Simpler error message

[varosi]

A proposal about error message:

Couldn't match expected multiplicity ‘'One’ of variable ‘a’ with actual multiplicity ‘'Omega’

Is it possible to show a simpler error message in such cases, as:

Variable ‘a’ is used more than once.

It'll be more understandable by more people.

[aspiwack]

Note: it's better to use tweag/ghc to report such issues.

There is already a conversation on error messages, by the way: tweag/ghc#5 .

The short story is: error messages are still not good, and certainly not definitive.

I've got issues with things like “more than once”, though:

• Is if b then x else x not “using” x more than once?
• Is Unrestricted x “using” x more than once?
• What if the expected multiplicity was a variable p, what would the message be in that case?

[varosi-chaosgroup]

Then may be: "not exactly once" ?

[varosi-chaosgroup]

In the ideal case there could be different error messages for different groups of errors, so they sound understandable for the average haskeller.