Use generics-rep

[paf31]

Fixes #15

@chexxor Would you mind taking a look over this?

[chexxor]

Requires 0.10.3 compiler? I'm getting an error on pulp test for the usage genericShow:

No type class instance was found for
                                     
    Data.Symbol.IsSymbol "TupleArray"

update: yeah, looks like compiling with 0.10.3 instead of 0.10.2 fixed that issue.

[paf31]

Yes, that's right, it needs the symbol deriving feature to derive Show.

[chexxor]

@paf31 - I'm not sure what exactly you wanted me to check for, so I looked for the bit I'm most interested in seeing fixed in this rewrite, which is adding the missing field name to the error.

It seems to work well in the success case on my one simple test, which is parsing a record, but the failure case still doesn't include the missing field's name. See my notes and test case in my branch, if you're curious: chexxor@21ad569

I have a suggested fix, but I'm not familiar with the expected return value and errors of the genericDecodeFieldsField instance. Following is its description.

y, the Foreign value at the expected field name, is 'undefined'. read undefined throws error: (TypeMismatch "String" "Undefined"). I'd like to add name, the field's name, to this error. By doing this, would this function's error message still describe the correct error in all cases of this function args?

[chexxor]

Using your suggestion, I added a fix to include the field name. It's in this commit: chexxor@c2cf778#diff-f4f396b6bb00857e2d3c4a669f217589R172

The error message is given a field name like: (Left (NonEmptyList (NonEmpty (ErrorAtIndex 0 (ErrorAtProperty "name" (TypeMismatch "String" "Undefined"))) Nil)))

You can pull that fix into your current branch, if you'd like.