"class []" proposal Re: [Haskell-cafe] One thought: Num to 0as ? to list?

[Brian Hulley]

Bulat Ziganshin wrote:
> from one my module:
>
> copyStream :: (BlockStream h1, BlockStream h2, Integral size)
>           => h1 -> h2 -> size -> IO ()

> in my library and found the way to simplify most of their signatures:
>
> copyStream :: BlockStream* -> BlockStream** -> Integral -> IO ()
>
> i think that second block of signatures is an order of magnitude more
> readable

I think template haskell could do this translation. Eg if you declared some 
dummy phantom types so the arg to the template function could be parsed, and 
as a way of telling the function which types were to be replaced by type 
variables with class constraints the block of signatures would just be 
declared in a splice:

$(expandSigs [d|

    data BlockStream'
    data BlockStream''
    data Integral'

    copyStream :: BlockStream' -> BlockStream'' -> Integral' -> IO ()
    ...
|])

Though I leave the definition of expandSigs :: Q [Dec] -> Q () as a little 
exercise for the reader... :-)

Best regards, Brian.
-- 
Logic empowers us and Love gives us purpose.
Yet still phantoms restless for eras long past,
congealed in the present in unthought forms,
strive mightily unseen to destroy us.

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