Actually I have some formula like "x + y"
, which is a String
.
I managed to replace the x/y
variable with specific values like "1.2"
, which is still String
type.
Now I have expression like "1 + 2"
.
So the problem is how to evaluate a expression of a string type and get the result.
ps: I wanna sth like read
, that can directly convert the whole string expression instead of handling the operator (+/-,etc) case by case. Is that possible?
Your question leaves a lot of room for interpretation. I'm taking a guess you aren't accustom to building a whole pipeline of lexing, parsing, maybe type checking, and evaluating. The long answer would involve you defining what language you wish to evaluate (Just integers with '+', perhaps all rationals with '+', '-' '*', '/', or even a larger language?) and perform each of the above steps for that language.
The short answer is: to evaluate Haskell expressions, which includes the basic math operators you're probably talking about, just use the "hint" package:
$ cabal install hint
...
$ ghci
> import Language.Haskell.Interpreter
> runInterpreter $ setImports ["Prelude"] >> eval "3 + 5"
Right "8"
Yay!