first initial; then alternately parse and insert/skip, some number of times, for each string literal and each substitution (respectively); and finally complete to produce a result. insert may never be invoked if there are no substitutions, but parse will always be invoked once more than insert.

For example, for a string with two substitutions, the invocation order would be:

initial -> parse -> insert -> parse -> insert -> parse -> complete

at runtime, or,

initial -> parse -> skip -> parse -> skip -> parse -> complete

at compiletime.

An object encoding the interpolator's state is returned by each of these method calls, and is passed as input to the next—with the exception of complete, which should return the final value that the interpolated string will evaluate to. This is where final checks can be carried out to check that the interpolated string is in a complete final state.

In other words, each segment of an interpolated string is read in turn, to incrementally build up a working representation of the incomplete information in the interpolated string. And at the end, it is converted into the return value.

The separation into parse and insert/skip calls means that the static parts of the interpolated string can be parsed the same way at both compiletime or runtime, while the dynamic parts may be interpreted at runtime when they're known, and their absence handled in some way at compiletime when they're not known.

Of course, skip could be implemented to delegate to insert using a dummy value.

Here are the signatures for each method in the Interpolator type:

trait Interpolator[Input, State, Result]:
  def initial: State
  def parse(state: State, next: String): State
  def insert(state: State, value: Input): State
  def skip(state: State): State
  def complete(value: State): Return

Three abstract types are used in their definitions: State represents the information passed from one method to the next, and could be as simple as Unit or String, or could be some complex document structure. Input is a type chosen to represent the types of all substitutions. String would be a common choice for this, but there may be utility in using richer types, too. And Return is the type that the interpolated string will ultimately evaluate to.

In addition to parse, insert, skip and complete taking State instances as input, note that parse always takes a String, and insert takes an Input.

Any of the methods may throw an InterpolationError exception, with a message. At compiletime, these will be caught, and turned into compile errors. Additionally, a range of characters may be specified to highlight precisely where the error occurs in an interpolated string.

Any interpolator needs to choose these three types, and implement these four methods.

For example, the interpolated string,


could be interpreted by a Contextual interpolator, in which case it would be checked at compiletime with the composed invocation,

complete(parse(insert(parse(insert(parse(initial, ""), None),
    "/images/"), None), ""))

and at runtime with something which is essentially this:

complete(parse(insert(parse(insert(parse(initial, ""), Some(dir)),
    "/images/"), Some(img)), ""))

Compile Errors

Throwing exceptions provides the flexibility to raise a compilation error just by examining the state value and/or the other inputs.

For example, insertions could be permitted only in appropriate positions, i.e. where the state value passed to the insert method indicates that the insertion can be made. That is knowable at compiletime, without even knowing the inserted value, and can be generated as a compile error by throwing an InterpolationError in the implementation of insert.

The compile error will point at the substituted expression.

Likewise, throwing an InterpolationError in parse will generate a compile error. The optional second parameter of InterpolationError allows an offset to be specified, relative to the start of the literal part currently being parsed, and a third parameter allows its length to be specified.

For example, if we were parsing url"https://example.ocm/$dir/images/$img", and wanted to highlight the mistake in the invalid TLD .ocm, we would throw,

InterpolationError("not a valid TLD", 15, 4)

during the first invocation of parse, and the Scala compiler would highlight .ocm as the error location: in this example, 15 is the offset from the start of this part of the string to the error location, and 4 is the length of the error.

Binding an interpolator

A small amount of boilerplate is needed to bind an Interpolator object, for example Abc, to a prefix, i.e. the letters abc in the interpolated string, abc"":

extension (inline ctx: StringContext)
  transparent inline def abc(inline parts: Any*): Return =
    ${Abc.expand('Abc, 'ctx, 'parts)}

This boilerplate should be modified as follows:

  • the method name, abc, should change to the desired prefix,
  • the method's return type, Return, should be changed to the return type of the complete method, and,
  • the interpolator object, Abc, should be specified (twice).

In particular, the type of parts, Any*, should be left unchanged. This does not mean that Any type may be substituted into an interpolated string; Contextual provides another way to constrain the set of acceptable types for insertions.


Contextual uses a typeclass interface to support insertions of different types. An insertion of a particular type, T, into an interpolator taking a value of type I requires a corresponding given Insertion[I, T] instance in scope.

This means that the set of types which may be inserted into an interpolated string can be defined ad-hoc. There is only the requirement that any inserted type, T, may be converted to an I, since I is a type known to the Interpolator implementation.

So, if an interpolator's general Input type is List[String], and we wanted to permit insertions of List[String], String and Int, then three given instances would be necessary:

given Insertion[List[String], String] = List(_)
given Insertion[List[String], List[String]] = identity(_)
given Insertion[List[String], Int] = int => List(int.toString)


A Substitution is a typeclass that's almost identical to Insertion (and is, indeed, a subtype of Insertion), but takes an additional type parameter: a singleton String literal. The behavior of a given Substitution will be identical to a given Insertion at runtime, but differs at compiletime:

During macro expansion, instead of invoking skip, the substitute method will be called instead, passing it the String value taken from the additional type parameter to Substitution.

For example the given definitions,

given Substitution[XInput, String, "\"\""] = str => StrInput(str)
given Substitution[XInput, Int, "0"] = int => IntInput(int)

would mean that an Int, int, and a String, str, substituted into an interpolated string would result in invocations of, substitute(state, "0") and substitute(state, "\"\"") respectively.

By default, the substitute method simply delegates to parse, which takes the same parameters, and will parse the substituted strings in a predictable way. Any user-defined substitute method implementation will therefore need the override modifier, but can provide its own implementation that is distinct from parse.

The benefit of Substitution over Insertion is that the compiletime interpretation of the interpolated string may be dependent on the types inserted, distinguishing between types on the basis of the singleton String literal included in the given's signature. This compares to the skip method which offers no more information about a substitution than its existence.

A First Interpolator

Here is a trivial interpolator which can parse, for example, hex"a948b0${x}710bff", and return an IArray[Byte]:

object Hex extends Interpolator[Long, String, IArray[Byte]]:
  def initial: String = ""

  def parse(state: String, next: String): String =
    if next.forall(hexChar(_)) then state+next
    else throw InterpolationError("not a valid hexadecimal character")
  def insert(state: String, value: Option[Long]): String =
    value match
      case None       => state+"0"
      case Some(long) => state+long.toHexString
  def complete(state: String): IArray[Byte] =
  private def hexChar(ch: Char): Boolean =
    ch.isDigit || ch >= 'a' && ch <= 'f' || ch >= 'A' && ch <: 'F'

Having defined this interpolator, we can bind it to the prefix, hex with:

extension (ctx: StringContext)
  transparent inline def hex(inline parts: Any*): IArray[Byte] =
    ${Hex.expand('Hex, 'ctx, 'parts)}

Note that this should be defined in a different source file from the object Hex.

Related Projects

The following Niveau libraries are dependencies of Contextual:


The following Niveau libraries are dependents of Contextual:



Contextual is classified as maturescent. Propensive defines the following five stability levels for open-source projects:

  • embryonic: for experimental or demonstrative purposes only, without any guarantees of longevity
  • fledgling: of proven utility, seeking contributions, but liable to significant redesigns
  • maturescent: major design decisions broady settled, seeking probatory adoption and refinement
  • dependable: production-ready, subject to controlled ongoing maintenance and enhancement; tagged as version 1.0 or later
  • adamantine: proven, reliable and production-ready, with no further breaking changes ever anticipated

Contextual is designed to be small. Its entire source code currently consists of 91 lines of code.


Contextual can be built on Linux or Mac OS with Vex, by running the vex script in the root directory:


This script will download vex the first time it is run, start a daemon process, and run the build. Subsequent invocations will be near-instantaneous.


Contributors to Contextual are welcome and encouraged. New contributors may like to look for issues marked label: good first issue.

We suggest that all contributors read the Contributing Guide to make the process of contributing to Contextual easier.

Please do not contact project maintainers privately with questions. While it can be tempting to repsond to such questions, private answers cannot be shared with a wider audience, and it can result in duplication of effort.


Contextual was designed and developed by Jon Pretty, and commercial support and training is available from Propensive OÜ.


Contextual takes its name from its ability to provide context-aware substitutions in interpolated strings.


Contextual is copyright © 2016-22 Jon Pretty & Propensive OÜ, and is made available under the Apache 2.0 License.