Re: [Caml-list] Option to fully expand types in error messages?

[Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>]

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