Unexpected if expression in function application [GHC-01239]

Unlike in many other languages, in Haskell the If-Then-Else construct is an expression, which means it returns a value that can be processed further.

ageMessage :: Int -> String 
ageMessage age = if age < 18 then "You are too young to enter" else "Welcome to the club"

putStrLn (ageMessage 10) -- You are too young to enter
putStrLn (ageMessage 20) -- Welcome to the club

Because If-Then-Else expressions return values, it makes sense to pass them as input to a function. However, without language extensions, Haskell’s grammar requires parentheses around them in function argument positions.

Examples

Unexpected if expression in function application

To pass If-Then-Else expressions as function arguments we either have to surround them in parentheses, use the function application operator ($) or enable the BlockArguments extension.

Error Message

IfInFunAppExpr.hs:4:18: error:
    Unexpected if expression in function application:
        if True then [1] else []
    Suggested fixes:
      • Use parentheses.
      • Perhaps you intended to use BlockArguments
  |
4 | example = length if True then [1] else []
  |                  ^^^^^^^^^^^^^^^^^^^^^^^^

IfInFunAppExpr.hs

Before

module IfInFunAppExpr where

example :: Int
example = length if True then [1] else []

After

{-# LANGUAGE BlockArguments #-}
module IfInFunAppExpr where

-- Requires BlockArguments language extension to work
example :: Int
example = length if True then [1] else []

-- Works without BlockArguments
example2 :: Int
example2 = length (if True then [1] else [])

-- Works without BlockArguments
example3 :: Int
example3 = length $ if True then [1] else []