Why do list comprehensions need a period? about coffeescript HOT 16 CLOSED

It's just the grammar at the moment -- it would definitely be better without. Just simply removing the period raises 36 reduce/reduce errors. If you can figure out a refactor to the grammar that allows it, I'd be glad to take the patch.

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Well, python's list comprehensions surround it with []'s, which makes sense as the result is always an array. Perhaps you can just do this?

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(This is also done by Haskell, ML and other languages with comprehensions)

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We could do that, but it would be no better than a period, wouldn't be very CoffeeScript-y, and would muck up the multiline array comprehensions proposed in the other issue. Better for the single liner just to lose the period, if we can fix the grammar.

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Hm, examining the grammar, i think the r/r's come from this:

IN PureExpression

The PureExpression seems to be contained, but then if you had an expression like this:

for x in "hi" + for y in "za" if true

You could parse it as either:

for x in ("hi" + for y in "za") if true

or:

for x in ("hi" + (for y in "za" if true))

Any ideas on fixes?

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The reason behind this problem is that PureExpression can still be a full Expression provided it includes an operator. If you made PureExpressions not include operators, this problem would be solved.

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Alternatively (perhaps better), make operators only take PureExpressions.

The downside of this is that this is necessary:

3 + (if a then b else c)

# not this
3 + if a then b else c

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If you resolve this, could we get while as an expression as well?

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So, tried this, and there's nothing too insurmountable about the grammar, but it does introduce a nasty ambiguity that parses wrong under the current precedence rules. Filters on the comprehensions get parsed as postfix if clauses instead:

highlight(row) for row, i in table if i % 2 is 0

Parses as:

(highlight(row) for row, i in table) if i % 2 is 0

Instead of:

(highlight(row) for row, i in table if i % 2 is 0)

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Couldn't this be solved by changing "if" to a different keyword in a comprehension?

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Yes, yes it could -- perhaps filter.

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how about "where"?

for row in table where row isnt null

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Even better.

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or 'when' to mirror the case statement ?

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Better yet -- I'll change it.

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This is now fixed on master, with significant whitespace. Use when to specify the filter over the array comprehension.

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