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Update loop documentation.

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Calvin Rose 2018-12-29 17:42:44 -05:00
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doc/Loop.md
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@ -2,13 +2,13 @@
A very common and essential operation in all programming is looping. Most
languages support looping of some kind, either with explicit loops or recursion.
Janet supports both recursion and a primitve `while` loop. While recursion is
Janet supports both recursion and a primitive `while` loop. While recursion is
useful in many cases, sometimes is more convenient to use a explicit loop to
iterate over a collection like an array.
## An Example - Iterating a Range
Supose you want to calculate the sum of the first 10 natural numbers
Suppose you want to calculate the sum of the first 10 natural numbers
0 through 9. There are many ways to carry out this explicit calculation
even with taking shortcuts. A succinct way in janet is
@ -16,7 +16,7 @@ even with taking shortcuts. A succinct way in janet is
(+ ;(range 10))
```
We will limit ourselves however to using explicit looping and no funcitions
We will limit ourselves however to using explicit looping and no functions
like `(range n)` which generate a list of natural numbers for us.
For our first version, we will use only the while macro to iterate, similar
@ -143,6 +143,32 @@ As before, we can evaluate this loop using only a while loop and the `next` func
(set key (next alphabook key))
```
For ou
However, we can do better than this with the loop macro using the `:pairs` or `:keys` verbs.
```
(loop [[letter word] :pairs alphabook]
(print letter " is for " word))
```
Using the `:keys` verb and the dot syntax for indexing
```
(loop [letter :keys alphabook]
(print letter " is for " alphabook.letter))
```
The symbol `alphabook.letter` is shorthand for `(get alphabook letter)`.
Note that the dot syntax of `alphabook.letter` is different than in many languages. In C or
ALGOL like languages, it is more akin to the indexing operator, and would be written `alphabook[letter]`.
The `.` character is part of the symbol and is recognized by the compiler.
We can also use the core library functions `keys` and `pairs` to get arrays of the keys and
pairs respectively of the alphabook.
```
(loop [[letter word] :in (pairs alphabook)]
(print letter " is for " word))
(loop [letter :in (keys alphabook)]
(print letter " is for " alphabook.letter))
```