'lub' and 'both' on strictness - what does it mean for products to have different arity?

[Ömer Sinan Ağacan]

> Could I ask that you add this example as a Note to the relevant functions, so
> that the next time someone asks this question they'll find the answer right
> there?

Yep, I'll do that soon.

2016-03-01 12:01 GMT-05:00 Simon Peyton Jones <simonpj at microsoft.com>:
> Omer
>
> Joachim is right.  The strictness analyser just looks inside casts, so these unexpectedly ill-typed cases could show up.  For example
>
> f :: T a -> a -> Int
> f x y = case x of
>           P1 -> case y of (a,b,c) -> a+b+c
>           P2 -> case y of (p,q) -> p+q
>
> data T a where
>   P1 :: T (Int,Int,Int)
>   P2 :: T (Int,Int)
>
> In the P1 branch we have that y::(Int,Int,Int), so we'll get a demand S(SSS).  And similarly in the P2 branch.  Now we combine them.  And there you have it.
>
>
> Could I ask that you add this example as a Note to the relevant functions, so that the next time someone asks this question they'll find the answer right there?
>
> Thanks
>
> Simon
>
> |  -----Original Message-----
> |  From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of Ömer
> |  Sinan Agacan
> |  Sent: 19 February 2016 17:27
> |  To: ghc-devs <ghc-devs at haskell.org>
> |  Subject: 'lub' and 'both' on strictness - what does it mean for
> |  products to have different arity?
> |
> |  I was looking at implementations of LUB and AND on demand signatures
> |  and I found this interesting case:
> |
> |      lubStr (SProd s1) (SProd s2)
> |          | length s1 == length s2   = mkSProd (zipWith lubArgStr s1 s2)
> |          | otherwise                = HeadStr
> |
> |  The "otherwise" case is interesting, I'd expect that to be an error.
> |  I'm trying to come up with an example, let's say I have a case
> |  expression:
> |
> |      case x of
> |         P1 -> RHS1
> |         P2 -> RHS2
> |
> |  and y is used in RHS1 with S(SS) and in RHS2 with S(SSS). This seems to
> |  me like a type error leaked into the demand analysis.
> |
> |  Same thing applies to `bothDmd` too. Funnily, it has this extra comment
> |  on this same code:
> |
> |      bothStr (SProd s1) (SProd s2)
> |          | length s1 == length s2   = mkSProd (zipWith bothArgStr s1 s2)
> |          | otherwise                = HyperStr  -- Weird
> |
> |  Does "Weird" here means "type error and/or bug" ?
> |
> |  Should I try replacing these cases with panics and try to validate?
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