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GHC 9.10.3 · lts/ghc-9.10.x · 248f8f0 · 2026-10-05

Modulebase-4.20.2.0Haskell2010

GHC.TypeError

This module exports:

  • The TypeError type family, which is used to provide custom type errors. This is a type-level analogue to the term level error function.

  • The ErrorMessage kind, used to define custom error messages.

  • The Unsatisfiable constraint, a more principled variant of TypeError which gives a more predictable way of reporting custom type errors.

  • 1 type
  • 1 class
  • 1 value
  • Packagebase-4.20.2.0
  • Exports5
  • LanguageHaskell2010
  • LicenceBSD-3-Clause
  • SourceTypeError.hs
familytype family TypeError (a :: ErrorMessage) :: b where
#

The type-level equivalent of error.

The polymorphic kind of this type allows it to be used in several settings. For instance, it can be used as a constraint, e.g. to provide a better error message for a non-existent instance,

-- in a context
instance TypeError (Text "Cannot Show functions." :$$:
                    Text "Perhaps there is a missing argument?")
      => Show (a -> b) where
    showsPrec = error "unreachable"

It can also be placed on the right-hand side of a type-level function to provide an error for an invalid case,

type family ByteSize x where
   ByteSize Word16   = 2
   ByteSize Word8    = 1
   ByteSize a        = TypeError (Text "The type " :<>: ShowType a :<>:
                                  Text " is not exportable.")
familytype family Assert (check :: Bool) (errMsg :: Constraint) :: Constraint where
#

A type-level assert function.

If the first argument evaluates to true, then the empty constraint is returned, otherwise the second argument (which is intended to be something which reduces to TypeError is used).

For example, given some type level predicate P' :: Type -> Bool, it is possible to write the type synonym

type P a = Assert (P' a) (NotPError a)

where NotPError reduces to a TypeError which is reported if the assertion fails.

Equations

valueunsatisfiable :: Unsatisfiable msg => a
#

Prove anything within a context with an Unsatisfiable constraint.

This is useful for filling in instance methods when there is an Unsatisfiable constraint in the instance head, e.g.:

instance Unsatisfiable (Text "No Eq instance for functions") => Eq (a -> b) where

(==) = unsatisfiable

since base-4.19.0.0.