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POHow can I parse a string to a function in Haskell?
 

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  1. POHow can I parse a string to a function in Haskell?
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    <p>I want a function that looks something like this </p> <pre><code>readFunc :: String -&gt; (Float -&gt; Float) </code></pre> <p>which operates something like this</p> <pre><code>&gt;(readFunc "sin") (pi/2) &gt;1.0 &gt;(readFunc "(+2)") 3.0 &gt;5.0 &gt;(readFunc "(\x -&gt; if x &gt; 5.0 then 5.0 else x)") 2.0 &gt;2.0 &gt;(readFunc "(\x -&gt; if x &gt; 5.0 then 5.0 else x)") 7.0 &gt;5.0 </code></pre> <p>The incredibly naive approach (note this must be compiled with <code>{-# LANGUAGE FlexibleContexts #-}</code>)</p> <pre><code>readFunc :: (Read (Float -&gt; Float)) =&gt; String -&gt; (Float -&gt; Float) readFunc s = read s </code></pre> <p>gives</p> <pre><code>No instance for (Read (Float -&gt; Float)) ... </code></pre> <p>Which makes sense since no such instance exists. I understand that I can parse the input string character by character by writing a map from <code>String</code> to <code>Float -&gt; Float</code> but I want to be able to parse at least the most common functions from prelude, and even that would be way more work than I want to commit to. Is there an easy way of doing this?</p> <h1>Just one solution using hint</h1> <pre><code>import Language.Haskell.Interpreter hiding (typeOf) import Data.Typeable (typeOf) data Domain = Dom Float Float Float Float Domain | SDom Float Float Float Float deriving (Show, Read) --gets all the points that will appear in the domain points (SDom a b c d) m = [(x, y)|x &lt;- [a, a+m .. b], y &lt;- [c, c+m .. d]] points (Dom a b c d next) m = points next m ++ [(x, y)|x &lt;- [a, a+m .. b], y &lt;- [c, c+m .. d]] readFunc = do putStrLn "Enter a domain (as Dom x-min x-max y-min y-max subdomain, or, SDom x-min x-max y-min y-max)" domain' &lt;- getLine let domain = (read domain') :: Domain -- putStrLn "Enter a mesh size" meshSize' &lt;- getLine let meshSize = (read meshSize') :: Float -- putStrLn "Enter an initial value function (as f(x,y))" func' &lt;- getLine values' &lt;- runInterpreter $ setImports["Prelude"] &gt;&gt; eval ("map (\\(x,y) -&gt; " ++ func' ++ ")" ++ show (points domain meshSize)) let values = (\(Right v) -&gt; (read v)::([Float])) values' --the haskell expression being evaluated putStrLn $ ("map (\\(x,y) -&gt; " ++ func' ++ ")" ++ show (points domain meshSize)) --prints the actual values putStrLn $ show values --the type is indeed [float] putStrLn $ show $ typeOf values </code></pre>
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    1. COWhat do you need it for? Maybe there are easier ways to solve the underlying problem.
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      1. POHow can I parse a string to a function in Haskell?
      1. USAndrewC
    2. COI am writing a program which implements several numerical techniques for solving differential equations in two dimensions. I would like a command line program which will take input from the user in the form of an anonymous Haskell function (in string format of course) in order to define the initial conditions of a problem.
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      1. POHow can I parse a string to a function in Haskell?
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    3. COIs it enough to interpret expressions? See http://stackoverflow.com/questions/5582926/haskell-how-to-evaluate-a-string-like-12/5584638#5584638
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      1. POHow can I parse a string to a function in Haskell?
      1. USThomas M. DuBuisson
 

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