Hello, parse-infix, being a macro, works on the code-as-datastructure it has as arguments.
So (parse-infix x) receives the symbol x , unevaluated, is in charge of returning a new datastructure (generally involving the symbol x). Only then, the compiler will evaluate the result of having called parse-infix with the symbol x. So you cannot have the result of having evaluated x at "compile time" (when parse-infix is called). 2010/1/8 tristan <tristan.k...@gmail.com>: > Hi guys, > > I've been working on a problem where I want the user to be able to > input an equation in infix notation which includes variables and > convert that to a clojure fn that my program can call later. > > i.e. the user inputs the string "a*(b+c)" and i generate the unnamed > function (fn [a b c] (* a (+ b c))). > note: rather than having (fn [a b c] body), (fn [inputs] body), where > inputs is a map and reference to variables is replaces with (get > inputs variable), is fine, and possibly even simpler to work with > later. > > Is this possible? or will i have to parse and evaluate the infix > string on each call to the function, or come up with some way to > compile the code and load it back in? > > Here is what I have attempted so far: > http://github.com/tristan/modelmaker/blob/41844b9376b57c05b457c02ac70dfc39c0935a03/infix_parser.clj > > My (parse) function expects a string containing an infix equation and > returns a vector where the first element is a list of symbols > reflecting the prefix version of the equation (i.e. (list '* 'a (list > '+ 'b 'c))) and the 2nd element is the list of variables (i.e. (list > 'a 'b 'c)). It is called by my macro (parse-infix) that takes the > result from (parse) and expands to an unnamed function. > > At first I thought I had solved it, as calling (parse-infix "a*(b+c)") > returned the desired function that i could call. However as soon as i > attempted to use it in the form (parse-infix users-input) it falls > over with a "Don't know how to create ISeq from: clojure.lang.Symbol" > error (examples in code at the bottom of the above file). I have > fallen victim here to my own lack of understanding of macros. I now > understand my folly and have a better understanding of how macro > expansion works, however now I'm stuck as to how to solve this > problem. > > Another thing i've attempted is as follows. I thought that I could > simply return an unnamed function which accepts a map (i.e {'a 1 'b 2 > 'c 3}) and change my (parse) function to insert something along the > lines of (list 'get 'inputs variable), such that i can just call (eval > prefix). So i wrote this: > > (defn parse-infix [infix] > (let [[f v] (parse infix)] > (fn [inputs] > (if (= (count inputs) (count v)) > (eval f) > (throw (Exception. (str "expected " (count v) " inputs, got " (count > inputs)))))))) > > but eval doesn't seem to use the same context as the Thread that calls > it, so it complains that it's "Unable to resolve symbol: inputs in > this context". I've played a bit with with-bindings as well but cannot > seem to get any results from it either. > > thanks > .Tristan > > -- > You received this message because you are subscribed to the Google > Groups "Clojure" group. > To post to this group, send email to clojure@googlegroups.com > Note that posts from new members are moderated - please be patient with your > first post. > To unsubscribe from this group, send email to > clojure+unsubscr...@googlegroups.com > For more options, visit this group at > http://groups.google.com/group/clojure?hl=en >
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