Untypeable Haskell 98
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Revision as of 15:19, 6 February 2021 by Gwern (talk | contribs) (Reverted edits by Tomjaguarpaw (talk) to last revision by DonStewart)
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Here we document code that looks like it should be valid Haskell98, but isn't typeable without extensions:
f :: a -> a f x = g x where -- g :: a -> a g y = bind x y bind :: a -> a -> a bind a b = a
The above Haskell code is Haskell 98, rank-1 types, but cannot be given a type signature. Try commenting out the type signature for g and everything will go wrong.
In GHC making the type of f :: forall a . a -> a, and adding -fglasgow-exts will make this code work. No such luck in Hugs.
With pattern type annotations, however, the code works in both Hugs and GHC:
f :: a -> a f (x :: a) = g x where g :: a -> a g y = bind x y bind :: a -> a -> a bind a b = a