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Inverted pattern unification error #6849

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vicuna opened this issue Apr 27, 2015 · 4 comments

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commented Apr 27, 2015

Original bug ID: 6849
Reporter: @lpw25
Assigned to: @garrigue
Status: closed (set by @xavierleroy on 2016-12-07T10:49:12Z)
Resolution: fixed
Priority: normal
Severity: minor
Version: 4.02.1
Target version: 4.02.2+dev / +rc1
Fixed in version: 4.02.2+dev / +rc1
Category: typing
Monitored by: @hcarty

Bug description

The following code:

type 'a t = Foo : int t

let f : int -> int = function
Foo -> 5;;

produces the following error:

Characters 34-37:
    Foo -> 5;;
    ^^^

Error: This pattern matches values of type int
but a pattern was expected which matches values of type 'a t

The types should of course be the other way around.

@vicuna

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commented Apr 27, 2015

Comment author: @gasche

That looks like a programming mistake to me, because your (int -> int) annotation is wrong, you should write (int t -> int) or drop it altogether as the compiler can infer it.

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commented Apr 27, 2015

Comment author: @lpw25

The code is nonsense. I just wanted a small repro case to illustrate that somewhere one of the calls to unify is back-to-front.

@vicuna

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commented Apr 27, 2015

Comment author: @gasche

Ah, of course: the error message should read

Error: This pattern matches values of type 'a t
but a pattern was expected which matches values of type int

Sorry for the noise.

@vicuna

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commented May 6, 2015

Comment author: @garrigue

Fixed in 4.02 at revision 16088.

@vicuna vicuna closed this Dec 7, 2016

@vicuna vicuna added the typing label Mar 14, 2019

@vicuna vicuna added this to the 4.02.2 milestone Mar 14, 2019

@vicuna vicuna added the bug label Mar 20, 2019

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