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📝 why OVERLAPPABLE.

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Rob Rix 2016-06-16 11:58:17 -04:00
parent cff9ac00ab
commit 63ed038654
1 changed files with 2 additions and 0 deletions
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src/Data/Record.hs
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@ -33,6 +33,8 @@ class HasField (fields :: [*]) (field :: *) where
-- Instances
-- OVERLAPPABLE is required for the HasField instances so that we can handle the two cases: either the head of the non-empty h-list is the requested field, or it isn’t. The third possible case (the h-list is empty) is rejected at compile-time.
instance {-# OVERLAPPABLE #-} HasField fields field => HasField (notIt ': fields) field where
getField (RCons _ t) = getField t