Refactored the error handling

[EliasC]

This commit reimplements the error handling of encorec. Before, an error
carried the message that should be displayed, meaning that the more
detailed a message you wanted, the more bloated the error handling code
got. The new model introduces a datatype Error which has one
constructor for each kind of error, and showing this value prints the
error message. For example, if there is a type mismatch error, instead
of writing

tcError $ "Expected type '" ++ show ty1 ++ "' but found '" ++ show ty2 ++ "'"

you now write

tcError $ TypeMismatchError ty1 ty2

The hope is that this will encourage writing better error messages, and
also that it will reduce code duplication when the same kind of error
message needs to be thrown by two different places in the code. As an
added bonus, we can now catch specific errors:

matchTypes (getResultType expected) (getResultType ty)
  `catchError` (\case
                 TCError (TypeMismatchError _ _) _ ->
                     tcError $ TypeMismatchError ty expected
                 TCError err _ -> tcError err
               )

If the first line above throws a TypeMismatchError, we will catch it
and throw a new error. If it's another error we will pass this error
along.

[kikofernandez]

If you want to be explicit about it, you are missing the classOrTraitOrCapability.
I am not sure if it's better to abstract away from these details and just name the function typeErrorName (you may need to switch the order of the nouns... in Spanish this sounds ok) since the intended use is: given a type (class, capability or trait), return the associated error name.

[EliasC]

It's really just a convenience function for writing "class Foo", "trait Foo" or "capability Foo + Bar" rather than "type Foo". Maybe refTypeName would be better?

[kikofernandez]

Maybe :) I thought that it is good if it refers to an abstraction, rather than being super specific

[EliasC]

Adressed the comments about ObjectCreationError thusly:

    show (ObjectCreationError ty)
        | isMainType ty = "Cannot create additional Main objects"
        | isCapabilityType ty =
            printf "Cannot create instance of %s (type must be a class)"
                   (refTypeName ty)
        | otherwise = printf "Cannot create object of type '%s'" (show ty)

[kikofernandez]

I will look at the changes (you need to push :) ) tomorrow and merge around 10 if there are no more issues

[EliasC]

Pushed now! Please squash when you are done :)

[albertnetymk]

I can't build on dev after this merging. Anyone else?

[kikofernandez]

you are completely right... weird... I'll look into it right away

[kaeluka]