tek / splain

better implicit errors for scala


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.


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

addCompilerPlugin("io.tryp" % "splain" % "0.5.7" 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 "0.5.7"
  ("io.tryp" %% "splain" % v cross CrossVersion.patch).withConfigurations(Some("plugin->default(compile)"))


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


param can be one of the following:

  • all
  • infix
  • foundreq
  • implicits
  • bounds (default off)
  • color
  • breakinfix (default 0)
  • tree
  • compact (default off)
  • boundsimplicits
  • truncrefined (default 0)
  • rewrite (string)
  • keepmodules (default 0)

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:

scalacOptions += "-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