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+ <title>The GHC Commentary - Cunning Prelude Code</title>
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+ <h1>The GHC Commentary - Cunning Prelude Code</h1>
+ <p>
+ GHC's uses a many optimsations and GHC specific techniques (unboxed
+ values, RULES pragmas, and so on) to make the heavily used Prelude code
+ as fast as possible.
+
+ <hr>
+ <h4>Par, seq, and lazy</h4>
+
+ In GHC.Conc you will dinf
+<blockquote><pre>
+ pseq a b = a `seq` lazy b
+</pre></blockquote>
+ What's this "lazy" thing. Well, <tt>pseq</tt> is a <tt>seq</tt> for a parallel setting.
+ We really mean "evaluate a, then b". But if the strictness analyser sees that pseq is strict
+ in b, then b might be evaluated <em>before</em> a, which is all wrong.
+<p>
+Solution: wrap the 'b' in a call to <tt>GHC.Base.lazy</tt>. This function is just the identity function,
+except that it's put into the built-in environment in MkId.lhs. That is, the MkId.lhs defn over-rides the
+inlining and strictness information that comes in from GHC.Base.hi. And that makes <tt>lazy</tt> look
+lazy, and have no inlining. So the strictness analyser gets no traction.
+<p>
+In the worker/wrapper phase, after strictness analysis, <tt>lazy</tt> is "manually" inlined (see WorkWrap.lhs),
+so we get all the efficiency back.
+<p>
+This supersedes an earlier scheme involving an even grosser hack in which par# and seq# returned an
+Int#. Now there is no seq# operator at all.
+
+
+ <hr>
+ <h4>fold/build</h4>
+ <p>
+ There is a lot of magic in <a
+ href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/lib/std/PrelBase.lhs"><code>PrelBase.lhs</code></a> -
+ among other things, the <a
+ href="http://haskell.cs.yale.edu/ghc/docs/latest/set/rewrite-rules.html">RULES
+ pragmas</a> implementing the <a
+ href="http://research.microsoft.com/Users/simonpj/Papers/deforestation-short-cut.ps.Z">fold/build</a>
+ optimisation. The code for <code>map</code> is
+ a good example for how it all works. In the prelude code for version
+ 5.03 it reads as follows:
+ <blockquote><pre>
+map :: (a -> b) -> [a] -> [b]
+map _ [] = []
+map f (x:xs) = f x : map f xs
+
+-- Note eta expanded
+mapFB :: (elt -> lst -> lst) -> (a -> elt) -> a -> lst -> lst
+{-# INLINE [0] mapFB #-}
+mapFB c f x ys = c (f x) ys
+
+{-# RULES
+"map" [~1] forall f xs. map f xs = build (\c n -> foldr (mapFB c f) n xs)
+"mapList" [1] forall f. foldr (mapFB (:) f) [] = map f
+"mapFB" forall c f g. mapFB (mapFB c f) g = mapFB c (f.g)
+ #-}</pre>
+ </blockquote>
+ <p>
+ Up to (but not including) phase 1, we use the <code>"map"</code> rule to
+ rewrite all saturated applications of <code>map</code> with its
+ build/fold form, hoping for fusion to happen. In phase 1 and 0, we
+ switch off that rule, inline build, and switch on the
+ <code>"mapList"</code> rule, which rewrites the foldr/mapFB thing back
+ into plain map.
+ <p>
+ It's important that these two rules aren't both active at once
+ (along with build's unfolding) else we'd get an infinite loop
+ in the rules. Hence the activation control using explicit phase numbers.
+ <p>
+ The "mapFB" rule optimises compositions of map.
+ <p>
+ The mechanism as described above is new in 5.03 since January 2002,
+ where the <code>[~</code><i>N</i><code>]</code> syntax for phase number
+ annotations at rules was introduced. Before that the whole arrangement
+ was more complicated, as the corresponding prelude code for version
+ 4.08.1 shows:
+ <blockquote><pre>
+map :: (a -> b) -> [a] -> [b]
+map = mapList
+
+-- Note eta expanded
+mapFB :: (elt -> lst -> lst) -> (a -> elt) -> a -> lst -> lst
+mapFB c f x ys = c (f x) ys
+
+mapList :: (a -> b) -> [a] -> [b]
+mapList _ [] = []
+mapList f (x:xs) = f x : mapList f xs
+
+{-# RULES
+"map" forall f xs. map f xs = build (\c n -> foldr (mapFB c f) n xs)
+"mapFB" forall c f g. mapFB (mapFB c f) g = mapFB c (f.g)
+"mapList" forall f. foldr (mapFB (:) f) [] = mapList f
+ #-}</pre>
+ </blockquote>
+ <p>
+ This code is structured as it is, because the "map" rule first
+ <em>breaks</em> the map <em>open,</em> which exposes it to the various
+ foldr/build rules, and if no foldr/build rule matches, the "mapList"
+ rule <em>closes</em> it again in a later phase of optimisation - after
+ build was inlined. As a consequence, the whole thing depends a bit on
+ the timing of the various optimsations (the map might be closed again
+ before any of the foldr/build rules fires). To make the timing
+ deterministic, <code>build</code> gets a <code>{-# INLINE 2 build
+ #-}</code> pragma, which delays <code>build</code>'s inlining, and thus,
+ the closing of the map. [NB: Phase numbering was forward at that time.]
+
+ <p><small>
+<!-- hhmts start -->
+Last modified: Mon Feb 11 20:00:49 EST 2002
+<!-- hhmts end -->
+ </small>
+ </body>
+</html>
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