On Sun, Aug 2, 2015 at 12:58 PM, Daniel Bergey <bergey@teallabs.org> wrote:

On 2015-07-31 at 08:59, Simon Peyton Jones <simonpj@microsoft.com> wrote:
> Daniel Bergey wrote:
> | How hard would it be to give a different error message instead of "No
> | instance ..." when the type variable is ambiguous? I always find this
> | error slightly misleading, since it seems to me that there are
> | multiple valid instances, not that there is "no instance".
>
> What would you like it to say? I think it likely we could make it say that!

Great! I'd like it to say "Multiple instances for ..." or "No unique
instance for ...". I have a slight preference for the former.

It may be worth noting that the existing error message is actually technically

correct, in the sense that what would be needed for the program to compile

is exactly an instance of the form "instance Foldable t where ...". Then the

compiler would know that the ambiguity in the type variable t0 doesn't matter.

It doesn't make any difference whether there are zero, one, or multiple instances

of Foldable for more specific types. (Except in that if there is at least one

such instance, then there can't also be an "instance Foldable t" assuming

that OverlappingInstances is not enabled.) Once you understand this, the error
message makes perfect sense. But it is often confusing to beginners.

"Multiple instances for (C t)" seems bad because there might not be any

instances for C at all. "No unique instance for (C t)" is better most of the time,

but it doesn't exactly get to the core of the issue, since there could be just one

instance of C, for a specific type, and then it is no better than "No instance for

(C t)". If I were to explain the situation, I would say "there is no single instance

(C t) that applies for every type t", but it seems a bit wordy for a compiler error...

Regards,

Reid Barton