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<h3>
Marge Bot pushed to branch master
at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
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<h4>
Commits:
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<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/c30cc0e9c3704b24ad0f6d9a0199bf8b5835bd40">c30cc0e9</a></strong>
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<span>by David Feuer</span>
<i>at 2020-09-02T15:55:31-04:00</i>
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<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Remove potential space leak from Data.List.transpose
Previously, `transpose` produced a list of heads
and a list of tails independently. This meant that
a function using only some heads, and only some tails,
could potentially leak space. Use `unzip` to work
around the problem by producing pairs and selector
thunks instead. Time and allocation behavior will
be worse, but there should be no more leak potential.</pre>
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<h4>1 changed file:</h4>
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<a href="#5fbb1164b5c674e4253da7e91b14facf09746bbd">
libraries/base/Data/OldList.hs
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<h4>Changes:</h4>
<li id="5fbb1164b5c674e4253da7e91b14facf09746bbd">
<a href="https://gitlab.haskell.org/ghc/ghc/-/commit/c30cc0e9c3704b24ad0f6d9a0199bf8b5835bd40#5fbb1164b5c674e4253da7e91b14facf09746bbd"><strong>libraries/base/Data/OldList.hs</strong></a>
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<td class="line_content match " style="padding-left: 0.5em; padding-right: 0.5em; color: rgba(0,0,0,0.3);" bgcolor="#fafafa">@@ -550,7 +550,13 @@ intercalate xs xss = concat (intersperse xs xss)</td>
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<pre style="margin: 0;"> <span id="LC550" class="line" lang="haskell"><span class="n" style="color: #333;">transpose</span> <span class="o" style="font-weight: 600;">::</span> <span class="p">[[</span><span class="n" style="color: #333;">a</span><span class="p">]]</span> <span class="o" style="font-weight: 600;">-></span> <span class="p">[[</span><span class="n" style="color: #333;">a</span><span class="p">]]</span></span>
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551
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<pre style="margin: 0;"> <span id="LC551" class="line" lang="haskell"><span class="n" style="color: #333;">transpose</span> <span class="kt" style="color: #458; font-weight: 600;">[]</span> <span class="o" style="font-weight: 600;">=</span> <span class="kt" style="color: #458; font-weight: 600;">[]</span></span>
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<pre style="margin: 0;"> <span id="LC552" class="line" lang="haskell"><span class="n" style="color: #333;">transpose</span> <span class="p">(</span><span class="kt" style="color: #458; font-weight: 600;">[]</span> <span class="o" style="font-weight: 600;">:</span> <span class="n" style="color: #333;">xss</span><span class="p">)</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">transpose</span> <span class="n" style="color: #333;">xss</span></span>
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<pre style="margin: 0;">-<span id="LC553" class="line" lang="haskell"><span class="n" style="color: #333;">transpose</span> <span class="p">((</span><span class="n" style="color: #333;">x</span><span class="o" style="font-weight: 600;">:</span><span class="n" style="color: #333;">xs</span><span class="p">)</span> <span class="o" style="font-weight: 600;">:</span> <span class="n" style="color: #333;">xss</span><span class="p">)</span> <span class="o" style="font-weight: 600;">=</span> <span class="p">(</span><span class="n" style="color: #333;">x</span> <span class="o" style="font-weight: 600;">:</span> <span class="p">[</span><span class="n" style="color: #333;">h</span> <span class="o" style="font-weight: 600;">|</span> <span class="p">(</span><span class="n" style="color: #333;">h</span><span class="o" style="font-weight: 600;">:</span><span class="kr" style="font-weight: 600;">_</span><span class="p">)</span> <span class="o" style="font-weight: 600;"><-</span> <span class="n" style="color: #333;">xss</span><span class="p">])</span> <span class="o" style="font-weight: 600;">:</span> <span class="n" style="color: #333;">transpose</span> <span class="p">(</span><span class="n" style="color: #333;">xs</span> <span class="o" style="font-weight: 600;">:</span> <span class="p">[</span> <span class="n" style="color: #333;">t</span> <span class="o" style="font-weight: 600;">|</span> <span class="p">(</span><span class="kr" style="font-weight: 600;">_</span><span class="o" style="font-weight: 600;">:</span><span class="n" style="color: #333;">t</span><span class="p">)</span> <span class="o" style="font-weight: 600;"><-</span> <span class="n" style="color: #333;">xss</span><span class="p">])</span></span>
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<pre style="margin: 0;">+<span id="LC553" class="line" lang="haskell"><span class="n" style="color: #333;">transpose</span> <span class="p">((</span><span class="n" style="color: #333;">x</span><span class="o" style="font-weight: 600;">:</span><span class="n" style="color: #333;">xs</span><span class="p">)</span> <span class="o" style="font-weight: 600;">:</span> <span class="n" style="color: #333;">xss</span><span class="p">)</span> <span class="o" style="font-weight: 600;">=</span> <span class="p">(</span><span class="n" style="color: #333;">x</span> <span class="o" style="font-weight: 600;">:</span> <span class="n" style="color: #333;">hds</span><span class="p">)</span> <span class="o" style="font-weight: 600;">:</span> <span class="n" style="color: #333;">transpose</span> <span class="p">(</span><span class="n" style="color: #333;">xs</span> <span class="o" style="font-weight: 600;">:</span> <span class="n" style="color: #333;">tls</span><span class="p">)</span></span>
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<pre style="margin: 0;">+<span id="LC554" class="line" lang="haskell"> <span class="kr" style="font-weight: 600;">where</span></span>
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<pre style="margin: 0;">+<span id="LC555" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- We tie the calculations of heads and tails together</span></span>
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<pre style="margin: 0;">+<span id="LC556" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- to prevent heads from leaking into tails and vice versa.</span></span>
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<pre style="margin: 0;">+<span id="LC557" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- unzip makes the selector thunk arrangements we need to</span></span>
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<pre style="margin: 0;">+<span id="LC558" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- ensure everything gets cleaned up properly.</span></span>
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<pre style="margin: 0;">+<span id="LC559" class="line" lang="haskell"> <span class="p">(</span><span class="n" style="color: #333;">hds</span><span class="p">,</span> <span class="n" style="color: #333;">tls</span><span class="p">)</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">unzip</span> <span class="p">[(</span><span class="n" style="color: #333;">hd</span><span class="p">,</span> <span class="n" style="color: #333;">tl</span><span class="p">)</span> <span class="o" style="font-weight: 600;">|</span> <span class="p">(</span><span class="n" style="color: #333;">hd</span><span class="o" style="font-weight: 600;">:</span><span class="n" style="color: #333;">tl</span><span class="p">)</span> <span class="o" style="font-weight: 600;"><-</span> <span class="n" style="color: #333;">xss</span><span class="p">]</span></span>
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<pre style="margin: 0;"> <span id="LC560" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC561" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC562" class="line" lang="haskell"><span class="cd">-- | The 'partition' function takes a predicate a list and returns</span></span>
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