Why can't you (completely) apply a synonym for a type that has arguments using a synonym for another type?

Question

We can define type synonyms with arguments, and this works great when used with actual types:

type MyType t = t String String

data Test a b = Test a b

f :: MyType Test
f = undefined

main = undefined

Compiling the results does not cause errors:

$ghc --make test.hs                                           
[1 of 1] Compiling Main             ( test.hs, test.o )                                          
Linking test ...   

However, this does not work if it Testis a type synonym:

type MyType t = t String String

data Test a b = Test a b

type Test' a b = Test a b

f :: MyType Test'
f = undefined

main = undefined

Which gives the following error:

$ghc --make test.hs
[1 of 1] Compiling Main             ( test.hs, test.o )

test.hs:7:6:
    Type synonym Test' should have 2 arguments, but has been given none
    In the type signature for `f': f :: MyType (Test')

What puzzles me is that Test' applies to two arguments, so why does the GHC complain that I didn't pass arguments?

Shouldn't a synonym be entered completely transparent and impossible to distinguish from other types?

Is there a way to achieve the expected behavior?

Answer 1

Haskell report :

T, ; T .

, , , :

MyType Test'

MyType , , Test' .

, GHC LiberalTypeSynonyms. , - , , .

^.

, , - :

Test' Int

- . , , .

, .