<div dir="ltr"><div class="markdown-here-wrapper" style=""><h2 id="motivation" style="margin:1.3em 0px 1em;padding:0px;font-weight:bold;font-size:1.4em;border-bottom:1px solid rgb(238,238,238)">Motivation</h2>
<p style="margin:0px 0px 1.2em!important">Partial functions in base (especially Prelude) often cause runtime errors and is hard to locate.</p>
<p style="margin:0px 0px 1.2em!important">(<a href="https://gist.github.com/luochen1990/6fcbb0fa4c0c7caa1b242188eb586d1f">here</a> is a document about the concept of totality and partial function, ignore this if you are familiar with them)</p>
<p style="margin:0px 0px 1.2em!important">For example, consider the following piece of code:</p>
<pre style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;font-size:1em;line-height:1.2em;margin:1.2em 0px"><code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline;white-space:pre;overflow:auto;border-radius:3px;border:1px solid rgb(204,204,204);padding:0.5em 0.7em;display:block!important">import GHC.Stack

foo :: HasCallStack => [Int] -> Int
foo xs = last xs + 1

xs :: [Int]
xs = []

main :: IO ()
main = do
    print $ foo xs
</code></pre><p style="margin:0px 0px 1.2em!important">In this case, the error message will tell nothing about <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">foo</code>, and the <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">HasCallStack</code> constraint is totally helpless, because the call stack is cut off by the call to <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">last</code> which without <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">HasCallStack</code> constraint.</p>
<p style="margin:0px 0px 1.2em!important">My current workaround is define my own wrapper functions with <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">HasCallStack</code> constraint for some mostly used partial functions to make them traceable, and use the wrapper (the traceable version) whenever I need them.</p>
<p style="margin:0px 0px 1.2em!important">e.g.</p>
<pre style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;font-size:1em;line-height:1.2em;margin:1.2em 0px"><code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline;white-space:pre;overflow:auto;border-radius:3px;border:1px solid rgb(204,204,204);padding:0.5em 0.7em;display:block!important">last' :: HasCallStack => [a] -> a
last' xs = case xs of [] -> error "abuse last"; _ -> last xs</code></pre><p style="margin:0px 0px 1.2em!important">So, IMHO, if our goal is to make errors in haskell traceable, then <strong>only providing <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">HasCallStack</code> mechanism is not enough, we have to provide traceable base package and prelude at the same time</strong>.</p>
<p style="margin:0px 0px 1.2em!important">Further more, all untraceable partial functions are considered to be harmful, and should not be exported by any package. Because an improper call to an untraceable partial function will cut off the call stack, and <a href="https://gist.github.com/luochen1990/94179a2492ff7d1ca45153645f1bb449">here is a demonstration about that</a>.</p>
<p style="margin:0px 0px 1.2em!important">On the other hand, is it ever necessary for us to add <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">HasCallStack</code> for a total function? Or we can ask, is it possible that a call to a total function cause runtime error?  Maybe it’s a NO, since a total function will also crash when the machine is Out Of Memory, but that is also the only situation I can find out. So I suggest that we add <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">HasCallStack</code> constraint only for partial functions, and IMHO this could be a good balance for better debugging experience and less runtime overhead.</p>
<h2 id="proposal" style="margin:1.3em 0px 1em;padding:0px;font-weight:bold;font-size:1.4em;border-bottom:1px solid rgb(238,238,238)">Proposal</h2>
<ol style="margin:1.2em 0px;padding-left:2em">
<li style="margin:0.5em 0px">add <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">HasCallStack</code> constraint for all partial functions in base package</li>
<li style="margin:0.5em 0px">suggest all programmers to add <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">HasCallStack</code> constraint for their exported partial functions when they release a package</li>
<li style="margin:0.5em 0px">provide a compiler option <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">-fignore-hascallstack</code> to toggle off the effect of <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">HasCallStack</code> constraint in case somebody need best performance</li>
</ol>
<h2 id="other-considerations" style="margin:1.3em 0px 1em;padding:0px;font-weight:bold;font-size:1.4em;border-bottom:1px solid rgb(238,238,238)">Other Considerations</h2>
<h3 id="how-to-get-a-full-list-of-partial-functions-provided-by-the-base-package-" style="margin:1.3em 0px 1em;padding:0px;font-weight:bold;font-size:1.3em">How to get a full list of partial functions provided by the base package?</h3>
<p style="margin:0px 0px 1.2em!important">I wanted to provide a full list of partial functions exported by the base package in this post, but I find it is hard, since Haskell have no totality checking mechanism like Idris have, and there are no consistent keyword like “total” or “partial” in document, so it takes a lot of work to list all the partial functions of a package by check every item manually. Maybe we can work on this list later — when it’s turned out to be worth after some discussion.</p>
<p style="margin:0px 0px 1.2em!important"><a href="https://gist.github.com/luochen1990/5b7a0844701413486da595b9b997f3c2">Here</a> is part of the list that I have tidied for several modules of the base package.</p>
<h3 id="how-to-encourage-all-package-contributors-to-obey-the-rule-see-proposal-2-" style="margin:1.3em 0px 1em;padding:0px;font-weight:bold;font-size:1.3em">How to encourage all package contributors to obey the rule (see proposal #2)?</h3>
<p style="margin:0px 0px 1.2em!important">I don’t know, but I think there may be some other rules to obey when contributing packages. Maybe we can just add this into the list.</p>
<p style="margin:0px 0px 1.2em!important">Obviously, the final perfect solution should be let the compiler to check the totality of functions, and automatically add <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">HasCallStack</code> for the ones which the compiler cannot confirm it’s totality. But this seems too far away from us, since we still doesn’t have dependent type haskell yet.</p>
<h3 id="how-to-deal-with-recursive-partial-functions-" style="margin:1.3em 0px 1em;padding:0px;font-weight:bold;font-size:1.3em">How to deal with recursive partial functions?</h3>
<p style="margin:0px 0px 1.2em!important">Since the <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">HasCallStack</code> constraint affects the performance (not only because of the runtime overhead, but also it’s influence on optimization strategy), It is best not to add <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">HasCallStack</code> on recursive functions.</p>
<p style="margin:0px 0px 1.2em!important">In most of the cases, we can just check the input shallowly before everything start, just like how we deal with the non-recursive ones.</p>
<p style="margin:0px 0px 1.2em!important">But in some other cases, we need to go deep to recognize the invalidity of the input. A trivial solution is just perform a deep check before everything start, but the checking phase seems expensive.</p>
<p style="margin:0px 0px 1.2em!important">The best solution, IMHO, is to make the recursive part a total function, wrap the return value into <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">Maybe</code> or some similar things, and the partial version is just the total version combined <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">fromJust</code>. In this way, we avoid the single input checking phase and left the error awareing logic where it was before this translation.</p>
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