base- Basic libraries
Safe HaskellTrustworthy



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.

Since: base-



data ErrorMessage Source #

A description of a custom type error.


Text Symbol

Show the text as is.

ShowType t

Pretty print the type. ShowType :: k -> ErrorMessage

ErrorMessage :<>: ErrorMessage infixl 6

Put two pieces of error message next to each other.

ErrorMessage :$$: ErrorMessage infixl 5

Stack two pieces of error message on top of each other.

type family TypeError (a :: ErrorMessage) :: b where ... Source #

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.")

Since: base-

type family Assert (check :: Bool) errMsg where ... Source #

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.

Since: base-


Assert 'True _1 = () 
Assert _1 errMsg = errMsg 

class Unsatisfiable (msg :: ErrorMessage) Source #

An unsatisfiable constraint. Similar to TypeError when used at the Constraint kind, but reports errors in a more predictable manner.

See also the unsatisfiable function.


Minimal complete definition


unsatisfiable :: forall (msg :: ErrorMessage) a. Unsatisfiable msg => a Source #

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