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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 A very common and essential operation in all programming is looping. Most
languages support looping of some kind, either with explicit loops or recursion. 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 useful in many cases, sometimes is more convenient to use a explicit loop to
iterate over a collection like an array. iterate over a collection like an array.
## An Example - Iterating a Range ## 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 0 through 9. There are many ways to carry out this explicit calculation
even with taking shortcuts. A succinct way in janet is 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)) (+ ;(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. 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 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)) (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))
```