Pattern Guards

Ticket: #56

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Brief Explanation

The syntax for guards is changed to

gd| qual1,, qualn

(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.



  • 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.


  • 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.
Last modified 9 years ago Last modified on Nov 18, 2009 10:51:30 AM