`MixedAssociativityError`

Example

purescript

module ShortFailingExample where

import Prelude ((<$>), (==))

feq f x y = f <$> x == f <$> y

Result:

Cannot parse an expression that uses operators of the same precedence but mixed associativity:

  Data.Functor.(<$>) is infixl
  Data.Eq.(==) is infix

Use parentheses to resolve this ambiguity.

Cause

This error arises when an expression involves operators of the same precedence but different associativities. In this example, <$> is infixl 4, that is, left-associative with precedence 4, whereas == is infix 4, that is, non-associative with precedence 4. The compiler is unable to determine how to bracket this expression because of the precendences and associativities involved; either the associativities must be the same or the precedences must be different. See Syntax - Binary Operators in the language reference for more information.

Fix

The most appropriate fix is usually to add brackets to remove the ambiguity:

feq f x y = (f <$> x) == (f <$> y)

An alternative fix is to change the precedence of one of the operators, in order to allow the expression to be parsed without adding parentheses.