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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">I don’t know any way to make a compositional story about nested ! annotations in types. As various of you have pointed out, it looks attractive and then runs
out of steam.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">Putting !’s on data constructor arguments works because it’s not really part of the type at all; rather, it just says “give this contstructor a wrapper that
makes it strict”. I think one could make a similar story for functions in general, so that<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"> f :: !a -> a -> ![a] -> Int<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"> f a b c = e<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">would mean “add a wrapper to f that makes it strict”; thus<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"> f a b c = a `seq` c `seq` e<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">But it’s not at all clear that this is worth the fuss.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">Strict Core is another matter. As the paper says, it’s an intermediate language that exposes more about what is evaluated and what is not, and how functions
are curried. As I said in an earlier reply, it would certainly be an interesting experiment to make GHC’s intermediate language be Strict Core, but it would not be a routine exercise. Fun to be had!
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> Johan Tibell [mailto:johan.tibell@gmail.com]
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<b>Sent:</b> 07 May 2013 21:33<br>
<b>To:</b> Gábor Lehel<br>
<b>Cc:</b> Simon Peyton-Jones; Max Bolingbroke; ghc-devs@haskell.org<br>
<b>Subject:</b> Re: Implementing Strict Core<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Tue, May 7, 2013 at 1:00 PM, Gábor Lehel <<a href="mailto:illissius@gmail.com" target="_blank">illissius@gmail.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal"><span style="color:#500050">On Tue, May 7, 2013 at 6:38 PM, Johan Tibell <<a href="mailto:johan.tibell@gmail.com" target="_blank">johan.tibell@gmail.com</a>> wrote:</span><o:p></o:p></p>
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<p class="MsoNormal">`a` and `!a` are compatible types. The compiler will insert the `seq`s where needed. I don't think this is useless.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">First, it serves as good documentation. Documenting strictness properties, e.g. as we do in the container library today, is awkward. Strictness annotations on types would make the documentation precise and succinct.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Second, it's a bit more declarative than having put manually put bang patterns at the right places in the function definition and having to remember (and teach beginners) how to do that correctly..<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Third, it might help use generate better code, especially if we also implement Strict Core.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Right. I completely agree. The idea is attractive to me for the same reasons. My questions basically relate to: how would you implement it?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">A function:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"> f :: !a -> ...<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">would be implemented as<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"> f !x = ...<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">i.e. the body of x would start with a `seq` of `x`.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I we had Strict Core we could do better and instead demand that the caller does the evaluation, removing cases of duplicate reevaluation and perhaps expose more unboxing.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">Right now GHC's type checker works by solving type equality constraints. Type equality is symmetric, and implies that the two types are substitutable for each other in all contexts: there's no concept of "mixing them up", because there's
nothing to get mixed up, they are the same. But `a` and `!a` aren't like that. You very much want to avoid mixing them up, so you can insert `seq`s in the right places. But how would that mesh with the present system?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I honestly don't know. Possibly we could just erase the strictness annotations after we have annotated `f` as being strict in its first argument.<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">Would you allow strictness annotations on type arguments in arbitrary positions ([!Int] and so forth)? That leads to the issues regarding "how could the compiler know how to strict-ify arbitrary types", and/or co/contra/invariance of type
parameters.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">My current thinking is that we would only allow `!` at the "top-level". Even if we defined a meaning for e.g. `f !a`, I don't know if it would be very useful as we wouldn't be able to say things about specific fields. For example, given<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"> data T a = C a a<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">should `T !a` mean that both fields are strict? What if we only want one to be strict? What if the type is opaque to the user, how is he/she supposed to know what `T !a` means without referring to implementation details? I believe Scala
has the same restriction.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">The only thing I would regret if `!` is only allowed at the top-level is that we couldn't reuse e.g. Haskell pairs for both lazy and strict pairs, which would be neat.<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal"><span style="color:#222222">Looks like it's the reverse actually:
</span><a href="http://stackoverflow.com/a/6297466" target="_blank">http://stackoverflow.com/a/6297 using466</a><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I might have used the word "analogous" incorrectly. I meant: similar to what Scala does, except that Scala is strict by-default and adds by-name parameters through annotations.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">-- Johan<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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