David Feuer wrote:

As far as I can tell, Haskell 2010 does not specify anything about the strictness of genericLength. Currently, it is maximally lazy. This is good, I suppose, if you want to support lists that are very long and are using floating point or some similarly broken Num instance.

The motivating example I've seen is using lazy natural numbers, data Nat = Z | S Nat instance Num Nat where ... Z + y = y S x + y = S (x + y) instance Ord Nat where compare Z Z = EQ compare Z _ = LT compare _ Z = GT compare (S x) (S y) = compare x y and then foo xs = genericLength xs < (5 :: Nat) which will evaluate no more than 5 conses of xs.

But this is not something many (any?) people have any interest in doing. As a result, the genericLength function is on a nice little list I found of Haskell functions one should never use. I therefore propose that we change it to something nice and simple, like

genericLength = foldl' 0 (\x _ -> x + 1)

Admittedly, this may not be optimal for Int8, Int16, Word8, or Word16, so we may need to use rules to rewrite these four to narrow the result of length (or some such).

That said, I'm neutral on this propsal. Cheers, Bertram

Hi, Am Samstag, den 02.08.2014, 22:02 -0400 schrieb David Feuer:

As far as I can tell, Haskell 2010 does not specify anything about the strictness of genericLength. Currently, it is maximally lazy. This is good, I suppose, if you want to support lists that are very long and are using floating point or some similarly broken Num instance.

But this is not something many (any?) people have any interest in doing. As a result, the genericLength function is on a nice little list I found of Haskell functions one should never use.

I think it’s ok to have it this way. Just because you and I don’t like broken Num instances, it doesn’t mean people who do should have to suffer. So in the interest of api stability, -1 from me here. Let’s just continue not to use genericLength in non-broken code and let playful people play. (We could improve the docs if required, of course.) Greetings, Joachim -- Joachim “nomeata” Breitner mail@joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata@joachim-breitner.de • GPG-Key: 0xF0FBF51F Debian Developer: nomeata@debian.org