IncorrectAnonymousArgument.md

IncorrectAnonymousArgument Error

Example

module Example where

sub3 = _ - 3

square = (_ * _)
result1 = map (_ * _) [1, 2, 3]

result2 = map (div _ 2) [1, 2, 3]

result3 = map (1 + _ / 2) [1, 2, 3]

Cause

An operator section, like (_ + 1), has the following requirements:

• The expression must be bracketed (surrounded by parens).
• An anonymous argument can be used only once.
• The anonymous argument must neighbor a binary operator.
• The anonymous argument must be the sole expression for one of the binary operator arguments.

Fix

Surround with parens:

sub3 = (_ - 3)

Replace multiple anonymous arguments with named arguments:

square a = a * a
result1 = map (\n -> n * n) [1, 2, 3]

Write the function as an operator using backticks:

result2 = map (_ `div` 2) [1, 2, 3]

or simply use the corresponding infix operator if one is available:

result2 = map (div _ 2) [1, 2, 3]

Replace the anonymous argument with a named argument:

result3 = map (\n -> 1 + n / 2) [1, 2, 3]

Notes

• While (_ + _) will give this error; \a -> (a + _) will not

• If you really want to have multiple anonymous arguments, it can be achieved like this:

add = (((+) $ _) $ _)