0005808: Relax syntax for let binding in order to simplify type-constraint

Notes
(0008427) gasche (developer) 2012-11-06 16:58 edited on: 2012-11-06 17:09	There is a let_pattern production rule that would appear to have been designed for this purpose, but isn't actually used in the let_binding rule. It has the advantage of picking the proposer location for the constraint node, rather than ghost as in your patch. I think we are paying the price of a small oversight of the OCaml syntax: the coercion syntax is not `simple_expr COLON type` but `LPAREN expr COLON type RPAREN` (and similarly for patterns), where parenthesis have a strict role that is not disambiguation -- the only thing parenthesis should be used for. It would not be conservative to add `simple_pattern COLON type` as a rule for patterns, because `(x, y : int)` would then be ambiguous as meaning either `(x,y) : int` (interpretation of the current syntax) or `x, (y : int)` (interpretation of the nested simple_pattern rule with the current rule for commas). I'm not fond of breaking the reassuring property that in the parsing of "let foo = ...", "foo" is exactly parsed as any pattern.

(0008428) frisch (developer) 2012-11-06 17:10	> I'm not fond of breaking the reassuring property that in the parsing of "let foo = ...", "foo" is exactly parsed as any pattern. This is not the case, since let x : t = ... is already accepted, and so are: let f x y = ... let f x y : t = ... In those cases, what's between "let" and "=" is not just a pattern. I simply propose to extend the subset of patterns for which: let p : t = ... is accepted (currently, only identifiers).

(0008429) gasche (developer) 2012-11-06 17:26 edited on: 2012-11-06 17:26	That's correct. Functions declarations have a specific syntax; I'm myself lobbying for a "fun p ... p : t ->" syntax. I agree that the restriction to simple_pattern (atomic or parenthesized) rather than all patterns is a good idea. Otherwise the potential for confusion between the pattern-syntax and the function-syntax is a bit dangerous (let lazy x : t = ..., let F x : t = ...).

(0008431) frisch (developer) 2012-11-06 17:40	> I'm myself lobbying for a "fun p ... p : t ->" syntax. I'm not against it, but do we agree that this is a different topic? (While we are at it, some time ago, I also proposed a patch to allow: let (f : t) x y z = .....)

(0008436) garrigue (manager) 2012-11-07 09:34	My understanding was that general patterns were not allowed there because of parsing conflicts. How did you avoid it?

(0008437) frisch (developer) 2012-11-07 09:48	I did not allow general patterns, only those recognized by the "simple_pattern" grammar entry. There was a conflict because the case of a single identifier is already recognized, so I've split the simple_pattern into two (introducing simple_pattern_not_ident, which was the old simple_pattern minus the ident case).

Issue History
Date Modified	Username	Field	Change
2012-11-05 14:59	frisch	New Issue
2012-11-05 14:59	frisch	File Added: patch_let.diff
2012-11-06 16:58	gasche	Note Added: 0008427
2012-11-06 17:09	gasche	Note Edited: 0008427	View Revisions
2012-11-06 17:10	frisch	Note Added: 0008428
2012-11-06 17:26	gasche	Note Added: 0008429
2012-11-06 17:26	gasche	Note Edited: 0008429	View Revisions
2012-11-06 17:40	frisch	Note Added: 0008431
2012-11-07 09:34	garrigue	Note Added: 0008436
2012-11-07 09:48	frisch	Note Added: 0008437
2013-01-03 17:06	doligez	Severity	minor => feature
2013-01-03 17:06	doligez	Status	new => acknowledged

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