6.2.16. Multi-way if-expressions

MultiWayIf
Since

7.6.1

Allow the use of multi-way-if syntax.

With 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 layout context. So 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 the semi-colons between guards in a multi-way if are optional. 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.