Too many type arguments in constructor pattern [GHC-01629]

Every type application has to be “consumed” by a type variable in the corresponding type. Just as it is an error to pass more than one argument to putStrLn, it is also an error to apply more types than there are type variables in a type. For instance, if a type has only 1 type parameter (e.g. Maybe a), we can only apply 1 type argument (e.g. applying @Int to produce Maybe Int); there is no unbound type parameter present for further applications.

Examples

Too many type arguments in constructor pattern

The @Bool is too much in this example. There is nothing to bind to it, so it is not allowed to be there.

Error Message

TyArgs.hs:6:4: error: [GHC-01629]
    • Too many type arguments in constructor pattern for ‘Just’
      Expected no more than 1; got 2
    • In the pattern: Just @Int @Bool x
      In an equation for ‘f’: f (Just @Int @Bool x) = x
  |
6 | f (Just @Int @Bool x) = x
  |    ^^^^^^^^^^^^^^^^^

TyArgs.hs

Before

{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
module TyArgs where

f :: Maybe Int -> Int
f (Just @Int @Bool x) = x
f Nothing = 10

After

{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
module TyArgs where

f :: Maybe Int -> Int
f (Just @Int x) = x
f Nothing = 10