.. _multi-way-if:
Multi-way if-expressions
------------------------
.. extension:: MultiWayIf
:shortdesc: Enable multi-way if-expressions.
:since: 7.6.1
Allow the use of multi-way-``if`` syntax.
With :extension:`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.