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copied!<p>Use a name other than <code>print</code> for your image to avoid collisions with the Prelude's <code>print</code>.</p> <pre><code>type Pixel = Int type Row = [Pixel] type PixelImage = [Row] img :: PixelImage img = [[208,152,240,29],[0,112,255,59],[76,185,0,152]] </code></pre> <p>This is an inefficient representation, but it will do for a learning exercise.</p> <p>You could print a <code>PixelImage</code> with the rows stacked on top of one another with a few imports at the top of your source and an I/O action:</p> <pre><code>import Control.Monad import Data.List import Text.Printf printImage :: PixelImage -> IO () printImage img = forM_ img $ putStrLn . intercalate " " . map (printf "%3d") </code></pre> <p>That may look intimidating, but everything there is familiar. The word order is just a little funny. For each <code>Row</code> in the <code>PixelImage</code> (<a href="http://www.haskell.org/ghc/docs/6.12.2/html/libraries/base-4.2.0.1/Control-Monad.html#v%3AforM_" rel="nofollow noreferrer"><code>forM_</code></a>, a lot like a <code>for</code> loop in other languages) we print (with <a href="http://haskell.org/ghc/docs/6.12.1/html/libraries/base-4.2.0.0/Prelude.html#v:putStrLn" rel="nofollow noreferrer"><code>putStrLn</code></a>) the list of <code>Pixel</code> values separated by two spaces (thanks to <a href="http://haskell.org/ghc/docs/6.12.1/html/libraries/base-4.2.0.0/Data-List.html#v:intercalate" rel="nofollow noreferrer"><code>intercalate</code></a>) and left-padded with spaces to make uniform 3-character fields (<a href="http://haskell.org/ghc/docs/6.12.1/html/libraries/base-4.2.0.0/Text-Printf.html#v:printf" rel="nofollow noreferrer"><code>printf</code></a>).</p> <p>With the image from your question, we get</p> <pre>ghci> printImage img 208 152 240 29 0 112 255 59 76 185 0 152</pre> <p>Haskell lists are <em>immutable</em>: you cannot modify one in-place or destructively. Instead, think of it in terms of making a different list that's identical to the original except for the specified row.</p> <pre><code>modifyRow :: PixelImage -> (Row -> Row) -> Int -> PixelImage modifyRow img f i = map go (zip [0..] img) where go (j,r) | i == j = f r | otherwise = r </code></pre> <p>This gives your function a chance to fire for each <code>Row</code> in the <code>PixelImage</code>. Say you want to zero out a particular row:</p> <pre>ghci> printImage $ modifyRow img (map $ const 0) 0 0 0 0 0 0 112 255 59 76 185 0 152</pre> <p>Reversing a row is</p> <pre>ghci> printImage $ modifyRow img reverse 0 29 240 152 208 0 112 255 59 76 185 0 152</pre> <p>In another language, you might write <code>img[2] = [1,2,3,4]</code>, but in Haskell it's</p> <pre>ghci> modifyRow img (const [1..4]) 2 [[208,152,240,29],[0,112,255,59],[1,2,3,4]]</pre> <p>That usage isn't terribly evocative, so we can defined <code>setRow</code> in terms of <code>modifyRow</code>, a common technique in functional programming.</p> <pre><code>setRow :: PixelImage -> Row -> Int -> PixelImage setRow img r i = modifyRow img (const r) i </code></pre> <p>Nicer:</p> <pre>ghci> printImage $ setRow img [4,3,2,1] 1 208 152 240 29 4 3 2 1 76 185 0 152</pre> <p>Maybe you want to scale the pixel values instead.</p> <pre><code>scaleRow :: (RealFrac a) => PixelImage -> a -> Int -> PixelImage scaleRow img x i = modifyRow img f i where f = let clamp z | z < 0 = 0 | z > 255 = 255 | otherwise = truncate z in map (clamp . (x *) . fromIntegral) </code></pre> <p>For example:</p> <pre>ghci> printImage $ scaleRow img 0.5 1 208 152 240 29 0 56 127 29 76 185 0 152</pre> <p>Adding <code>scaleImage</code> to apply a scaling factor to each <code>Pixel</code> in a <code>PixelImage</code> means a bit of refactoring to avoid repeating the same code in multiple places. We'd like to be able to use</p> <pre><code>scaleImage :: (RealFrac a) => a -> PixelImage -> PixelImage scaleImage x = map $ scaleOneRow x </code></pre> <p>to get, say</p> <pre>ghci> printImage $ scaleImage 3 img 255 255 255 87 0 255 255 177 228 255 0 255</pre> <p>This means <code>scaleOneRow</code> should be</p> <pre><code>scaleOneRow :: (RealFrac a) => a -> Row -> Row scaleOneRow x = map (clamp . (x *) . fromIntegral) </code></pre> <p>which promotes <code>clamp</code> to a toplevel function on <code>Pixel</code> values.</p> <pre><code>clamp :: (RealFrac a) => a -> Pixel clamp z | z < 0 = 0 | z > 255 = 255 | otherwise = truncate z </code></pre> <p>This in turn simplifies <code>scaleRow</code>:</p> <pre><code>scaleRow :: (RealFrac a) => PixelImage -> a -> Int -> PixelImage scaleRow img x i = modifyRow img (scaleOneRow x) i </code></pre>

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