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4341160
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2010-12-02T23:39:48.520
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2010-12-02T23:48:32.160
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You're problem doesn't have anything to do with <code>Floating</code>, though the typeclass does make your error harder to understand. Take the below code as an example: <pre><code>test1 :: Int -> Char -> Int test1 = undefined test2 :: Int -> Int test2 x = undefined testBoth = test2 . test1 </code></pre> What is the type of testBoth? Well, we take the type of <code>(.) :: (b -> c) -> (a -> b) -> a -> c</code> and turn the crank to get: <ol> <li><code>b ~ Int</code> (the argument of <code>test2</code> unified with the first argument of <code>(.)</code>)</li> <li><code>c ~ Int</code> (the result of <code>test2</code> unified with the result of the first argument of <code>(.)</code>)</li> <li><code>a ~ Int</code> (<code>test1</code> argument 1 unified with argument 2 of <code>(.)</code>)</li> <li><code>b ~ Char -> Int</code> (result of <code>test1</code> unified with argument 2 of <code>(.)</code>)</li> </ol> but wait! that type variable, 'b' (#4, <code>Char -> Int</code>), has to unify with the argument type of <code>test2</code> (#1, <code>Int</code>). Oh No! How should you do this? A correct solution is: <pre><code>testBoth x = test2 . test1 x </code></pre> There are other ways, but I consider this the most readable. Edit: So what was the error trying to tell you? It was saying that unifying <code>Floating a => a -> a</code> with <code>Floating b => b</code> requires an <code>instance Floating (a -> a)</code> ... while that's true, you really didn't want GHC to try and treat a function as a floating point number.
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1. POHaskell: composing function with two floating arguments fails
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1. USThomas M. DuBuisson
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1. USThomas M. DuBuisson
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1. COThanks, this is an intuitive explanation. When I left out the typeclass, that must have meant that testBoth would take x and pass it to test1, then take the resulting function f : y |-> x and pass it to test2, which would then return f. That's clearly not what I wanted.
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