If-Then-Else in pattern match [GHC-45696]

When pattern matching, (if .. then .. else) expressions are not allowed as patterns to be matched against.

If your intention is to have conditional pattern matching, consider either pattern matching on tuples or to move the if statement outside of the pattern.

Examples

If-Then-Else in pattern match

When pattern matching, (if .. then .. else) expressions are not allowed as patterns to be matched against.

In this example, if a is True then we want to pattern match b to "something"; if a is False, we want to pattern match b with "something else" instead. For instance, f True "something else" is False and f False "something else" is True.

However we cannot use an if expression within a pattern match; the solution can be to either to match on a tuple of (a, b) or to move the if expression outside of the pattern.

Error Message

IfThenElseInPattern.hs:5:6: error: [GHC-45696]
    (if ... then ... else ...)-syntax in pattern
  |
5 |     (if a then b == "something" else b == "something else") -> True
  |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

IfThenElseInPattern.hs

Before

module IfThenElseInPattern where

f :: Bool -> String -> Bool
f a b = case a of
    (if a then b == "something" else b == "something else") -> True
    _ -> False

After

module IfThenElseInPattern where

f :: Bool -> String -> Bool
f a b = case (a, b) of
    (True, "something") -> True
    (False, "something else") -> True
    _ -> False

f' :: Bool -> String -> Bool
f' a b = (a && b == "something") || (not a && b == "something else")