Expressions

Impala is an expression-oriented language. This means, that if-, while- and for-constructs are not Statements but Expressions that yield values. It is perfectly fine and even encouraged to nest expressions.

Syntax

expr ::= primary_expr
       | block_expr
       | for_expr
       | if_expr
       | while_expr
       | with_expr
       | definite_array_expr
       | function_expr
       | indefinite_array_expr
       | simd_expr
       | struct_expr
       | tuple_expr
       | infix_expr
       | postfix_expr
       | prefix_expr
       | cast_expr
       | field_expr
       | map_expr
       | type_application_expr

Precedence

Nesting of expressions is disambiguated according to this table:

Comparison with C

• The as binds strongest of all binary operators. This one does not exist in C.
• &, ^ and | bind stronger than assignments. This is more intuitive.

• All relations share the same precedence. This means

    a == b < c
  

  binds in Impala like this because all infix operators bind from left to right:

    (a == b) < c
  

  Whereas C binds like this because == binds weaker than <:

    a == (b < c)
  

  Rationale: This is such a subtle thing hardly anyone knows. Some C-compilers emit a warning if the programmer relies on this precedence (-Wparentheses implied by -Wall in gcc).

• Assignments’ associativity is left-to-right instead of right-to-left. So

    a += b += c
  

  binds in Impala like this:

    (a += b) += c
  

  Whereas C binds like this:

    a += (b += c)
  

  Rationale: This makes all binary operator consistently left-to-right associative. However, the expression doesn’t type anyway because all assignments yield unit (). So there is not much point for complaints.