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# Seq magic
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The innocent-looking `seq` operator causes all manner of mayhem in GHC. This page summarises the issues.
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## The baseline position
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Our initial story was that `(seq e1 e2)` meant precisely
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```wiki
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case e1 of { _ -> e2 }
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```
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Indeed this was `seq`'s inlining. This translation validates some important rules
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```wiki
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* `seq` is strict in both its arguments
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* (e1 `seq` e2) e3 ===> e1 `seq` (e2 e3)
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* case (e1 `seq` e2) of alts ===> e1 `seq` (case e2 of alts)
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* value `seq` e ===> e
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```
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## Problems
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Here are some of the problems that showed up. See `Note [Deguaring seq]` in `DsUtils`.
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Trac [\#1031](https://gitlab.haskell.org//ghc/ghc/issues/1031). Consider
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```wiki
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f x y = x `seq` (y `seq` (# x,y #))
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```
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The `[CoreSyn let/app invariant]` means that, other things being equal, because
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the argument to the outer `seq` has an unlifted type, we'll use call-by-value thus:
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>
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> f x y = case (y `seq` (\# x,y \#)) of v -\> x `seq` v
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But that is bad for two reasons:
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>
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> (a) we now evaluate y before x, and
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> (b) we can't bind v to an unboxed pair
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Seq is very, very special! So we recognise it right here, and desugar to
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```wiki
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case x of _ -> case y of _ -> (# x,y #)
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```
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Note \[Desugaring seq (2)\] cf Trac [\#2273](https://gitlab.haskell.org//ghc/ghc/issues/2273)~~~~~~~~~~~~~~~~~~~~~~~~\~
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Consider
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>
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> let chp = case b of { True -\> fst x; False -\> 0 }
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> in chp `seq` ...chp...
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Here the seq is designed to plug the space leak of retaining (snd x)
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for too long.
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If we rely on the ordinary inlining of seq, we'll get
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>
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> let chp = case b of { True -\> fst x; False -\> 0 }
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> case chp of _ { I\# -\> ...chp... }
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But since chp is cheap, and the case is an alluring contet, we'll
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inline chp into the case scrutinee. Now there is only one use of chp,
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so we'll inline a second copy. Alas, we've now ruined the purpose of
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the seq, by re-introducing the space leak:
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>
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> case (case b of {True -\> fst x; False -\> 0}) of
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>
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> >
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> > I\# _ -\> ...case b of {True -\> fst x; False -\> 0}...
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We can try to avoid doing this by ensuring that the binder-swap in the
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case happens, so we get his at an early stage:
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>
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> case chp of chp2 { I\# -\> ...chp2... }
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But this is fragile. The real culprit is the source program. Perhaps we
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should have said explicitly
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>
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> let !chp2 = chp in ...chp2...
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But that's painful. So the code here does a little hack to make seq
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more robust: a saturated application of 'seq' is turned \*directly\* into
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the case expression, thus:
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>
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> x `seq` e2 ==\> case x of x -\> e2 -- Note shadowing!
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> e1 `seq` e2 ==\> case x of _ -\> e2
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So we desugar our example to:
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>
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> let chp = case b of { True -\> fst x; False -\> 0 }
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> case chp of chp { I\# -\> ...chp... }
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And now all is well.
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The reason it's a hack is because if you define mySeq=seq, the hack
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won't work on mySeq.
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Note \[Desugaring seq (3)\] cf Trac [\#2409](https://gitlab.haskell.org//ghc/ghc/issues/2409)~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\~
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The isLocalId ensures that we don't turn
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~~
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>
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> True `seq` e
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into
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>
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> case True of True { ... }
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which stupidly tries to bind the datacon 'True'.
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### The need for special rules
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Roman found situations where he had
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```wiki
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case (f n) of _ -> e
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```
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where he knew that `f` (which was strict in `n`) would terminate if n did.
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Notice that the result of `(f n)` is discarded. So it makes sense to
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transform to
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```wiki
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case n of _ -> e
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```
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Rather than attempt some general analysis to support this, I've added
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enough support that you can do this using a rewrite rule:
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```wiki
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RULE "f/seq" forall n. seq (f n) e = seq n e
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```
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You write that rule. When GHC sees a case expression that discards
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its result, it mentally transforms it to a call to `seq` and looks for
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a RULE. (This is done in `Simplify.rebuildCase`.) As usual, the
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correctness of the rule is up to you.
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To make this work, we need to be careful that `seq` is **not** desguared
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into a case expression on the LHS of a rule.
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To increase applicability of these user-defined rules, we also have the following built-in rule for `seq`
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```wiki
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seq (x |> co) y = seq x y
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```
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This eliminates unnecessary casts and also allows other seq rules to
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match more often. Notably,
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```wiki
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seq (f x |> co) y --> seq (f x) y
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```
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and now a user-defined rule for `seq` may fire. |