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Merge pull request #202 from urbit/funcprog

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Add functional programming note.
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@ -138,3 +138,14 @@ One solution is to “dry out” the wet gate using [`++bake`](/reference/hoon/s
> ((curr (bake reel ,[(list @) _add]) add) `(list @)`[1 2 3 4 ~])
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```
## Classic Operations
Functional programmers frequently rely on three design patterns to produce operations on collections of data:
1. Map. The Map operation describes applying a function to each item of a set or iterable object, resulting in the same final number of items transformed. In Hoon terms, we would say slamming a gate on each member of a `list` or `set`. The standard library arms that accomplish this include [`++turn`](https://developers.urbit.org/reference/hoon/stdlib/2b#turn) for a `list`, [`++run:in`](https://developers.urbit.org/reference/hoon/stdlib/2h#repin) for a `set`, and [`++run:by`](https://developers.urbit.org/reference/hoon/stdlib/2i#runby) for a `map`.
2. Reduce. The Reduce operation applies a function as a sequence of pairwise operations to each item, resulting in one summary value. The standard library arms that accomplish this are [`++roll`](https://developers.urbit.org/reference/hoon/stdlib/2b#roll) and [`++reel`](https://developers.urbit.org/reference/hoon/stdlib/2b#reel) for a `list`, [`++rep:in`](https://developers.urbit.org/reference/hoon/stdlib/2h#repin) for a `set`, and [`++rep:by`](https://developers.urbit.org/reference/hoon/stdlib/2i#repby) for a `map`.
3. Filter. The Filter operation applies a true/false function to each member of a collection, resulting in some number of items equal to or fewer than the size of the original set. In Hoon, the library arms that carry this out include [`++skim`](https://developers.urbit.org/reference/hoon/stdlib/2b#skim), [`++skid`](https://developers.urbit.org/reference/hoon/stdlib/2b#skid), [`++murn`](https://developers.urbit.org/reference/hoon/stdlib/2b#murn) for a `list`, and [`++rib:by`](https://developers.urbit.org/reference/hoon/stdlib/2i#ribby) for a `map`.