Chained comparisons

[DartBot]

This issue was originally filed by @seaneagan

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Often one needs to compare a chain of values, for example to see if they are all ordered correctly. In most programming languages it is quite ugly and unreadable:

var x = foo() , y = bar();
a < x && x <= y && y == d

however, in the following languages:

CoffeeScript: http://coffeescript.org/#comparisons
Python: http://docs.python.org/reference/expressions.html#not-in
Perl 6: http://en.wikipedia.org/wiki/Perl_6#Chained_comparisons
Standard math notation

the above can be written simply as:

a < foo() <= bar() == d

Dart mostly disallows this syntax with the exception that a relationalOperator can appear directly before an equalityOperator:

a < b == c

but it is merely interpreted as:

(a < b) == c

Such interpretations of chained comparisons are not very useful, since single comparisons return bool, which does not itself define relational operators, and for which equality operators are only used in place of XOR/XNOR, which are uncommon use cases.

Thus, this syntax should either be supported with the correct semantics (logical conjunction of each comparison), or not supported at all (which would at least allow it to be supported with the correct semantics in a later release).

Both options require merging "equalityOperator" and "relationalOperator" into "comparisonOperator", and "equalityExpression" and "relationalExpression" into "comparisonExpression".

Regardless of whether it is supported, it would a good idea to issue a static type warning if any comparison operator returns a type other than bool (or Dynamic).

The option to support chained comparisons would then additionally require something like:

comparisonExpression:
     shiftExpression (isOperator type | (comparisonOperator shiftExpression)+)?
   | super (comparisonOperator shiftExpression)+
   ;

[dgrove]

Removed Type-Defect label.
Added Type-Enhancement, Area-Language labels.

[dgrove]

Added Triaged label.

[gbracha]

Set owner to @gbracha.
Added Accepted label.

[gbracha]

We're trying to keep the number of special cases and sugars down within reasonable limits.

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Added WontFix label.