| Safe Haskell | Trustworthy |
|---|---|
| Language | Haskell2010 |
GHC.TypeError
Description
This module exports the TypeError family, which is used to provide custom type errors, and the ErrorMessage kind used to define these custom error messages. This is a type-level analogue to the term level error function.
Since: base-4.16.0.0
Synopsis
- data ErrorMessage
- = Text Symbol
- | forall t. ShowType t
- | ErrorMessage :<>: ErrorMessage
- | ErrorMessage :$$: ErrorMessage
- type family TypeError (a :: ErrorMessage) :: b where ...
- type family Assert check errMsg where ...
Documentation
data ErrorMessage Source #
A description of a custom type error.
Constructors
| Text Symbol | Show the text as is. |
| forall t. ShowType t | Pretty print the type.
|
| ErrorMessage :<>: ErrorMessage infixl 6 | Put two pieces of error message next to each other. |
| ErrorMessage :$$: 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-4.9.0.0
type family Assert check 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-4.16.0.0