10.4. Precedence and Associativity

Precedence and associativity of tokens

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Use %left, %right and %nonassoc instead of %token to declare precedence and associativity of tokens. Lines

%left     TOK_FROG
%right    TOK_TOAD
%nonassoc TOK_SALAMANDER

• TOK_FROG is left-associative,
• TOK_TOAD is right-associative and
• TOK_SALAMENDER has no associativity, meaning its precedence should not be allowed to be compared to itself.

Precedence is from low to high, in the order written. So the above lines indicate that TOK_SALAMANDER has highest precedence, then TOK_TOAD, and TOK_FROG has the lowest precedence.

You can declare a few tokens to have the same precedence and associativity. For example,

%left '+' '-'

Precedence and associativity of productions

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By default, the precedence of a production is the same as the precedence of the first token on its right-hand side, if there is one.

If you want to choose your own precedence, follow the production by

  %prec t

expression : expression '-' expression
             %prec '+'