[Haskell-cafe] Expression dye

Vo Minh Thu noteed at gmail.com
Wed Jul 14 17:55:44 EDT 2010

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...)


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.


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