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    <p>From the Racket <a href="http://docs.racket-lang.org/reference/pairs.html" rel="nofollow">documentation</a>, the description of <code>foldl</code>:</p> <pre><code>(foldl proc init lst ...+) → any/c </code></pre> <p>Two points of interest for your question are mentioned:</p> <blockquote> <p>the input lsts are traversed from left to right</p> </blockquote> <p>And</p> <blockquote> <p>foldl processes the lsts in constant space</p> </blockquote> <p>I'm gonna speculate on how the implementation for that might look like, with a single list for simplicity's sake:</p> <pre><code>(define (my-foldl proc init lst) (define (iter lst acc) (if (null? lst) acc (iter (cdr lst) (proc (car lst) acc)))) (iter lst init)) </code></pre> <p>As you can see, the requirements of left-to-right traversal and constant space are met (notice the tail recursion in <code>iter</code>), but the <em>order</em> of the arguments for <code>proc</code> was never specified in the description. Hence, the result of calling the above code would be:</p> <pre><code>(my-foldl - 0 '(1 2 3 4)) &gt; 2 </code></pre> <p>If we had specified the order of the arguments for <code>proc</code> in this way:</p> <pre><code>(proc acc (car lst)) </code></pre> <p>Then the result would be:</p> <pre><code>(my-foldl - 0 '(1 2 3 4)) &gt; -10 </code></pre> <p>My point is, the documentation for <code>foldl</code> doesn't make any assumptions on the evaluation order of the arguments for <code>proc</code>, it only has to guarantee that constant space is used and that the elements in the list are evaluated from left to right.</p> <p>As a side note, you can get the desired evaluation order for your expression by simply writing this:</p> <pre><code>(- 0 1 2 3 4) &gt; -10 </code></pre>
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    1. POWhy is foldl defined in a strange way in Racket?
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    1. USWill Ness
    1. USÓscar López
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    1. COPersonally, I would much rather the description for a function was specific about the result it returns than how much stack space it uses!
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    2. CO@Ben [the doc does show an example application](http://docs.racket-lang.org/reference/pairs.html#%28def._%28%28lib._racket%2Fprivate%2Flist..rkt%29._foldl%29%29) of `> (foldl cons '() '(1 2 3 4))` ==> `'(4 3 2 1)` which can only be with one specific order of args. They could stress it some more, I agree.
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