Difference between revisions of "Untypeable Haskell 98"

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Here we document code that looks like it should be valid Haskell98, but
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isn't typeable without extensions:
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<haskell>
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f :: a -> a
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f x = g x
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where
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-- g :: a -> a
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g y = bind x y
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bind :: a -> a -> a
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bind a b = a
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</haskell>
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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.
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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.
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With pattern type annotations, however, the code works in both Hugs and GHC:
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<haskell>
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f :: a -> a
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f (x :: a) = g x
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where
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g :: a -> a
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g y = bind x y
  +
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bind :: a -> a -> a
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bind a b = a
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</haskell>
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[[Category:Proposals]]

Latest revision as of 15:19, 6 February 2021

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