= Pattern Guards =
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'''Ticket:''' #56

See ExtensionDescriptionHowto for information on how to write these extension descriptions.  Please add any new extensions to the list of HaskellExtensions.

== Brief Explanation ==

The syntax for guards is changed to

   ''gd''  ->  `|` ''qual'',,1,,`,` ...`,` ''qual'',,n,,

(Haskell 98 allows only a single, Boolean, guard.)
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.

== References ==
 * [http://www.haskell.org/onlinereport/exps.html#case Case Expressions] and [http://www.haskell.org/onlinereport/exps.html#case-semantics Formal Semantics of Pattern Matching] in the Haskell 98 Report
 * [http://research.microsoft.com/~simonpj/Haskell/guards.html Simon's original proposal], 1997.
 * [http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#pattern-guards Pattern guards] in the GHC User's Guide
 * [http://research.microsoft.com/Users/simonpj/Papers/pat.htm Pattern Guards and Transformational Patterns], Martin Erwig and Simon Peyton Jones, Haskell Workshop, 2000.

== Pros ==
 * avoids awkward `case` constructs in certain situations
 * a relatively small and orthogonal extension
 * the required changes to the Report are clearly laid out in the above paper.

== Cons ==
 * 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.
   {{{
f p | x = y = z
}}}

== Results of committee discussion for Haskell 2010 ==

 * in the first bullet in 3.13, there's a double period, and I don't
   think the contextual difference can be called "minor".

 * it would help to present the three types of guard in the same order
   each time they're discussed.

 * in 3.17, the rule for clause (w) has parentheses around p <- e0,
   which is not permitted by the syntax.