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Why don't all collection functions exist on all the collection types?
f#compiler

I recently shared a f# collection function cheat sheet that revels what functions are on what collection.

Turns out its the lack of a function on a collection type that's the more interesting question.

[link:dl.dropbox.com]

For example, it make sense to me for the Seq.foldback not to exist since Seq's can't go backwards.

But the List.foldback and List.scanback do exist. I read that the f# List type was a single forward linked design so shouldn't these functions also not be available?

I suspect that either the compiler is inserting a List.rev command before the List.foldback or the List type is really has a duel forward and back linked structure.

If it's the List.rev answer, then why wasn't the same trick applied to Seq module?

Another weird one is the fact that the groupby function only exists for Seq's. Seems too useful to have it on only Seq's.

Anywho I hope the cheetsheet is useful for you.

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You could ask the opposite question -- if the fn's are available for Seq's, why even put them anywhere else when you could treat the DS as a seq? I'd assume that answer is notational convenience, and in some cases efficiency, when the "shape" of the DS dictates it, or for some other reasons.

As for Seq vs. List operations - a Seq can be infinite and is often computed lazily, while lists are finite and concrete - these reasons make it a poor choice to actualize into a concrete structure that can be operated on in reverse.

You could argue that lists shouldn't have an indexing operator because it's O(n) and not O(1) like arrays, but again, it's for convenience and it's up to the programmer to assume this is used on small lists, outside of loops, etc.

By on 11/15/2010 5:45 PM ()

Thank you.

I forgot about the Seq lazy infinite angle. That now makes perfect sense to me, which also explains why Seq.rev doesn't exist either.

When I was first learning F#, I did wonder why I had to place the collection type before the function.(like List.map and Seq.map). Certainly the function could be overloaded so the compiler could call the right one. Now I believe it was done that way to help the type inference engine and to make you think are you using the correct data structure for the problem at hand.

Thanks again.

By on 11/15/2010 6:24 PM ()

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