AnyDSL

AnyDSL

Expressions

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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:

Operator Description Associativity
++ --
()
[]
.		 Postfix Expression (increment/decrement)
Map Expression
Type Application Expression
Field Expression		 left-to-right
++ --
+ -
!
~
*
& &mut
|/*....*/|		 Prefix Expression (increment/decrement)
Prefix Expression (unary plus/minus)
Prefix Expression (logical/bitwise NOT)
Prefix Expression (alloc)
Prefix Expression (dereference)
Prefix Expression (address-of/mutable address-of)
Function Expression		 right-to-left
as Cast Expression left-to-right
* / % Infix Expression (multiplication/division/remainder) left-to-right
+ - Infix Expression (addition/subtraction) left-to-right
<< >> Infix Expression (bitwise left/right shift) left-to-right
& Infix Expression (bitwise AND) left-to-right
^ Infix Expression (bitwise XOR) left-to-right
| Infix Expression (bitwise OR) left-to-right
== !=
< <= > >=		 Infix Expression (equal/not equal)
Infix Expression (less/less equal/greater/greater equal)		 left-to-right
&& Infix Expression (logical AND) left-to-right
|| Infix Expression (logical OR) left-to-right
=
*= /= %=
+= -=
<<= >>=
&= ^= |=		 Infix Expression (assignment)
Infix Expression (assign by sum/difference)
Infix Expression (assign by product/quotient/remainder)
Infix Expression (assign by bitwise left/right shift)
Infix Expression (assign by bitwise AND/XOR/OR)		 left-to-right

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.