for...in-loops & iterators

[phorward]

To implement the for...in-loop construct in Tokay, it is required to make iterators available for values like dict, list or string.

[phorward]

#90 and d1f2f80 show up general problems with Tokay's parentheses-less syntax attempt.

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The syntax of for-loops is currently a stopgap and trade-off between C/awk-style, Rust/Python-style and Tokay's own syntactic specialities. The iterator-part can only be a Statement, not a Sequence, as expressed in tokay.tok:

Atomic : @{
    # ...
    'for' _SeparatedIdentifier (Sequence | Nop) ';' _ (Sequence | Nop) ';' _ Statement _ Block  ast("op_for")
    'for' _SeparatedIdentifier (Sequence | Nop) ';' _ (Sequence | Nop) ';' _ Nop _ Block  ast("op_for")
    # ...
}

This allows for simple

for i = 0; i < 10; i++ { print(i) }

but it's getting stuck here

for i = 0, j= 100; i < 10; i++, j-- { print(i, j) }

which will throw a syntax error with d1f2f80. Note, that previously, even less was possible, though!
Anyway, the syntax error is because Statement is used rather than Sequence on the iterator part, and a block is part of a Sequence, so this even more causes a syntax error.

The syntax is disambiguating here, and

for i = 0, j= 100; i < 10; (i++, j--) { print(i, j) }

is an acceptable, but not nice workaround.

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Also, generally adding parentheses to the syntax would solve the problem.

Atomic : @{
    # ...
    'for' _ '(' _ (Sequence | Nop) ';' _ (Sequence | Nop) ';' _ (Sequence | Nop) ')' _ Block  ast("op_for")
    # ...
}

would enforce the original C/awk-style syntax

for (i = 0, j= 100; i < 10; i++, j--) { print(i, j) }

but this isn't wanted due personal preference, and Tokay's "different" syntax from existing syntax.

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Generally, maybe its better to reinforced for...in and use this as the one-and-only-for syntax.

[phorward]

#101 is the first pull request on this topic.

[phorward]

#101 merged, issue fixed.