6.7.1. Pattern guards — Glasgow Haskell Compiler 9.14.0.20251028 User's Guide

# 6.7.1. Pattern guards

PatternGuards
  Since:

    6.8.1

  Status:

    Disabled in `Haskell98`, enabled in `Haskell2010` and later.

  Haskell 2010 allows pattern match guards to contain pattern guards:

      case animal of
        Fish { name=nm }
          | "bob" <- nm  -> "it's Bob!"
        --  ^^^^^^^^^^^ a pattern guard

  This extension controls whether pattern guard syntax is allowed, independent of language edition.