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Explaining the custom Eq, Ord instances for Binder #3271

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Merged
kritzcreek merged 2 commits into from
Mar 9, 2018
Merged

Explaining the custom Eq, Ord instances for Binder #3271

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Explaining the custom Eq, Ord instances for Binder
bitemyapp Mar 7, 2018
194e1b3
fix type and stretch to 80 characters
kritzcreek Mar 9, 2018
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7 changes: 7 additions & 0 deletions src/Language/PureScript/AST/Binders.hs
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Expand Up @@ -65,6 +65,13 @@ data Binder
| TypedBinder Type Binder
deriving (Show)

-- Manual Eq and Ord instances for `Binder` were added on 2018-03-05. Comparing
-- the `SourceSpan` values embedded in some of the data constructors of `Binder`
-- was expensive. This made exhaustiveness checking observably slow for code
-- such as the `explode` function in `examples/passing/LargeSumTypes.purs`.
-- Custom instances were written to skip comparing the `SourceSpan` values. Only
-- the `Ord` instance was needed for the speed-up, but I did not want the `Eq`
-- to have mismatched behavior.
instance Eq Binder where
(==) NullBinder NullBinder = True
(==) NullBinder _ = False
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