document utility functions

docs/docs/lips/functional-helpers.md

@@ -4,3 +4,374 @@ description: Code that allows to do more with less
 ---
 
 # Functional and other utils
+
+LIPS Scheme provide various of utility functions. Some of them are inspired by [Ramda.js
+library](https://ramdajs.com/) and [Lodash](https://lodash.com/). Some of those functions are
+defined in [SRFI-1](https://srfi.schemers.org/srfi-1/srfi-1.html).
+
+## Curry
+
+[Curry](https://en.wikipedia.org/wiki/Currying) is a common function in functional programing that
+return a new function with predefined arguments. The classic version, returns functions that accept
+one argumnet and keep returning new function until all argumments are passed. In LIPS there is more
+useful verion of curry, that allow to pass more than one argument at the time. This is a common way
+curry is implemented. This is working in LIPS becase Scheme lambdas has length property that
+indecate number of argumnets.
+
+```scheme
+(define (sum a b c)
+  (+ a b c))
+
+(define add1 (curry sum 1))
+
+(print (add1 2 4))
+;; ==> #<procedure>
+```
+
+## Take and drop
+
+Take and drop procedures operates on lists and return a new list with:
+* `take` - only first n elements
+* `drop` - first elements removed
+
+```scheme
+(define lst '(1 2 3 4 5))
+
+(take lst 3)
+;; ==> (1 2 3)
+(drop lst 3)
+;; ==> (4 5)
+(equal? lst (append! (take lst 3) (drop lst 3)))
+;; ==> #t
+```
+
+## range
+
+This is common function from Python. LIPS Scheme version works exactly the same and return list of numbers:
+
+```scheme
+(range 10)
+;; ==> (0 1 2 3 4 5 6 7 8 9)
+(range 5 10)
+;; ==> (5 6 7 8 9)
+(range 0 10 2)
+;; ==> (0 2 4 6 8)
+```
+
+If used with one argument it returns `n` numbers starting from `0`.
+If used with two argumnets, the first one is starting number and the second one is the limit.
+When used with tree arguments, the last argumnets is a step between the result numbers.
+
+## filter
+This is a common function in JavaScript and Python. But LIPS procedure also allow accepting
+regular expression as a filter.
+
+```scheme
+(filter odd? (range 10))
+;; ==> (1 3 5 7 9)
+(filter #/0|7/ (range 10))
+;; ==> (0 7)
+```
+
+## complement
+Function return the oposite of the predicate:
+
+```scheme
+(define not-null? (complement null?))
+(not-null? #null)
+;; ==> #f
+(not-null? 10)
+;; ==> #t
+```
+
+## pluck
+This is a procedure that operate on JavaScript objects it return procedure that return specified fields from an object.
+If you pass single argumnent the function will return the value. Of you pass more than one argument it will return
+an object with only those keys provided as arguments. Arguments can be strings or symbols.
+
+```scheme
+(define jack &(:first "Jack" :last "Kirby" :age 77))
+(define stan &(:first "Stan" :last "Lee" :age 96))
+(define steve &(:first "Steve" :last "Ditko" :age 91))
+
+(define get-name (pluck "first"))
+(string-append (get-name stan)
+               " likes "
+               (get-name jack)
+               " and "
+               (get-name steve))
+;; ==> "Stan likes Jack and Steve"
+
+(define full-name-data (pluck "first" "last"))
+
+
+(map full-name-data (list jack stan))
+;; ==> (&(:first "Jack" :last "Kirby") &(:first "Stan" :last "Lee"))
+
+(define (full-name person)
+  (let ((person (full-name-data person)))
+    (--> (Object.values person) (join " "))))
+
+(map full-name (list jack stan steve))
+;; ==> ("Jack Kirby" "Stan Lee" "Steve Ditko")
+```
+
+The last example use interop with JavaScript see [Integration with
+JavaScript](/docs/lips/intro#integration-with-javascript) to know more.
+
+## flip
+This is very useful procedure that return new procedure with swaped first two arguments.
+
+```scheme
+(take '(1 2 3 4) 2)
+;; ==> (1 2)
+(define take (flip take))
+(take 2 '(1 2 3 4))
+;; ==> (1 2)
+(unset! take)
+```
+
+This function helps in composing functions with functions like `curry`. `unset!` is a function that
+removes the biding of the first appearance of the object. Here we define take inside the current
+environment and you can `unset!` it, to get the original value back.
+
+## Combinations of functions
+
+You can use curry with filter to create filter procedures:
+
+```scheme
+(define non-zero (curry filter (complement zero?)))
+
+(non-zero '(1 0 2 3 0 4 5 0 6 7 0 8 9))
+;; ==> (1 2 3 4 5 6 7 8 9)
+```
+
+Another thing you can do is curry the take procedure to get only two elemets from the list with help of flip:
+
+```scheme
+(define first-two (curry (flip take) 2))
+(first-two '(1 2 3 4))
+;; ==> (1 2)
+```
+
+## Unary, binary, and n-ary
+With LIPS you can change the arrity of the function by forcing only specific number of arguments.
+
+This is a classic error:
+
+```scheme
+(--> #("1" "2" "3" "4") (map string->number))
+;; ==> #(1 +nan.0 +nan.0 +nan.0)
+```
+
+The error happen becase Array::map pass additional argumnets not only a value, and second argument
+to `string->number` is Radix of the number (number base, default `10`). To fix the issue you can use `unary`:
+
+```scheme
+(--> #("1" "2" "3" "4") (map (unary string->number)))
+;; ==> #(1 2 3 4)
+```
+
+Binary and n-ary work similary but limit the number of arguments to 2 or any number.
+
+## always and once
+`always` is a procedure that return procedure that alwasy return same value
+
+```scheme
+(map (always 1) (range 10))
+;; ==> (1 1 1 1 1 1 1 1 1 1)
+```
+
+And `once` is a procedure that execute target procedure only once.
+
+Let's create a macro that will count how many times the code is executed:
+
+```scheme
+(define-macro (with-time label expr)
+  (let ((result #:result))
+    `(begin
+       (console.time ,label)
+       (try
+        ,expr
+        (finally
+         (console.timeEnd ,label))))))
+```
+
+**NOTE** `#:result` expression is [auto gensym](/docs/lips/extension#autogensyms) as one of
+builtin [syntax extensions](/docs/lips/extension#syntax-extensions).
+
+We can use this code to make sure that the function has been executed only once:
+
+```scheme
+(define (expensive value)
+  (new Promise (lambda (resolve)
+                 (setTimeout (curry resolve value) 1000))))
+
+(with-time "expensive" (expensive "hello"))
+;; ==> expensive: 1.003s
+;; ==> "hello"
+(define only-once (once expensive))
+
+(with-time "expensive" (only-once "message"))
+;; ==> expensive: 1.023s
+;; ==> "message"
+
+(with-time "expensive" (only-once "message"))
+;; ==> expensive: 9.609ms
+;; ==> "message"
+
+(with-time "expensive" (only-once "different message"))
+;; ==> expensive: 10.022ms
+;; ==> "message"
+```
+
+## some and every
+Work like JavaScript
+[Array::some](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/some)
+and
+[Array::every](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/every).
+But the operate on lists and return true when predicate return true of any element (`some`) or for
+all elemebts (`every`).
+
+```scheme
+(some null? '(1 2 3 #null))
+;; ==> #t
+(some null? '(1 2 3))
+;; ==> #f
+```
+
+```scheme
+(every number? '(1 2 3 4))
+;; ==> #t
+(every number? '(1 2 3 "4"))
+;; ==> #f
+```
+
+## pipe
+
+Pipe is higher ordder procedure that accept functiosn and aguments and return a new function that apply those function in order:
+
+```scheme
+(define non-zero (curry filter (complement zero?)))
+(define (fraction x)
+  (/ 1 x))
+
+(define calculate (pipe non-zero (curry map fraction)))
+
+(calculate '(1 2 0 3 4 0 5 6 0 7 8 0 9))
+;; ==> (1 1/2 1/3 1/4 1/5 1/6 1/7 1/8 1/9)
+```
+
+The code removes the zeros from list before applying the fraction that will throw an error on zero.
+
+## folding
+
+LIPS define function `reduce` that is an alias to standard Scheme procedure `fold-right` and fold that is the same
+as `fold-left`. Both procedures works similary. The take a procedure and a list and reduce it into a single value.
+
+**NOTE** the reduce works differently than in JavaScript, the callback function get accumulator in last argument.
+
+
+```scheme
+(reduce cons '() '(1 2 3 4 5))
+;; ==> (5 4 3 2 1)
+```
+
+The function also accept more than one list.
+
+```scheme
+(reduce (lambda (num str acc)
+          (cons num (cons str acc)))
+        '()
+        '(1 2 3 4 5)
+        '("foo" "bar" "baz" "quuz"))
+;; ==> (4 "quuz" 3 "baz" 2 "bar" 1 "foo")
+```
+
+`fold` function work similary but the order of execution is reversed.
+
+```scheme
+(fold cons '() '(1 2 3 4))
+;; ==> (1 2 3 4)
+```
+
+Same as reduce it accept more than one list:
+
+```scheme
+(fold (lambda (num str acc)
+          (cons num (cons str acc)))
+        '()
+        '(1 2 3 4 5)
+        '("foo" "bar" "baz" "quuz"))
+;; ==> (1 "foo" 2 "bar" 3 "baz" 4 "quuz")
+```
+
+**NOTE**: here we use `cons` to create a list, `cons` construct the list in reverse order so `reduce`
+start from first element and `fold` is reversed:
+
+```scheme
+(reduce (lambda (item acc)
+          (print acc)
+          (cons item acc))
+        '()
+        '(1 2 3 4))
+;; ==> (1)
+;; ==> (2 1)
+;; ==> (3 2 1)
+;; ==> (4 3 2 1)
+```
+
+```scheme
+(fold (lambda (item acc)
+          (print acc)
+          (cons item acc))
+        '()
+        '(1 2 3 4))
+;; ==> (4)
+;; ==> (3 4)
+;; ==> (2 3 4)
+;; ==> (1 2 3 4)
+```
+
+## unfold
+Unfold is the opposite of folding. You can use it to create a list based on single function.
+The [callback function](https://en.wikipedia.org/wiki/Callback_(computer_programming)) accept next item and should return
+#f when done or pair of two values element that should be added to the output list and next value.
+
+Here is example of a procedure that use:
+
+```scheme
+(unfold (lambda (n)
+          (if (> n 50)
+              false
+              (cons n (+ n 10))))
+        10)
+;; ==> (10 20 30 40 50)
+```
+
+And here is more complex example with a utility function:
+
+```scheme
+(define (range-between a b)
+  (let* ((inc (< a b))
+         (test (if inc > <))
+         (op (if inc + -)))
+    (unfold (lambda (x)
+                  (if (test x b)
+                      false
+                      (cons x (op x 1))))
+            a)))
+
+(range-between 10 20)
+;; ==> (10 11 12 13 14 15 16 17 18 19 20)
+```
+
+## flatten
+This procedure flattens the tree structure into flat list, similar to
+[`Array::flat` in JavaScript](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flat).
+
+```scheme
+(flatten '(((((1) 2) 3) 4) 5))
+;; ==> (1 2 3 4 5)
+```
+