Generalise FromJSON (Map k v) instances

[phadej]

Could we change

instance FromJSON v => FromJSON (Map String v)
instance FromJSON v => FromJSON (HashMap String v)

to

instance (IsText k, Ord k, FromJSON v) => FromJSON (Map k v)
instance (IsText k, Eq k, Hashable k, FromJSON v) => FromJSON (HashMap k v)

unfortunately Text doesn't provided IsText (yet?)

[bos]

No, I've never even heard of this typeclass.

[phadej]

Could it be created inside Aeson for this purpose?

[mgsloan]

Incidentally, I just copy+modified a FromJSON instance for Map yesterday, and in the comment, described the same idea: https://github.com/commercialhaskell/stack/blob/45fa84da555c1e1a02da70d056f8286c25d68dba/src/Stack/Types/Compiler.hs#L46

Naming-wise, maybe something like FromJSONName and ToJSONName classes.

[phadej]

@mgsloan I agree. Having Text -> Parser a would be more useful than bare Text -> a.