From anthony_clayden at clear.net.nz Mon Mar 25 05:36:54 2019
From: anthony_clayden at clear.net.nz (Anthony Clayden)
Date: Mon, 25 Mar 2019 18:36:54 +1300
Subject: [Hugs-users] props for Hugs over System FC
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heh heh. I've got to record this for posterity. I've never known an example
before.
Some code that compiles in Hugs and works fine; but GHC (8.6.4) can't
typecheck so rejects.
It's an example in the 2011 'System F with Type Equality Coercions',
section 2.3 discussing FunDeps; and used to justify the extra power of type
inference in Type Families as opposed to FunDeps. Full details discussed
here: https://gitlab.haskell.org/ghc/ghc/issues/16430#note_189393
class F a b | a -> b
instance F Int Bool
class D a where { op :: F a b => a -> b }
instance D Int where { op _ = True }
True that doesn't compile as given. Hugs says: 'Inferred type is not
general enough'.
GHC says 'Couldn't match expected type `b' with actual type `Bool''/
'`b' is a rigid type variable'. (So essentially the same failure of
typechecking.)
With a little help for the type inference, this compiles in Hugs.
class F a b | a -> b
instance F Int Bool
class D a where { op :: (F a b) => a -> b }
instance (TypeCast Bool b') => D Int where { op _ = typeCast True }
With `TypeCast` defined as for HList.
That dangling type variable `b'` in the constraint is weird.
But GHC still rejects it; and rejects a version with a `(~)`
constraint instead of the `TypeCast`.
AntC
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