[Haskell-cafe] Expression dye

[Vo Minh Thu]

2010/7/15 Vo Minh Thu <noteed at gmail.com>:
> 2010/7/14 Vo Minh Thu <noteed at gmail.com>:
>> 2010/7/14 Andrew Coppin <andrewcoppin at btinternet.com>:
>>> I'm trying to write a function that builds a series of results in a very
>>> complicated way. Eventually I ended up writing things like
>>>
>>>> newtype Dye = Dye String deriving (Eq, Show)
>>>>
>>>> instance Num Dye where
>>>>   (Dye x) + (Dye y) = Dye (x ++ " + " ++ y)
>>>>   (Dye x) - (Dye y) = Dye (x ++ " - " ++ y)
>>>>   (Dye x) * (Dye y) = Dye (x ++ " * " ++ y)
>>>>   abs (Dye x) = Dye ("abs " ++ x)
>>>
>>> and so on. In this way, you can do something like
>>>
>>>> sum [Dye "x", Dye "y", Dye"z"]
>>>
>>> and get "0 + x + y + z" as the result. (In reality you probably want to keep
>>> track of bracketing and so forth.) In this way, you can take functions that
>>> accept any Num instance and feed the "dye" through them to see what they're
>>> actually computing on a given run.
>>>
>>> Has anybody ever put anything like this on Hackage? I'd prefer to not invent
>>> this stuff if somebody has already done it...
>>>
>>> (The small problem with the approach above, of course, is that as soon as
>>> the function wants to do comparisons or take flow control decisions, you've
>>> got trouble. It's not impossible to solve, but it *is* a lot of work...)
>>
>> Hi,
>>
>> Why not make some kinf of AST and pretty-print it ? Also you can use
>> -XOverloadedStrings to write "x" + "y" instead of Dye "x" + Dye "y".
>>
>> If the goal is to see some common expressions as text, I believe there
>> is such a package on Hackage but can't remember its name.
>
> Oh, maybe not on Hackage, I think what I had in mind was in fact a blog post:
> http://tom.lokhorst.eu/2009/09/deeply-embedded-dsls

Found it:
http://hackage.haskell.org/package/repr

Cheers,
Thu