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

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 :

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@haskell.org] On Behalf Of Ömer | Sinan Agacan | Sent: 19 February 2016 17:27 | To: ghc-devs | 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? | _______________________________________________ | ghc-devs mailing list | ghc-devs@haskell.org | https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fmail.ha | skell.org%2fcgi-bin%2fmailman%2flistinfo%2fghc- | devs&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7cdd8ae326a8e14ef8 | 9e8608d339520cb9%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=sRFRKgj3y | zQdEZT4y7KLk18cP43Rv1J%2bx8oPZyr1QzA%3d