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Haskell printer generator should not use deprecated OverlappingInstances extension #233

andreasabel opened this issue Sep 25, 2018 · 1 comment


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Haskell printer generator should not use the deprecated OverlappingInstances extension on recent GHCs. E.g., on ghc-8.2:

CPP/Print.hs:1:33: warning:
   -XOverlappingInstances is deprecated: instead use per-instance pragmas OVERLAPPING/OVERLAPPABLE/OVERLAPS
1 | {-# LANGUAGE FlexibleInstances, OverlappingInstances #-}
 |                                 ^^^^^^^^^^^^^^^^^^^^

For backwards compatiblity, we could use #if pragmas.

From ghc-8.6 on, OverlappingInstances may trigger an error.

@andreasabel andreasabel added this to the 2.8.2 milestone Sep 25, 2018
@andreasabel andreasabel mentioned this issue Sep 25, 2018
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@andreasabel andreasabel modified the milestones: 2.8.2, 2.8.3 Nov 4, 2018
@andreasabel andreasabel self-assigned this Feb 3, 2019
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I am changing the Haskell backend to use the {-# OVERLAPPABLE #-} pragma.
As a consequence, BNFC-2.8.3 will only work for GHC >= 7.10.

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