Mark partial functions as such

[David Feuer]

Why isn't the call to error in div what you mean?

On Fri, Aug 31, 2018, 10:49 AM Richard Eisenberg <rae at cs.brynmawr.edu>
wrote:

> In a response not cc'd to the list, a contributor (not sure if they want
> public identification) suggests:
>
> > I think we want something like "partial even given input you can
> successfully DeepSeq"
>
> That's the specification of the feature I'm after. I think all the
> commentary about infinite lists, etc., would lead also to good
> documentation additions. (For example, it would be fantastic if every
> function precisely documented its strictness, preferably with some standard
> notation, but this is not the problem I'm trying to solve here.)
>
> Also, it was suggested that the documentation be checked -- that is, we
> could imagine a {-# TOTAL ... #-} or {-# PARTIAL ... #-} pragma that GHC
> could check on compilation and Haddock could include in the documentation.
> This would also be great, but much more than I'm proposing here.
>
> Maybe here's a concrete example:
>
> > -- | /Contains a call to 'error'./ Extract the first element of a list,
> which must be non-empty.
> > head                    :: [a] -> a
>
> In the end, it's the call to error that I want noted. Of course, having a
> similar note on functions like div (where the problem isn't a call to
> error) and length (that will loop on infinite lists) is good, but not
> really what I'm proposing here.
>
> Thanks,
> Richard
>
> > On Aug 30, 2018, at 8:10 PM, Richard Eisenberg <rae at cs.brynmawr.edu>
> 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
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