Expressions.md

title	parent	weight
Expressions	Impala.md	7

Impala is an expression-oriented language. This means, that [if-]({% link If-Expression.md %}), [while-]({% link While-Expression.md %}) and [for-constructs]({% link For-Expression.md %}) are not [Statements] but [Expressions] that yield values. It is perfectly fine and even encouraged to nest expressions.

[Expressions]: {% link Expressions.md %} [Statements]: {% link Statements.md %}

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 `	/..../	`
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)
== != < <= > >=	[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] (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)

[Postfix Expression]: {% link Postfix-Expression.md %} [Map Expression]: {% link Map-Expression.md %} [Type Application Expression]: {% link Type-Application-Expression.md %} [Field Expression]: {% link Field-Expression.md %} [Prefix Expression]: {% link Prefix-Expression.md %} [Function Expression]: {% link Function-Expression.md %} [Cast Expression]: {% link Cast-Expression.md %} [Infix Expression]: {% link Infix-Expression.md %}

Comparison with C

• The [as][Cast Expression] 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][Infix Expression] 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.