# [Haskell-cafe] Expression dye

Vo Minh Thu noteed at gmail.com
Wed Jul 14 18:02:46 EDT 2010

```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

Cheers,
Thu
```