# Difference between revisions of "Untypeable Haskell 98"

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## 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
```