Diffs are often useful in visualizing such things.  not sure how helpful it
is in this case, but it does at least make it a bit clearer what's lined
up.  This was generated with our patdiff tool.

@@@@@@@@@@ -1,7 +1,7 @@@@@@@@@@
           val traverseexercise :-|           *'a* container
PseudoCode.abstract ->-|           (*'a* PseudoCode.abstract ->-|
      *'b* PseudoCode.abstract -> (*'c* * *'b*) PseudoCode.abstract)
->-|           *'b* PseudoCode.abstract* ->*-|*           'd
container* ->-|           (*'d* *container* *-*> *'c container*
PseudoCode.*abstract -> 'e) -> 'e*+|           loopdata container
PseudoCode.abstract ->+|           (loopdata PseudoCode.abstract ->+|
          loopdata PseudoCode.abstract ->+|            (loopdata *
loopdata) PseudoCode.abstract) ->+|           loopdata
PseudoCode.abstract ->+|           (< answer : 'a; state : 'b; .. >,
loopdata) PseudoCode.cmonad


On Fri, Jun 3, 2016 at 11:42 AM, Carette, Jacques <carette@mcmaster.ca>
wrote:

> So here is an actual example.  The error I get is
> ocamlc -short-paths -c reproduce.ml
> File "reproduce.ml", line 148, characters 21-25:
> Error: Signature mismatch:
>        ...
>        Values do not match:
>          val traverseexercise :
>            'a container PseudoCode.abstract ->
>            ('a PseudoCode.abstract ->
>             'b PseudoCode.abstract -> ('c * 'b) PseudoCode.abstract) ->
>            'b PseudoCode.abstract ->
>            'd container ->
>            ('d container -> 'c container PseudoCode.abstract -> 'e) -> 'e
>        is not included in
>          val traverseexercise :
>            loopdata container PseudoCode.abstract ->
>            (loopdata PseudoCode.abstract ->
>             loopdata PseudoCode.abstract ->
>             (loopdata * loopdata) PseudoCode.abstract) ->
>            loopdata PseudoCode.abstract ->
>            (< answer : 'a; state : 'b; .. >, loopdata) PseudoCode.cmonad
>        File "reproduce.ml", line 111, characters 6-206: Expected
> declaration
>        File "reproduce.ml", line 125, characters 8-24: Actual declaration
>
> I can't tell from the above error what the actual cause is.  The full code
> is attached.  [This code is quite reduced already.  It is kept at this size
> to show in more detail the kinds of errors we're seeing.]
>
> Any 'hint' from OCaml as to the precise nature of the non-match would sure
> be appreciated.
>
> Jacques
>
> ________________________________________
> From: Jacques Garrigue [garrigue@math.nagoya-u.ac.jp]
> Sent: June 2, 2016 19:59
> To: Carette, Jacques
> Cc: OCaML List Mailing
> Subject: Re: [Caml-list] Option to fully expand types in error messages?
>
> On 2016/06/03 04:18, "Carette, Jacques" wrote:
> >
> > In writing some code which uses a lot of monads with underlying types
> which use constraints, even simple errors can lead to extremely hard to
> read error messages.  The main reason is that the two types given in errors
> are partially expanded, to different levels.  This frequently means that
> the part where the type checker detects a mismatch is (extremely) opaque to
> human eyes.
> >
> > In that case, it would actually be preferable to fully expand the
> types.  Yes, that will produce wallpaper.  But at least the mismatch should
> be considerably easier to catch.
> >
> > Does this already exist, or should I submit a feature request?
> >
> > Jacques
>
> In the error message, types are expanded just enough to get down to the
> conflict.
> If the conflict is not visible at that point, this is probably a scoping
> error (and there should be an extra line stating that);
> otherwise this should be seen as a bug.
> As Yaron pointed, -short-paths can help by at least giving a normal form
> for paths (which may not be the expansion, but should be unique in the
> error context). But it will not expand a type if the expansion is not a
> type constructor, or if the parameters are different.
>
> Jacques
>
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