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Error messages for 1-tuples are confusing #6388

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vicuna opened this Issue May 1, 2014 · 2 comments

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commented May 1, 2014

Original bug ID: 6388
Reporter: @whitequark
Assigned to: @alainfrisch
Status: closed (set by @xavierleroy on 2015-12-11T18:27:53Z)
Resolution: fixed
Priority: normal
Severity: minor
Fixed in version: 4.02.0+dev
Category: ~DO NOT USE (was: OCaml general)
Related to: #6424

Bug description

If I construct an 1-tuple directly via AST, e.g. with Ast_helper.Exp.tuple [a], the error message from typechecker is as follows:

Error: This expression has type int but an expression was expected of type int

Also, the pretty-printer prints it as an expression, which doesn't help.

I think the Pprintast should be extended to print 1-tuples (and perhaps 0-tuples?) in a special way; if I did not know exactly what the bug were, I would have spent hours figuring it out.

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commented May 2, 2014

Comment author: @alainfrisch

Instead of adapting the printer, I think it would make more sense to check in the type-checker the invariants which are normally enforced by the parser, but could be broken if one uses -pp/-ppx processors.

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commented Jun 3, 2014

Comment author: @alainfrisch

Commit 14949 in 4.02 checks that tuples have at least 2 elements. (Please shout if you have a legitimate use of "unary tuples" in generated code.) The same commit adds some more invariant checks.

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