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POHaskell, terminal call optimisation and lazy evaluation
 

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  1. POHaskell, terminal call optimisation and lazy evaluation
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    <p>I'm trying to write a <code>findIndexBy</code> which would return the index of an element selected in a list by an ordering function. This function is equivalent to sorting the list and returning the top element, but I want to implement it to be able to process lists without size limit.</p> <pre class="lang-hs prettyprint-override"><code>findIndexBy :: (Ord a) =&gt; (a -&gt; a -&gt; Bool) -&gt; [a] -&gt; Integer findIndexBy f (x:xs) = findIndexBy' xs x 1 0 where findIndexBy' [] _ _ i = i findIndexBy' (x:xs) y xi yi = if f x y then findIndexBy' xs x (xi + 1) xi else findIndexBy' xs y (xi + 1) yi </code></pre> <p>With this implementation, I get a <code>Stack space overflow</code> when processing big list, as in the following example (trivial):</p> <pre class="lang-hs prettyprint-override"><code>findIndexBy (&gt;) [1..1000000] </code></pre> <p>I know there should be more elegant solutions to solve this problem, and I'm interested in knowing the most idiomatic and efficient ones, but I really want to understand what is wrong with my function.</p> <p>I might be wrong, but I think my implementation of <code>findIndexBy'</code> is based on terminal recursion, so I don't really understand why the compiler doesn't seem to optimize the tail call.</p> <p>I thought it might be due to the if/then/else and also trying the following, which results in the same error:</p> <pre class="lang-hs prettyprint-override"><code>findIndexBy :: (Ord a) =&gt; (a -&gt; a -&gt; Bool) -&gt; [a] -&gt; Integer findIndexBy f (x:xs) = findIndexBy' xs x 1 0 where findIndexBy' [] _ _ i = i findIndexBy' (x:xs) y xi yi = findIndexBy' xs (if f x y then x else y) (xi + 1) (if f x y then xi else yi) </code></pre> <p>Is there a simple way to ask the compiler to show where tail-call optimization is (not) performed?</p> <p>For reference, below is the equivalent function that I wrote in Clojure, and that I am now trying to port to Haskell:</p> <pre class="lang-lisp prettyprint-override"><code>(defn index-of [keep-func, coll] (loop [i 0 a (first coll) l (rest coll) keep-i i] (if (empty? l) keep-i (let [keep (keep-func a (first l))] (recur (inc i) (if keep a (first l)) (rest l) (if keep keep-i (inc i))))))) </code></pre> <p>For information, the previously quoted Haskell code was compiled using the <code>-O3</code> flag.</p> <p><strong>[edit after leventov answer]</strong></p> <p>The problem seems to be related to lazy evaluation. Although I found about <code>$!</code> and <code>seq</code>, I wonder what is the best practice when using them to fix the original code.</p> <p>I'm still interested with more idiomatic implementations relying on functions from <code>Data.List</code>.</p> <p><strong>[edit]</strong></p> <p>The simplest fix is to add <code>yi `seq`</code> in the first snippet before the <code>if</code> statement.</p>
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    1. COYou won't find anything conceptually different. `findIndexBy f (x:xs) = snd $ fst $ foldl' (\(i, found@(foundI, foundX)) x -> (i + 1, if f x foundX (i + 1, x) else found)) xs (1, (0, x))`
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