I'm having a problem getting code with datatype contexts to build with GHC
HEAD. The code is from FGL, but even a simplified code like:
module Heap(
Heap(..)
) where
data Ord a => Heap a b = Empty | Node a b [Heap a b]
deriving Eq
does not work. I get the compile error:
Heap.hs:1:1:
No instance for (Ord a)
arising from a use of `Node'
In the pattern: Node _ _ _
In an equation for `$con2tag_Heap': $con2tag_Heap (Node _ _ _) = 1#
I checked the release notes and saw that Datatype contexts are now treated as
an extension. It says it will be on by default, but even passing
-XDatattypeContexts or adding the pragma {-# LANGUAGE DatatypeContexts #-} does
not help.
The problem started after applying the patches related to the new type checker
(around 13 Sept).
-David
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