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Why is this not happening?

This is a toy example.hs:

{-# LANGUAGE ImpredicativeTypes #-}

import Control.Arrow

data From = From (forall a. Arrow a => a Int Char -> a [Int] String)

data Fine = Fine (forall a. Arrow a => a Int Char -> a () String)

data Broken = Broken (Maybe (forall a. Arrow a => a Int Char -> a () String))

fine :: From -> Fine
fine (From f) = Fine g
  where g :: forall a. Arrow a => a Int Char -> a () String
        g x = f x <<< arr (const [1..5])

broken :: From -> Broken
broken (From f) = Broken (Just g) -- line 17
  where g :: forall a. Arrow a => a Int Char -> a () String
        g x = f x <<< arr (const [1..5])

And this is what ghci thinks about it:

GHCi, version 7.0.3: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading package ffi-1.0 ... linking ... done.
Prelude> :l toy-example.hs 
[1 of 1] Compiling Main             ( toy-example.hs, interpreted )

toy-example.hs:17:32:
    Couldn't match expected type `forall (a :: * -> * -> *).
                                  Arrow a =>
                                  a Int Char -> a () String'
                with actual type `a0 Int Char -> a0 () String'
    In the first argument of `Just', namely `g'
    In the first argument of `Broken', namely `(Just g)'
    In the expression: Broken (Just g)
Failed, modules loaded: none.

Why finetypecheck is brokennot working yet?

How do I get brokento typecheck?

(In my real code, I can add a type parameter ato broken, if I need, instead of universally defining it inside the constructor, but I would like to avoid this if possible.)

Edit: If I change the definition brokento

data Broken = Broken (forall a. Arrow a => Maybe (a Int Char -> a () String))

then brokentypechecks. Hooray!

But if I then add the following function

munge :: Broken -> String
munge (Broken Nothing) = "something"  -- line 23
munge (Broken (Just f)) = f chr ()

then I get an error

toy-example.hs:23:15:
    Ambiguous type variable `a0' in the constraint:
      (Arrow a0) arising from a pattern
    Probable fix: add a type signature that fixes these type variable(s)
    In the pattern: Nothing
    In the pattern: Broken Nothing
    In an equation for `munge': munge (Broken Nothing) = "something"

How do I get mungefor typecheck?

2nd edit: Broken (Maybe ...) BrokenNothing BrokenJust ... ( ), , .

+5
1

ImpredicativeTypes , GHC - , , .

Maybe, , , , .

, munge , Broken RHS, , , :

munge (Broken x@(Just _)) = fromJust x chr ()

, .

+2

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