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Make tuple expression type errors more specific #5347

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vicuna opened this Issue Aug 24, 2011 · 2 comments

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commented Aug 24, 2011

Original bug ID: 5347
Reporter: bartjacobs
Assigned to: @garrigue
Status: closed (set by @garrigue on 2012-06-01T07:10:08Z)
Resolution: fixed
Priority: normal
Severity: feature
Version: 3.12.1
Fixed in version: 3.13.0+dev
Category: ~DO NOT USE (was: OCaml general)
Monitored by: bartjacobs "Pascal Cuoq"

Bug description

Currently, the error message generated by OCaml in case of an ill-typed tuple expression is less specific than the error message generated in case of an analogous ill-typed variant expression. For example:

let x: int * int = 1, "Hello";;

Characters 19-29:
let x: int * int = 1, "Hello";;
^^^^^^^^^^
Error: This expression has type int * string
but an expression was expected of type int * int

type ('a, 'b) pair = Pair of 'a * 'b;;

type ('a, 'b) pair = Pair of 'a * 'b

let x: (int, int) pair = Pair (1, "Hello");;

Characters 34-41:
let x: (int, int) pair = Pair (1, "Hello");;
^^^^^^^
Error: This expression has type string but an expression was expected of type int

Notice that the error message for the tuple does not indicate which element is ill-typed, whereas the error message for the variant does.

Since I use lots of long tuples in my code, it would be very useful if OCaml gave more specific error messages in case of ill-typed tuple expressions.

@vicuna

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commented Sep 6, 2011

Comment author: @xavierleroy

Thanks for the suggestion. I guess we could have a special "propagate expected type" mode for tuple expressions, just like we have one for constructors, functions, etc. But I'll let our typechecker experts decide.

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commented Jun 1, 2012

Comment author: @garrigue

The use of type_expect has been generalized for type inference, and as a result one now gets the behavior you expected.

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