Custom Infix Functions in Elixir

The post explains that the author needed a nearly-equal float test for an Elixir Math module and discovered that the apparent custom infix function was just a regular function definition using one of Elixir’s predefined infix operators. It then shows that Elixir’s parser recognizes a fixed set of operators, so new infix symbols cannot be added without changing and recompiling Elixir itself.

Reading notes

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• The author needed tests for floats that are very close to equal.
• The sample nearly-equal test is written as an ordinary function definition.
• Elixir 1.2.0 allows only a fixed set of infix operators such as \, <-, |, ~>>, <~>, |||, &&&, and ^^^.
• Attempts to define a new custom infix operator produce a syntax error.
• The parser, not Kernel, is where the operators are special-cased.
• The parser file elixir_parser.yrl lists operator precedence and the built-in infix forms.
• Because parsing happens before bootstrapping, new infix operators cannot be defined in user code.
• José Valim confirmed that the unused operators were included for this use case.
• Kernel operators can be redefined after unimporting them, but that can break expectations in other Elixir code.
• The pipespect library uses this technique to inspect each stage in a |> pipeline.