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2011-12-20T17:01:57.933
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2011-12-20T17:01:57.933
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The tricky thing to understand about Haskell is how to recognize and compose IO actions. Let's look at some type signatures. <pre><code>dir :: FilePath allFilesIn :: FilePath -> IO [FilePath] perFileFunc :: Int -> FilePath -> IO (Map.Map [Char] Double) </code></pre> Now then, you said that for <code>myFunc</code>: <blockquote> I was expecting a list of maps </blockquote> So for that function, you want the type signature <pre><code>myFunc :: Int -> FilePath -> [Map.Map String Double] </code></pre> Of course, the return type can't be just <code>[Map.Map String Double]</code>, because we need to perform some IO in order to evaluate <code>myFunc</code>. So given an <code>Int</code> and a <code>FilePath</code>, we actually want the return type to be an IO action that produces a <code>[Map.Map String Double]</code>: <pre><code>myFunc :: Int -> FilePath -> IO [Map.Map String Double] </code></pre> Now then, let's look at the IO actions we will be composing to create this function. <pre><code>allFilesIn dir :: IO [FilePath] perFileFunc n :: FilePath -> IO (Map.Map String Double) </code></pre> <code>perFileFunc</code> isn't actually an IO action, but it is a function that, given a <code>FilePath</code>, produces an IO action. So let's see...if we run the <code>allFilesIn</code> action, then we can work with that list and run <code>perFileFunc n</code> on each of its elements. <pre><code>myFunc dir n = do files <- allFilesIn dir ??? </code></pre> So what goes in the <code>???</code> spot? We have a <code>[FilePath]</code> at our disposal, since we used <code><-</code> to run the action <code>allFilesIn dir</code>. And we have a function, <code>perFileFunc n :: FilePath -> IO (Map.Map String Double)</code>. And we want the result to have the type <code>IO [Map.Map String Double]</code>. Stop...Hoogle time! Generalizing the components we have (<code>a = FilePath</code>, <code>b = Map.Map String Double</code>), we <a href="http://haskell.org/hoogle/?hoogle=%5Ba%5D+-%3E+%28a+-%3E+IO+b%29+-%3E+IO+%5Bb%5D" rel="nofollow">hoogle for <code>[a] -> (a -> IO b) -> IO [b]</code></a> (pretending we haven't seen ehird's answer yet). Lo and behold, <code>mapM</code> is the magical solution we were looking for! (or <code>forM</code>, which is just <code>flip mapM</code>) <pre><code>myFunc dir n = do files <- allFilesIn dir mapM (perFileFunc n) files </code></pre> If you desugar this do notation, you'll find it reduces to ehird's answer: <pre><code>myFunc dir n = allFilesIn dir >>= (\files -> mapM (perFileFunc n) files) -- eta reduce (\x -> f x) ==> f myFunc dir n = allFilesIn dir >>= mapM (perFileFunc n) </code></pre>
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1. POIterating over files using Haskell
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1. USDan Burton
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1. CO+1, mechanically deriving the correct answer is definitely the way to approach problems like this.
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