Re: Mark partial functions as such

I feel like there's a difference between partial functions in the sense of length and partial functions in the sense of head. If you have a partiality monad like the free monad over Identity: data Partial a = Done a | NotYet (Partial a) instance Monad Partial where return = Done Done a >>= f = f a NotYet m >>= f = NotYet $ m >>= f then length has a sensible and productive implementation in terms of it: partialLength :: [a] -> Partial Int partialLength = go 0 where go n ls = seq n $ case ls of [] -> Done n _:ls' -> NotYet $ go (n + 1) ls' This is similar to the mechanism that Agda and Idris use to denote a potentially non-terminating result; with it, these languages are Turing complete. head and tail aren't like that, and should be marked differently in the documentation. On Fri, Aug 31, 2018 at 7:28 AM, Andrew Martin wrote:

I'm strongly +1 on this.

On Thu, Aug 30, 2018 at 8:10 PM Richard Eisenberg wrote:

...

Proposal: Mark partial functions in `base` as partial

Motivation: I'm about to teach Haskell to a classful of beginners. In my experience, they will soon reach for functions like `head` and `tail`, because pattern-matching is foreign to them. I would love just to be able to say "Don't use partial functions", but many students will not easily be able to tell partial functions from total ones.

I do expect this problem to work itself out rather quickly, and then students will be able to identify partial functions, but loudly marking partial functions as partial seems like a small service to everyone and a bigger one to newbies. I don't see any downsides.

Thoughts?

Thanks, Richard _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

-- -Andrew Thaddeus Martin

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