nimcity/doc/part02/ch13.md

Chapter 13. Syntax

This chapter describes in detail the second phase of the translation process called parsing. How the parser handles the indentation is described in Chapter 12, Lexical analysis. Nim allows user-definable operators. Binary operators have 11 different levels of precedence. 13.1. Associativity Binary operators whose first character is ^ are right-associative, all other binary operators are left-associative. proc ^/(x, y: float): float =

# a right-associative division operator
result = x / y

echo 12 ^/ 4 ^/ 8 # 24.0 (4 / 8 = 0.5, then 12 / 0.5 = 24.0) echo 12 / 4 / 8 # 0.375 (12 / 4 = 3.0, then 3 / 8 = 0.375) 13.1.1. Precedence Unary operators always bind stronger than any binary operator: $a + b is ($a) + b and not $(a + b). If a unary operator’s first character is @, it is a sigil-like operator which binds stronger than a primarySuffix: @x.abc is parsed as (@x).abc whereas $x.abc is parsed as $(x.abc). For binary operators that are not keywords, the precedence is determined by

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the following rules: Operators ending in either →, ~> or => are called arrow-like, and have the lowest precedence of all operators. If the operator ends with = and its first character is none of <, >, !, =, ~, ?, it is an assignment operator which has the second-lowest precedence. Otherwise, precedence is determined by the first character. Table 4. Precedence levels Precedence Operators First Terminal level character symbol 10 (highest) $ ^ OP10 9 * / div mod shl shr % * % \ / OP9 8 + - + - ~ | OP8 7 & & OP7 6 .. . OP6 5 == <= < >= > != in notin is = < > ! OP5

             isnot not of as from

4 and OP4 3 or xor OP3 2 @ : ? OP2 1 assignment operator (like +=, OP1

             *=)

0 (lowest) arrow-like operator (like ->, OP0

             =>)

Whether an operator is used as a prefix operator is also affected by preceding whitespace: echo $foo # is parsed as echo($foo) 64

          Spacing also determines whether (a, b) is parsed as an
          argument list of a call or whether it is parsed as a tuple
          constructor:

 echo(1, 2) # pass 1 and 2 to echo

            echo (1, 2) # pass the tuple (1, 2) to echo

13.2. Dot-like operators Terminal symbol in the grammar: DOTLIKEOP. Dot-like operators are operators starting with ., but not with .., for example .?. Dot-like operators have the same precedence as ., so that a.?b.c is parsed as (a.?b).c instead of a.?(b.c).

          The rules of operator precedence were designed to be as
          “intuitive” as possible and so that you can avoid many
          parenthesis in practice. However, you are not supposed to

 learn the rules by heart, if in doubt use parenthesis explicitly:

            (a and b) or c  # is more readable than
            a and b or c

Other syntax rules are described in the following sections along the semantics of described construct like an if statement. The complete and formal grammar can found in Appendix A, Grammar.