Re: Names for small functions: just say no... Re: Data.List.join

On Thu, 9 Nov 2006 17:27:39 +0000 you wrote:

Jon Fairbairn wrote:

...

Then there's the specific question of whether intercalate is actually a better abstraction than intersperse. "concat . intersperse" is hardly difficult to read,

I have found over the years, that pretty-much every time I write "intersperse" in a program, I get a type error. It almost always turns out that I really wanted "concat . intersperse", and so I write that instead.

That is certainly an interesting datapoint. Unfortunately I don't believe I've ever made that particular mistake, so we cancel each other out ;-). But it raises a point about collecting data for this kind of argument. What we want is a history of the mistakes people made; /then/ we could argue from searches in such histories which constructs were more error-prone. Until you wrote this I was thinking that perhaps a search of version-control histories would do, but since people don't (I hope!) check in code that doesn't compile, your sort of problem goes unrecorded.

So perhaps this just demonstrates that I have the wrong idea about what intersperse is supposed to do. But the very fact that I keep making the same mistake suggests that there is indeed a natural and intuitive function hiding there, and I keep reaching for it, but with the wrong name.

I suspect what's natural and intuitive differs from person to person too much for us to make progress on that front without the sort of data that I say above is hard to get, which is why I'd prefer to see arguments about mathematical properties. Largely pointless rumination follows. One of the properties is the type; I can immediately think of three types that the "intuitive" function might have: intersperse :: a -> [a] -> [a] inter_concat :: [a] -> [a] -> [a] -- not the best name? intercalate :: [a] -> [[a]] -> [a] from that I should prefer intersperse on the grounds that it has a simpler type, but when I tried writing each in terms of the other, I found that the definitions of intersperse and intercalate in terms of inter_concat were both nice and obvious. Defining inter_concat in terms of intersperse is a bit messy, and inter_concat in terms of intercalate only slightly less so, which bend me towards the belief that inter_concat is the most primitive! The only conclusion I can really draw from this is that the argument that wins for me is still: intersperse got there first, it may not be ideal, but the grounds for adding another are too weak. Jón -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk 31 Chalmers Road jf@cl.cam.ac.uk Cambridge CB1 3SZ +44 1223 570179 (after 14:00 only, please!)