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A description of a custom type error.

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

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-4.17.0.0

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.

since base-4.19.0.0.

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.