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<p class="MsoNormal"><span style="font-size:12.0pt">Johan<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Consider this with -XStrict<o:p></o:p></span></p>
<p class="Code">f y = let Just x = blah[y] in body[y,x]<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Suppose that in a call to f, <o:p>
</o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l1 level1 lfo1"><![if !supportLists]><span style="font-size:12.0pt;font-family:Symbol"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:12.0pt">blah returns Nothing<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l1 level1 lfo1"><![if !supportLists]><span style="font-size:12.0pt;font-family:Symbol"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:12.0pt">but body does not use x<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Should f succeed? For sure, blah will be evaluated to HNF before body is started, but is the match against Just done strictly too?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">According to our current semantics, in the match against Just is not done strictly, so the call should succeed. I think that’s unexpected and probably wrong.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Here’s the semantics <a href="http://downloads.haskell.org/~ghc/master/users-guide/glasgow_exts.html#recursive-and-polymorphic-let-bindings">
http://downloads.haskell.org/~ghc/master/users-guide/glasgow_exts.html#recursive-and-polymorphic-let-bindings</a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">The translation for <o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> !(Just x) = blah<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo2"><![if !supportLists]><span style="font-size:12.0pt;font-family:Wingdings"><span style="mso-list:Ignore">ð<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:12.0pt">(FORCE) v = blah; Just x = v (and add a seq on v)<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo2"><![if !supportLists]><span style="font-size:12.0pt;font-family:Wingdings"><span style="mso-list:Ignore">ð<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:12.0pt">(SPLIT) v = blah; x = case v of Just x -> x<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">So we finish up with<o:p></o:p></span></p>
<p class="Code">f y = let v = blah[y] in<o:p></o:p></p>
<p class="Code"> let x = case v of Just x -> x in<o:p></o:p></p>
<p class="Code"> v `seq` body[y,x]<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt">I don’t think that’s what you intended.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">If the pattern can fail, I think we want the FORCE step to say this:<o:p></o:p></span></p>
<p class="last" style="margin-left:36.0pt">Replace any binding <span class="pre">
<span style="font-size:10.0pt;font-family:"Courier New"">!p</span></span><code><span style="font-size:10.0pt">
</span></code><span class="pre"><span style="font-size:10.0pt;font-family:"Courier New"">=</span></span><code><span style="font-size:10.0pt">
</span></code><span class="pre"><span style="font-size:10.0pt;font-family:"Courier New"">e</span></span> with
<o:p></o:p></p>
<p class="last" style="margin-left:36.0pt"><span class="pre"><span style="font-size:10.0pt;font-family:"Courier New"">v</span></span><code><span style="font-size:10.0pt">
</span></code><span class="pre"><span style="font-size:10.0pt;font-family:"Courier New"">=</span></span><code><span style="font-size:10.0pt"> case
</span></code><span class="pre"><span style="font-size:10.0pt;font-family:"Courier New"">e of p -> (v1,..,vn);</span></span><code><span style="font-size:10.0pt">
</span></code><span class="pre"><span style="font-size:10.0pt;font-family:"Courier New"">(v1,..,vn)</span></span><code><span style="font-size:10.0pt">
</span></code><span class="pre"><span style="font-size:10.0pt;font-family:"Courier New"">=</span></span><code><span style="font-size:10.0pt">
</span></code><span class="pre"><span style="font-size:10.0pt;font-family:"Courier New"">v</span></span>
<o:p></o:p></p>
<p class="last" style="margin-left:36.0pt">and replace <span class="pre"><span style="font-size:10.0pt;font-family:"Courier New"">e0</span></span> with
<span class="pre"><span style="font-size:10.0pt;font-family:"Courier New"">v</span></span><code><span style="font-size:10.0pt">
</span></code><span class="pre"><span style="font-size:10.0pt;font-family:"Courier New"">seq</span></span><code><span style="font-size:10.0pt">
</span></code><span class="pre"><span style="font-size:10.0pt;font-family:"Courier New"">e0</span></span>, where
<span class="pre"><span style="font-size:10.0pt;font-family:"Courier New"">v</span></span> is fresh and v1..vn are the variable(s) bound by p<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt">(Compare with the text at the above link.)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Do you agree?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
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