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

[Simon Peyton Jones]

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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