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2580442
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2010-04-05T19:02:43.917
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2010-04-05T19:32:59.713
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2010-04-05T19:32:59.713
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<code>ghci</code> is your friend: <pre>*Main> :t map head map head :: [[a]] -> [a] *Main> :t map tail map tail :: [[a]] -> [[a]]</pre> Even if you don't understand <a href="http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3Amap" rel="nofollow noreferrer"><code>map</code></a> (a problem you'd want to correct quickly!), the types of these expressions tell much about how they work. The first is a single list taken from a list of lists, so let's feed a simple vector to it to see what happens. You might want to write <pre><code>*Main> map head [1,2,3] </code></pre> but that fails to typecheck: <pre><interactive>:1:14: No instance for (Num [a]) arising from the literal `3' at :1:14 Possible fix: add an instance declaration for (Num [a]) In the expression: 3 In the second argument of `map', namely `[1, 2, 3]' In the expression: map head [1, 2, 3]</pre> Remember, the argument's type is a list of lists, so <pre><code>*Main> map head [[1,2,3]] [1] </code></pre> Getting a bit more complex <pre><code>*Main> map head [[1,2,3],[4,5,6]] [1,4] </code></pre> Doing the same but with <a href="http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3Atail" rel="nofollow noreferrer"><code>tail</code></a> instead of <a href="http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3Ahead" rel="nofollow noreferrer"><code>head</code></a> gives <pre><code>*Main> map tail [[1,2,3],[4,5,6]] [[2,3],[5,6]] </code></pre> As you can see, the definition of <code>transpose</code> is repeatedly slicing off the first “row” with <code>map head x</code> and transposing the rest, which is <code>map tail x</code>.
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1. POUnderstanding this matrix transposition function in Haskell
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1. USGreg Bacon
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1. USGreg Bacon
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