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# Pattern Guards
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**Ticket:** [\#56](https://gitlab.haskell.org//haskell/prime/issues/56)
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See [ExtensionDescriptionHowto](extension-description-howto) for information on how to write these extension descriptions. Please add any new extensions to the list of [HaskellExtensions](haskell-extensions).
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## Brief Explanation
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The syntax for guards is changed to
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>
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>
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> *gd* → `|` *qual*<sub>1</sub>`,` …`,` *qual*<sub>n</sub>
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>
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>
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(Haskell 98 allows only a single, Boolean, guard.)
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In pattern guards of the form *p* `<-` *e*, the two sides must have the same type, and if the value if *e* does not match *p*, the guards fails.
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## References
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- [ Case Expressions](http://www.haskell.org/onlinereport/exps.html#case) and [ Formal Semantics of Pattern Matching](http://www.haskell.org/onlinereport/exps.html#case-semantics) in the Haskell 98 Report
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- [ Simon's original proposal](http://research.microsoft.com/~simonpj/Haskell/guards.html), 1997.
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- [ Pattern guards](http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#pattern-guards) in the GHC User's Guide
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- [ Pattern Guards and Transformational Patterns](http://research.microsoft.com/Users/simonpj/Papers/pat.htm), Martin Erwig and Simon Peyton Jones, Haskell Workshop, 2000.
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## Pros
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- avoids awkward `case` constructs in certain situations
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- a relatively small and orthogonal extension
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- the required changes to the Report are clearly laid out in the above paper.
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## Cons
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- the re-use of monadic binding notation from list comprehensions and `do`-notation for pure matching is confusing, but the more rational `=` would create parsing difficulties, e.g.
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```wiki
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f p | x = y = z
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```
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## Results of committee discussion for Haskell 2010
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- in the first bullet in 3.13, there's a double period, and I don't
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think the contextual difference can be called "minor".
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- it would help to present the three types of guard in the same order
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each time they're discussed.
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- in 3.17, the rule for clause (w) has parentheses around p ← e0,
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which is not permitted by the syntax. |
