6.4.16. Type abstractions

TypeAbstractions
Since:9.8.1
Status:Partially implemented

Allow the use of type abstraction syntax.

The TypeAbstractions extension provides a way to explicitly bind scoped type or kind variables using the @a syntax. The feature is only partially implemented, and this text covers only the implemented parts, whereas the full specification can be found in GHC Proposals #448 and #425.

6.4.16.1. Type Abstractions in Patterns

The type abstraction syntax can be used in patterns that match a data constructor. The syntax can’t be used with record patterns or infix patterns. This is useful in particular to bind existential type variables associated with a GADT data constructor as in the following example:

{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeApplications #-}
import Data.Proxy

data Foo where
  Foo :: forall a. (Show a, Num a) => Foo

test :: Foo -> String
test x = case x of
  Foo @t -> show @t 0

main :: IO ()
main = print $ test (Foo @Float)

In this example, the case in test` is binding an existential variable introduced by Foo that otherwise could not be named and used.

It’s possible to bind variables to any part of the type arguments to a constructor; there is no need for them to be existential. In addition, it’s possible to “match” against part of the type argument using type constructors.

For a somewhat-contrived example:

foo :: (Num a) => Maybe [a] -> String
foo (Nothing @[t]) = show (0 :: t)
foo (Just @[t] xs) = show (sum xs :: t)

Here, we’re binding the type variable t to be the type of the elements of the list type which is itself the argument to Maybe.

The order of the type arguments specified by the type applications in a pattern is the same as that for an expression: either the order as given by the user in an explicit forall in the definition of the data constructor, or if that is not present, the order in which the type variables appear in its type signature from left to right.

For example if we have the following declaration in GADT syntax:

data Foo :: * -> * where
  A :: forall s t. [(t,s)] -> Foo (t,s)
  B :: (t,s) -> Foo (t,s)

Then the type arguments to A will match first s and then t, while the type arguments to B will match first t and then s.

Type arguments appearing in patterns can influence the inferred type of a definition:

foo (Nothing @Int) = 0
foo (Just x) = x

will have inferred type:

foo :: Maybe Int -> Int

which is more restricted than what it would be without the application:

foo :: Num a => Maybe a -> a

For more information and detail regarding type applications in patterns, see the paper Type variables in patterns by Eisenberg, Breitner and Peyton Jones. Relative to that paper, the implementation in GHC for now at least makes one additional conservative restriction, that type variables occurring in patterns must not already be in scope, and so are always new variables that only bind whatever type is matched, rather than ever referring to a variable from an outer scope. Type wildcards _ may be used in any place where no new variable needs binding.