How do you use TypeApplications in Haskell?

Question

With -XTypeApplications in GHC 8.0, you can specify types explicitly with @ preceding function arguments. What types does it exactly specify, especially when several @ are introduced?

Answer 1

If you look at the type of a function

elem :: (Foldable t, Eq a) => a -> t a -> Bool

we see it has two polymorphic variables, t and a. These variables are what the @ type applications specify. It seems that variables introduced in the context — where typeclass constraints go — affect order, and hence the first @ specifies the t, and the second the a. In functions without context variables

const :: a -> b -> a

the order is more obvious, the a is first and b is second. As Cactus mentioned in a comment above, you can also use explicit foralls to specify the order yourself.

myConst :: forall b a. a -> b -> a

Now the first type application will specify the b and the second the a.

You may run into this problem of needing to specify types particularly if you're using overloaded strings or lists

elem c "abc...xyz" -- What string type is this?
elem c ['a' .. 'z'] -- What list constructor is this?

therefore we use explicit type applications

elem @[] @Char c ['a' .. 'z']

in this case we only have to specify the @[] and say "this is a [] list type constructor" because GHC infers Char from the list elements, so @Char can be omitted here.

If a polymorphic argument GHC is able to infer happens to come first you can leverage -XPartialTypeSignatures which allows you to use _ in type signatures including type application signatures, telling GHC to just infer that [part of the] type, to make things less verbose.

f @_ @[]