Version 1 (modified by simonpj, 6 years ago) (diff)

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# Seq magic

The innocent-looking `seq` operator causes all manner of mayhem in GHC. This page summarises the issues.

## The baseline position

Our initial story was that `(seq e1 e2)` meant precisely

```   case e1 of { _ -> e2 }
```

Indeed this was `seq`'s inlining. This translation validates some important rules

``` * `seq` is strict in both its arguments
* (e1 `seq` e2) e3            ===>   e1 `seq` (e2 e3)
* case (e1 `seq` e2) of alts  ===>   e1 `seq` (case e2 of alts)
* value `seq` e               ===>   e
```

## Problems

Here are some of the problems that showed up. See `Note [Deguaring seq]` in `DsUtils`.

Trac #1031. Consider

```   f x y = x `seq` (y `seq` (# x,y #))
```

The `[CoreSyn let/app invariant]` means that, other things being equal, because the argument to the outer `seq` has an unlifted type, we'll use call-by-value thus:

f x y = case (y `seq` (# x,y #)) of v -> x `seq` v

But that is bad for two reasons:

(a) we now evaluate y before x, and (b) we can't bind v to an unboxed pair

Seq is very, very special! So we recognise it right here, and desugar to

```        case x of _ -> case y of _ -> (# x,y #)
```

Note [Desugaring seq (2)] cf Trac #2273 ~ Consider

let chp = case b of { True -> fst x; False -> 0 } in chp `seq` ...chp...

Here the seq is designed to plug the space leak of retaining (snd x) for too long.

If we rely on the ordinary inlining of seq, we'll get

let chp = case b of { True -> fst x; False -> 0 } case chp of _ { I# -> ...chp... }

But since chp is cheap, and the case is an alluring contet, we'll inline chp into the case scrutinee. Now there is only one use of chp, so we'll inline a second copy. Alas, we've now ruined the purpose of the seq, by re-introducing the space leak:

case (case b of {True -> fst x; False -> 0}) of

I# _ -> ...case b of {True -> fst x; False -> 0}...

We can try to avoid doing this by ensuring that the binder-swap in the case happens, so we get his at an early stage:

case chp of chp2 { I# -> ...chp2... }

But this is fragile. The real culprit is the source program. Perhaps we should have said explicitly

let !chp2 = chp in ...chp2...

But that's painful. So the code here does a little hack to make seq more robust: a saturated application of 'seq' is turned *directly* into the case expression, thus:

x `seq` e2 ==> case x of x -> e2 -- Note shadowing! e1 `seq` e2 ==> case x of _ -> e2

So we desugar our example to:

let chp = case b of { True -> fst x; False -> 0 } case chp of chp { I# -> ...chp... }

And now all is well.

The reason it's a hack is because if you define mySeq=seq, the hack won't work on mySeq.

Note [Desugaring seq (3)] cf Trac #2409 ~ The isLocalId ensures that we don't turn

True `seq` e

into

case True of True { ... }

which stupidly tries to bind the datacon 'True'.

### The need for special rules

Roman found situations where he had

```      case (f n) of _ -> e
```

where he knew that `f` (which was strict in `n`) would terminate if n did. Notice that the result of `(f n)` is discarded. So it makes sense to transform to

```      case n of _ -> e
```

Rather than attempt some general analysis to support this, I've added enough support that you can do this using a rewrite rule:

```  RULE "f/seq" forall n.  seq (f n) e = seq n e
```

You write that rule. When GHC sees a case expression that discards its result, it mentally transforms it to a call to `seq` and looks for a RULE. (This is done in `Simplify.rebuildCase`.) As usual, the correctness of the rule is up to you.

To make this work, we need to be careful that `seq` is not desguared into a case expression on the LHS of a rule.

To increase applicability of these user-defined rules, we also have the following built-in rule for `seq`

```  seq (x |> co) y = seq x y
```

This eliminates unnecessary casts and also allows other seq rules to match more often. Notably,

```   seq (f x |> co) y  -->  seq (f x) y
```

and now a user-defined rule for `seq` may fire.