0005839: Camlp4 incorrectly expands a quotation containing an 'assert false' expression.

Notes
(0008551) pveber (reporter) 2012-12-02 22:57	Just one remark, changing line 15 value input () = assert $`bool:false$; by value input () = failwith ""; or value input () = failwith $`str:""$; solves the problem. So the issue seems related to the use of assert.

(0008552) gasche (developer) 2012-12-02 23:29	This is not a Camlp4 bug but an oddity in the semantics of "assert false". OCaml recognizes two different syntactic constructors: - assert false (that generates the parstree Pexp_assertfalse) - assert foo (for foo <> false) (Pexp_assert (...)) The first has type 'a, the second has type unit. You can check this with: # fun x -> assert x;; - : bool -> unit = <fun> Now your code actually produces the second construction, rather than the first, because of the antiquotation indirection (which prevents the parser from recognizing the special case). The expression has type unit and therefore the module doesn't type-check. Now if you print the Camlp4 output to a file, you get "assert false" which is re-parsed into the special-case construct. This is not a problem with Camlp4 itself, but rather with assert. (The option -dparsetree is fairly useful in debugging those problems: you get to see a dump of the OCaml parsetree after the camlp4 preprocessing step. I was reminded of the difference by comparing the output of -dparsetree for the two ways to compile your file.)

(0008553) hongboz (developer) 2012-12-03 06:13	we may give a warning for this case

(0008554) pveber (reporter) 2012-12-03 09:46	Thanks Gabriel for the explanation, I wasn't aware of this peculiarity. Sorry for wasting your time!

(0008557) frisch (developer) 2012-12-03 18:44	A typical example where writing correct Camlp4 code requires expert knowledge about OCaml's AST, even though Camlp4 tries hard to hide this AST from you. Well, in this case, I would have expected Camlp4 to try to fix this weird behavior of OCaml syntax, by having a single node for "assert", mapping to the correct one according to whether the argument turns out to be "False" or not. This would introduce some "discontinuity", but this might be less surprising for the user.

Issue History
Date Modified	Username	Field	Change
2012-12-02 22:45	pveber	New Issue
2012-12-02 22:45	pveber	File Added: table_bug.tar.gz
2012-12-02 22:57	pveber	Note Added: 0008551
2012-12-02 23:29	gasche	Note Added: 0008552
2012-12-02 23:29	gasche	Status	new => resolved
2012-12-02 23:29	gasche	Resolution	open => not fixable
2012-12-02 23:29	gasche	Assigned To	=> gasche
2012-12-03 06:13	hongboz	Note Added: 0008553
2012-12-03 09:46	pveber	Note Added: 0008554
2012-12-03 18:44	frisch	Note Added: 0008557

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