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POBug in while loop
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16993831
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2013-06-07T22:24:28.507
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2013-06-07T22:29:39.727
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2013-06-07T22:29:39.727
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I can't figure out what the bug is in the toy <code>while</code> loop I wrote below. It works for one input, but hangs for other inputs. Here is the code - the while loop takes a vector, a predicate on vector, and transformation function on vector, and returns another vector: <pre><code>import Data.Vector.Unboxed as U while :: Vector Int -> (Vector Int -> Bool) -> (Vector Int -> Vector Int) -> Vector Int while v p f = go 0 v where go n x = if (p x) then go (n+1) (f x) else x test :: Vector Int -> Vector Int test a = while a (\x -> (x!0) < 2) (\x -> U.map (+1) a) main = print $ test (U.fromList [0]) </code></pre> This hangs on execution of <code>main</code>. On the other hand, if I change <code>test</code> to: <pre><code>test a = while a (\x -> (x!0) < 1) (\x -> U.map (+1) a) </code></pre> I get a termination with result (<code>ghci</code> output below): <pre><code>λ: main fromList [1] </code></pre> I feel I must be missing something. I looked hard at the function, but can't figure out what I am doing wrong. It looks like predicate can't be executed more than twice. BTW, it works with other types like <code>Int</code>. <pre><code> while :: Int -> (Int -> Bool) -> (Int -> Int) -> Int while i p f = go 0 i where go n x = if (p x) then go (n+1) (f x) else x test :: Int -> Int test a = while a (\x -> x < 2) (\x -> x+1) main = print $ test 0 </code></pre> ghci output: <pre><code>λ: main 2 </code></pre> <code>GHC version: 7.6.1</code> <code>Vector version: 0.10.0.1</code>
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<loops><haskell>
Title
Bug in while loop
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1. PO
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