6.2.15. Multi-way if-expressions

MultiWayIf

Since:

7.6.1

Allow the use of multi-way-if syntax.

With the MultiWayIf extension GHC accepts conditional expressions with multiple branches:

if | guard1 -> expr1
   | ...
   | guardN -> exprN

which is roughly equivalent to

case () of
  _ | guard1 -> expr1
  ...
  _ | guardN -> exprN

Multi-way if expressions introduce a new kind of layout context that does not generate semicolons. The example above is equivalent to:

if { | guard1 -> expr1
     | ...
     | guardN -> exprN
   }

The following behaves as expected:

if | guard1 -> if | guard2 -> expr2
                  | guard3 -> expr3
   | guard4 -> expr4

because layout translates it as

if { | guard1 -> if { | guard2 -> expr2
                      | guard3 -> expr3
                    }
     | guard4 -> expr4
   }

Layout with multi-way if works in the same way as other layout contexts, except that desugaring does not insert semicolons. So it is not necessary to line up all the guards at the same column; this is consistent with the way guards work in function definitions and case expressions.

Note that multi-way if supports guards other than boolean conditions:

if | parseNumbers settings
   , Just (exponent, mantissa) <- decomposeNumber str
   , let (integralPart, fractionPart) = parse mantissa
   , integralPart >= 0 -> ...
   | otherwise -> ...