inferred type is not accepted when included

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A function which compiles and cabal repl even indicates what a specific type should be (rel) produces an error if the inferred type is included in the code.

coords2faces  :: (MorphsHQ rel, Eq rel, Eq obj,   Ord obj)  => State (Store obj rel)  [(obj, obj)]
coords2faces = do 
    f <- rel3 (hqFace :: rel)
    return  f

How to include a type in code in such a case?

The error message is

app/TheMain.hs:107:16: error:
    • Could not deduce (MorphsHQ rel1) arising from a use of ‘hqFace’
      from the context: (MorphsHQ rel, Eq rel, Ord obj)
        bound by the type signature for:
                   coords2faces :: forall rel obj.
                                   (MorphsHQ rel, Eq rel, Eq obj, Ord obj) =>
                                   State (Store obj rel) [(obj, obj)]
        at app/TheMain.hs:105:1-98
      Possible fix:
        add (MorphsHQ rel1) to the context of
          an expression type signature:
            forall rel1. rel1
    • In the first argument of ‘rel3’, namely ‘(hqFace :: rel)’
      In a stmt of a 'do' block: f <- rel3 (hqFace :: rel)
      In the expression:
        do f <- rel3 (hqFace :: rel)
           return f
    |
107 |     f <- rel3 (hqFace :: rel)
haskell
1 Answers

This is because the rel in the type declared for coords2faces is not the same as the rel in the type declared for hqFace. To make this more obvious, you can see that the compiler renamed the type variable to rel1 in the error message. By default, type variables in separate signatures are always independent.

In order to make the code compile, you can either remove the type signature for hqFace,

coords2faces :: (MorphsHQ rel, Eq rel, Eq obj, Ord obj) => State (Store obj rel) [(obj, obj)]
coords2faces = do
    f <- rel3 hqFace
    return f

or you can enable the ScopedTypeVariables language extension and declare the rel type variable in a forall to specify the scope.

{-# LANGUAGE ScopedTypeVariables #-}

coords2faces :: forall rel obj. (MorphsHQ rel, Eq rel, Eq obj, Ord obj) => State (Store obj rel) [(obj, obj)]
coords2faces = do 
    f <- rel3 (hqFace :: rel)
    return f
4castle0
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