6.4.18. Custom compile-time errors¶
When designing embedded domain specific languages in Haskell, it is useful to
have something like error
at the type level. In this way, the EDSL designer
may show a type error that is specific to the DSL, rather than the standard GHC
type error.
For example, consider a type class that is not intended to be used with functions, but the user accidentally used it at a function type, perhaps because they missed an argument to some function. Then, instead of getting the standard GHC message about a missing instance, it would be nicer to emit a more friendly message specific to the EDSL. Similarly, the reduction of a type-level function may get stuck due to an error, at which point it would be nice to report an EDSL specific error, rather than a generic error about an ambiguous type.
To solve this, GHC provides a single type-level function,
type family TypeError (msg :: ErrorMessage) :: k
along with a small type-level language (via DataKinds
)
for constructing pretty-printed error messages,
-- ErrorMessage is intended to be used as a kind
data ErrorMessage =
Text Symbol -- Show this text as is
| forall t. ShowType t -- Pretty print a type
| ErrorMessage :<>: ErrorMessage -- Put two chunks of error message next to each other
| ErrorMessage :$$: ErrorMessage -- Put two chunks of error message above each other
in the GHC.TypeLits module.
For instance, we might use this interface to provide a more useful error
message for applications of show
on unsaturated functions like this,
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE UndecidableInstances #-}
import GHC.TypeLits
instance TypeError (Text "Cannot 'Show' functions." :$$:
Text "Perhaps there is a missing argument?")
=> Show (a -> b) where
showsPrec = error "unreachable"
main = print negate
Which will produce the following compile-time error,
Test.hs:12:8: error:
• Cannot 'Show' functions.
Perhaps there is a missing argument?
• In the expression: print negate
In an equation for ‘main’: main = print negate