On Tue, Oct 28, 2008 at 07:00:00PM -0400, Tony Hannan wrote:
On Mon, Oct 27, 2008 at 11:26 PM, Brandon S. Allbery KF8NH < allbery@ece.cmu.edu> wrote:
On 2008 Oct 27, at 23:25, Andrew Sackville-West wrote:
this raises a question for me, being a bit of a schemer. Is there any parallel in haskell to the data is code model of the lisp family? For example, playing around in scheme with a symbolic differentiator, it is trivial to then evaluate the differentiated s-expression at arbitrary value by representing the expression, and it's derivative as a regular scheme expression.
Is this something that can be done in haskell? My initial impression is no, that you'd have to parse it as an expression and evaluate it as you would in regular imperative languages. I'd love to hear otherwise.
You get this in a type-safe form with Template Haskell; you can operate on expressions at the AST level.
Yeah, but can you do this at run time? I though Template Haskell can only be used at compile time.
That's what I'm talking about, run time evaluation of code/data. In scheme, I can take as input (+ (exp x 2) (* 2 x)) a representation of x^2 + 2x I can feed that "data" into a function (derive input) and get back (+ (* 2 x) 2) a representation of 2x +2, the derivative of the above. The function derive can manipulate that "Data" which is just a list, or s-expression like any other data (for example accessing the first element in the list, determining that it's a '+' and so calling derive recursively on the two following elements). then I can feed that result into (eval) with some information about what the value of x is and it will be evaluated just like a regular scheme expression. something like (it's been a while): (let ((x 2)) (eval default-environment (derive (input)))) where default-environment lets you get the information about the current environment into the eval environment so that x has a value. So now, what was "Data" a moment ago is treated as executable within the eval. The above (let) would spit out(in this case) a value of 6. Is such a thing possible in haskell? (I'm really just curious, I have no need of this functionality at this point). Oh, and I apologise for the fingernail clippings... A --