[Haskell-cafe] Overloaded Quotes for Template Haskell

[Michael D. Adams]

I'm not very familiar with monad-control, but it looks like the
equivalent of liftWith from monad-control would be a function that has
type "StateT S Q a -> Q (S -> (a, S))".  IIUC, you are suggesting that
the code would look something like "restoreT [| ... $( liftWith ( ...
) ) ... |]".

Unfortunately, I don't think that would work because GHC requires that
the contents of "$( ... )" have exactly the type "Q Exp" (or "Q Type",
etc.).  Using liftWith in that way makes the contents of "$( ... )"
have type "Q (S -> (a, S))".

On Fri, Dec 30, 2011 at 5:55 AM, James Cook <mokus at deepbondi.net> wrote:
> One possible option would be to make a library that has all the combinators lifted to your more general type and use "lift" or "runQ" or something similar for any quotes that need lifting, along with operations from monad-control or monad-peel to lift quotes that also need access to the StateT layer in the splice.  It's a bit messier and those libraries are brain-bending at first (although definitely worth learning about), but it would allow you to code in the style you're talking about with a relatively small amount of extra syntactic clutter.
>
> -- James
>
> On Dec 29, 2011, at 9:51 PM, Michael D. Adams wrote:
>
>> What would it take to get an "-XOverloadedQuotes" flag of the same
>> sort as the "-XOverloadedStrings" flag?  I.e. "[| ... |]" would have
>> type "Quasi m => m Exp" instead of "Q Exp" and any splices in that
>> quotation expect the contents of that splice to have type "m Exp".
>> (Obviously, top level splices would still have type "Q Exp" otherwise
>> the compiler wouldn't know how to evaluate the monad.)
>>
>> I ran into this problem when writing a Template Haskell program in
>> which part of it operates in a "StateT S Q a" monad instead of the
>> usual "Q a" monad.  (The "S" type stores the state of a memoization
>> table of code fragments already generated.  Without it, the code would
>> loop infinitely when processing certain recursive structures.)
>>
>> It is fairly easy to declare an instance of "Quasi" for "StateT S Q",
>> so in order to keep the code clean, I'd like to use quotations with
>> splices in them (i.e. [| ... $( ... ) ... |] ) for expressing the
>> generated code.  However, quotations and splices are tied to the "Q"
>> monad which means that as it is now I have to manually write "LamE ...
>> VarP ... VarE ... etc." instead of using the much nicer quotation
>> syntax.
>>
>> Michael D. Adams
>> mdmkolbe at gmail.com
>>
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>