Commit 4f8369bf authored by Committed by Austin SeippBrowse files
Implement pattern synonyms
This patch implements Pattern Synonyms (enabled by -XPatternSynonyms), allowing y ou to assign names to a pattern and abstract over it. The rundown is this: * Named patterns are introduced by the new 'pattern' keyword, and can be either *unidirectional* or *bidirectional*. A unidirectional pattern is, in the simplest sense, simply an 'alias' for a pattern, where the LHS may mention variables to occur in the RHS. A bidirectional pattern synonym occurs when a pattern may also be used in expression context. * Unidirectional patterns are declared like thus: pattern P x <- x:_ The synonym 'P' may only occur in a pattern context: foo :: [Int] -> Maybe Int foo (P x) = Just x foo _ = Nothing * Bidirectional patterns are declared like thus: pattern P x y = [x, y] Here, P may not only occur as a pattern, but also as an expression when given values for 'x' and 'y', i.e. bar :: Int -> [Int] ...
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