Introduce SwaggerItems GADT to simplify ParamSchema

[fizruk]

Removes one type parameter from ParamSchema and allows to use less type annotations.

For instance, prior to this change this code required extra type annotation and gives a not very helpful type error:

>>> encode $ toParamSchema (Proxy :: Proxy Integer)

<interactive>:5:1:
    No instance for (ToJSON i0) arising from a use of ‘encode’
    The type variable ‘i0’ is ambiguous
    Note: there are several potential instances:
      instance ToJSON ApiKeyLocation
        -- Defined at /Users/nickolaykudasov/git/getshoptv/swagger2/src/Data/Swagger/Internal.hs:719:1
      instance ToJSON ApiKeyParams
        -- Defined at /Users/nickolaykudasov/git/getshoptv/swagger2/src/Data/Swagger/Internal.hs:720:1
      instance ToJSON (CollectionFormat t)
        -- Defined at /Users/nickolaykudasov/git/getshoptv/swagger2/src/Data/Swagger/Internal.hs:840:10
      ...plus 36 others
    In the expression: encode
    In the expression: encode $ toParamSchema (Proxy :: Proxy Int8)
    In an equation for ‘it’:
        it = encode $ toParamSchema (Proxy :: Proxy Int8)

This can be fixed by adding explicitly the (not very good looking) result type of toParamSchema:

>>> encode (toParamSchema (Proxy :: Proxy Integer) :: ParamSchema t Items)
"{\"type\":\"integer\"}"

With SwaggerItems, ambiguity disappears:

>>> encode $ toParamSchema (Proxy :: Proxy Integer)
"{\"type\":\"integer\"}"

SwaggerItems has a tiny disadvantage of allowing Items to be items type for Schema, but since it can always be converted naturally into Schema, it does not look bad. Besides, I don't expect library users to ever notice this or actually work with items directly.

[fizruk]

@dmjio I would like to hear your opinion on this one before merging :)

[dmjio]

@fizruk I'll look at it today ! apologies I've been traveling for work =)

On Mon, Dec 14, 2015 at 4:25 AM, Nickolay Kudasov notifications@github.com
wrote:

@dmjio https://github.com/dmjio I would like to hear your opinion on
this one before merging :)

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