Guard Operators

Avoid using a mix of andalso, orelse, ,, and ;, by choosing a consistent coding style for guard expressions.

Avoid

case some:expression() of
    X when X == a; X =< 10 -> {clause, 1};
    X when X == b orelse X == 10 -> {clause, 2};
    X when (X == c orelse X == d); X >= 10 -> {clause, 3}
end

Prefer

case some:expression() of
    X when X == a; X =< 10 -> {clause, 1};
    X when X == b; X == 10 -> {clause, 2};
    X when X == c; X == d; X >= 10 -> {clause, 3}
end

Or…

case some:expression() of
    X when X == a orelse X =< 10 -> {clause, 1};
    X when X == b orelse X == 10 -> {clause, 2};
    X when X == c orelse X == d orelse X >= 10 -> {clause, 3}
end

Or, at least…

case some:expression() of
    X when X == a; X =< 10 -> {clause, 1};
    X when X == b orelse X == 10 -> {clause, 2};
    X when X == c; X == d; X >= 10 -> {clause, 3}
end

(where within each guard the choice of operators is consistent)

Rationale

Choosing a specific guard operator style enhances consistency across your codebase.

Options

• preferred_syntax :: punctuation | words | per_expression | per_clause

  • default: per_expression
  • punctuation - always use , or ;.
  • words - always use andalso or orelse.
  • per_expression - do not mix words and punctuation in an expression.
  • per_clause - do not mix words and punctuation in a guard.

Example configurations

Using these functions as an example:

first_function(X) when X == a; X =< 10 -> {clause, 1};
first_function(X) when X == b orelse X == 10 -> {clause, 2};
first_function(X) when (X == c orelse X == d); X >= 10 -> {clause, 3}.

second_function(X) when X == a; X =< 10 -> {clause, 1};
second_function(X) when X == b; X == 10 -> {clause, 2};
second_function(X) when X == c; X == d; X >= 10 -> {clause, 3}.

third_function(X) when X == a orelse X =< 10 -> {clause, 1};
third_function(X) when X == b orelse X == 10 -> {clause, 2};
third_function(X) when X == c orelse X == d orelse X >= 10 -> {clause, 3}.

{elvis_style, guard_operators, #{preferred_syntax => punctuation}}

This configuration will emit a warning for:

• clauses 2 and 3 in the first function
• all clauses in the third function

{elvis_style, guard_operators, #{preferred_syntax => words}}

This configuration will emit a warning for:

• clauses 1 and 3 in the first function
• all clauses in the second function

{elvis_style, guard_operators, #{preferred_syntax => per_expression}}

This configuration will emit a warning for:

• clauses 2 and 3 in the first function

{elvis_style, guard_operators, #{preferred_syntax => per_clause}}

This configuration will emit a warning for:

• clause 3 in the first function