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3.12.0+dev18: "module type of" leaves weak polymorphic types in module type #5036

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vicuna opened this Issue Apr 24, 2010 · 3 comments

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vicuna commented Apr 24, 2010

Original bug ID: 5036
Reporter: kaustuv
Status: closed (set by @garrigue on 2010-04-30T05:13:39Z)
Resolution: fixed
Priority: normal
Severity: minor
Fixed in version: 3.12.0+dev
Category: ~DO NOT USE (was: OCaml general)
Monitored by: "Julien Signoles" @hcarty

Bug description

module type S =

   module type of (struct
                    let f = (fun x -> x) (fun x -> x)
                   end) ;;

module type S = sig val f : '_a -> '_a end

Light experimentation suggests that only modules that define a value f : 'a. 'a -> 'a will ascribe to the above signature, so "module type of" should promote weak polymorphic types to strong polymorphic types.

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vicuna commented Apr 28, 2010

Comment author: @xavierleroy

I'm not comfortable with generalizing these type variables automatically. Added a check to reject "module type of M" if "M" contains nongeneralized type variables.

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vicuna commented Apr 28, 2010

Comment author: kaustuv

Many apologies for reopening this with a trivial nitpick, but you got the test for closedness and the simplification (#5037) in the wrong order in typing/typemod.ml/type_module_type_of. Witness this startling result:

module M = struct let f = (fun x -> x) (fun x -> x) let f x = x end ;;

module M : sig val f : 'a -> 'a end

module type S = module type of M ;;

Error: The type of this module, sig val f : '_a -> '_a val f : 'a -> 'a end,
contains type variables that cannot be generalized

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vicuna commented Apr 30, 2010

Comment author: @garrigue

Indeed, type_implementation was doing it in the opposite order.
Fixed in revision 10330.

@vicuna vicuna closed this Apr 30, 2010

@vicuna vicuna added the bug label Mar 20, 2019

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