Type

Documentation

type Type = *

The kind of types with values. For example Int :: Type.

type 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.")

Since: base-4.9.0.0

data ErrorMessage where

A description of a custom type error.

Constructors

Text :: ErrorMessage

Show the text as is.

ShowType :: ErrorMessage

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

(:<>:) :: ErrorMessage infixl 6

Put two pieces of error message next to each other.

(:$$:) :: ErrorMessage infixl 5

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