[Haskell] evaluate function as a string

Sebastian Sylvan sylvan at dtek.chalmers.se
Fri Apr 23 16:02:32 EDT 2004

Tomasz Zielonka wrote:

> On Fri, Apr 23, 2004 at 03:32:54AM +0200, Sebastian Sylvan wrote:
>>First you need to parse the expression into a Haskell data type. For 
>>this I would recommend Parsec (see Haskell.org). The Haskell data type 
>>would be something like
>>data Expr = AtomD Double | AtomI Integer | Add Expr Expr | Mul Expr Expr 
>>| Sub Expr Expr | ....
> There is no need to introduce a datatype - for one parameter functions
> you can parse text directly to a (Double -> Double) function. Of course
> if you want to do more things with your functions than just evaluation
> (like printing, symbolic differentiation), introducing a datatype is a
> good idea.

The original poster used the term "equation" which to me indicated that 
he might have things like unknown variables in stuff. In other words: He 
would have to do some form of manipulation of the expression in order to 
get the result.


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