k-language

npm i '@fraczak/k'

From javascript:

import k from "@fraczak/k"
    
const fn = k.compile(`   -- complete "pure" k-program
  $ nat = < {} o, nat i >;     -- unary natural number encoding
  succ = {() i} $nat;          -- increment relation definition
  succ succ succ               -- +3 (in the above encoding)
`);
const three = {i:{i:{i:{o:{}}}}};
const six = fn(three);
console.log(JSON.stringify( six ));
// {"i":{"i":{"i":{"i":{"i":{"i":{"o":{}}}}}}}}

Working with REPL:

---

k - the way of building and manipulating JSON-like data

Technically, k is a notation for defining first-order partial functions.

An example of a partial function is the projection, e.g., ".toto", which maps an object to its property named toto, or it is not defined if the property doesn't exist. E.g.,

1   .toto :
2        {"toto": 5, "titi": 10}  --> 5
3        {"titi": 10}             ... undefined // it is not a value!

Note: The above 3 lines should be read as follows. A k-expression is printed in the first line (before ":"). The following lines are examples of the function defined by the k-expression applayed to JSON values (first part of each line). If the function for the value is defined, then the result is printed after "-->" (line 2 in the above example). If the function is not defined for the value, then "... undefined" is printed (line 3).

---

Combining "partial functions"

There are three ways of combining functions:

1. composition: (f1 f2 ...), e.g. (.toto .titi) extracts nested field.

       (.toto .titi) :
           {"toto": {"titi": 10}}   --> 10
           {"toto": {"other": 8}}   ... undefined
           {"other": {}}            ... undefined
   

2. merge: < f1, f2,... >, e.g., <.toto, .titi> extracts field toto if present; otherwise extracts titi.

       < .x.y, .z.y > :
           {"x":{"y": 5}, "z": {"y":10}}  --> 5
           {"x":{"o":{}}, "z": {"y":10}}  --> 10
           {"x":{"o":{}}, "z": {"o":{}}}  ... undefined
   

3. product: { f1 label1, f2 label2, ...}, e.g., {.toto TOTO, .titi TITI} extracts two fields and builds a record out of them.

       {.toto TOTO, .titi TITI} :
           {"toto": 5, "titi": 10, "x": 3}  --> {"TOTO": 5, "TITI": 10}
   

---

QUIZ: What is:

• empty composition: () ?
• empty merge: <> ?
• empty product: {} ?
• {{{{} s} s} s} ?
• ({{{() a} b} c} .c .b .a) ?

---

Syntactic sugar

• parenthesis can be omitted, except for the empty composition (),
• dot (.) in "projection" acts as a separator so the space around it can be omitted.

For example, (.toto .titi (.0 .1)) can be written as .toto.titi.0.1.

Comments can be introduced by //, --, %, or # and extends to the end of line. Multiline C-like comments, /* ... */, are also supported.

Basic extensions

Constants, i.e., literals for string, int, bool, and null

A constant defines a function which ignores its argument and produces the constant value. E.g.:

{123 int, "kScript" str, true bool, null null} :
    "any"  --> {"int":123,"str":"kScript","bool":true,"null":null}

Those values, i.e., strings, integers, booleans, and null, admit the projection, via (.), to the canonical string representation of the value, e.g.:

."a string" :
    "a string" --> {}
    "other"    ... undefined
    11         ... undefined
    null       ... undefined
.2 :
    2          --> {}
    "2"        --> {}
    4          ... undefined
    true       ... undefined

.true :
    true   --> {}
    "true" --> {}
    4      ... undefined
    "toto" ... undefined

.null :
    null   --> {}
    "null" --> {}
    4      ... undefined
    false  ... undefined

Vector product

Vector product can be seen as an abbreviation for product whose all fields have the same type, and the filed names are integers starting from zero.
E.g., {.toto 0, .titi 1, 123 2} can be written as
[.toto, .titi, 123].

[.toto, .titi, 12] :
    {"toto": 5, "titi": 10 }  --> [5, 10, 12] 

. 2 :
    ["A","B","C"]  --> "C"
    ["a"]          ... undefined

---

Pragmatic extensions, aka "standard library"

• GT -- identity for lists of decreasing elements; undefined otherwise

  GT:
    [4,3]     --> [4,3]
    [3,4]     ... undefined
    []        --> []
    [4,3,0]   --> [4,3,0]
  

• EQ -- identity for lists of equal elements; undefined otherwise

  EQ:     
    [4,4]     --> [4,4]
    [4,5]     ... undefined
    [4,4,4]   --> [4,4,4]
    []        --> []
  

• PLUS and TIMES -- sum and product of lists of numbers

  {PLUS plus, TIMES times} :
    [1,2]     --> {"plus":3,"times":2}
    [2,2,2]   --> {"plus":6,"times":8}
    []        --> {"plus":0,"times":1}
  

• CONCAT -- concatenation of lists of strings

  CONCAT:
    ["a","bc","d"] --> "abcd"
    []             --> ""
  

• toJSON and fromJSON -- conversion to and from JSON strings

  toJSON:
    {"a": 12} --> "{\"a\":12}"
  
  fromJSON:   
    "{\"a\":12}"       --> {"a":12}
  

• other predefined parial functions are: DIV, FDIV, CONS, SNOC, toVEC, toDateMsec, toDateStr, and _log!.

---

Function and code (i.e., type) definitions

  dec = [(),-1] PLUS;
  max = <SNOC [.car, .cdr max] <GT.0, .1>, .0> ;
  factorial = < [(),0] GT .0 [dec factorial, ()] TIMES, 1 >;
  6 factorial
--> 720

Codes (schemas, i.e., types)

Codes can be defined by tagged union and product. E.g.:

  $ nat = < nat 1, {} 0 >;
  $ pair = { nat x, nat y };

  suc = { $ nat 1 };
  add = $ pair <{.x.1 x, .y suc y} add, .y>;

Code definition statement starts with $. Also, any code expression occurrig within a k-expression is prefixed by $.

Two codes are considered equal when they are isomorphic (preserving union/product and field names). E.g., $pair, ${nat y, nat x}, and ${<nat 1, {} 0> x, nat y} are all equivalent and corresponds its cannonical form $C0={C1"x",C1"y"};$C1=<C2"0",C1"1">;$C2={};.

Try in k-repl:

> $ nat = < nat 1, {} 0 >; \
  $ pair = { nat x, nat y };
--> {}
> --C pair
 $ IxLVRLECv = {BADJOX x, BADJOX y}; -- $C0={C1"x",C1"y"};$C1=<C2"0",C1"1">;$C2={};
$ myCode = {<nat 1, {} 0> x, nat y};
--> {}
> --C myCode
 $ IxLVRLECv = {BADJOX x, BADJOX y}; -- $C0={C1"x",C1"y"};$C1=<C2"0",C1"1">;$C2={};

Basic extension codes

Since basic extension introduces integers, booleans, and strings, there are three predefined types: int, bool, and string. A vector product code can also be defined by [ codeExp ]. All members of the vector are the same code. E.g.,

  $ intVector = [ int ];
  $ boolVector = [ bool ];
  $ tree = [ tree ];

Code derivation and Patterns

Intuitively, a pattern represents some set of contraints on codes. For example, expression .toto is a projection from a value of a product or a union code with field toto. Therefore, expression .toto introduces two patterns p_i and p_o (for input and output codes, respectively); p_o imposes no constraint on code, however p_i is a product or union code with field toto leading to a code fulfilling p_o, which will be denoted by:

> toto = .toto;
--> {}
> --R toto
  toto : ?(X0 toto, ...)  -->  ?X0
  toto = .toto;

In a more complex expression each (occurrence of) subexpression will introduce some new patterns.
For example, concider rel = <.toto, ()>;.

    <    .toto    ,    ()    >
 p1   p2       p3   p4    p5   p6

where p1 is the pattern for the input code of the whole expression, and p6 is the pattern for the output. We can deduce that patterns (p1, p2, p4) defines the same code, as well as (p6, p5, p3), because union combines partial functions with the same input and output codes. Identity, (), implies that (p4, p5) define the same code. In k-repl:

> rel = < .toto, () >;
--> {}
> --R rel
  rel : ?(X0 toto, ...)=X0  -->  ?X0
  rel = <.toto, ()>;

For a given kScript, code derivation (as any static analysis) can fail, indicating that the script is invalid. In some other cases the code derivation can succeed even to the point of reducig every pattern to a single code making the program fully annotated by codes.

The language k supports type patterns, which is used for code derivation, however the patterns are not concidered in the evaluation of the program.

    treePattern = ?<(...) leaf, {T left, T right} tree> = T;

---

List comprehension

A vector can be "open" by PIPE (|) operator so the following partial function is applied to each element of the vector, yielding another open value. An open value can be "closed", i.e., turn into a regular vector using CARET (^) operator. E.g.:

    | .x  ^ :
       [{x:12}, {x:8}, {y:98}]       -->  [12,8]
       ["a","x","b","x"]             -->  [{},{}]

The PIPE operator can be used for defining the Cartesian product:

    [.0 |, .1 |] ^ :
        [[1,2], [3,4]]    -->  [[1,3],[1,4],[2,3],[2,4]]

or

    { | x, | y } ^ :
        [1,2]             --> [{"x":1,"y":1},{"x":2,"y":1},{"x":1,"y":2},{"x":2,"y":2}]

QUIZ: Write a function which will take a list of integers and an integer x, and count how many times value x appears in the list. (see Examples/list-comprehension.k for a solution)

    count_occurrences =
    $ { [int] list, int x } 
      -- ???
    $ int;        

WARNING: When using CARET operator paranthesis may be required, e.g., like in:

    | ( [|,|] ^ ) ^ :
        [[1,2],[3,4]]     -->  [[[1,1],[1,2],[2,1],[2,2]],[[3,3],[3,4],[4,3],[4,4]]]             

---

Examples

1. projection:

    .x
    . "field name"
    (. 4)
   

   The function is defined if its argument "has" the field.

2. constants, literals for Strings, Booleans, and Integers. Examples:

    "a string"
    'another "String"'
    123
    false
    null
   

3. "built-in" functions:

    [1, 2, 3] PLUS       -- integer constant function 6
    [4, 4] TIMES toJSON  -- string constant function "16"
    [3, 2] GT            -- vector constant function [3, 2]
    [3, 4] GT            -- ... undefined
   

   A more interesting example could be:

    < GT .0, .1>
   

   which selects the maximum element in two element vector, i.e.,

    [3,8] < GT .0, .1 >    --> 8
   

User defined functions

k-expression can be prefixed by function definitions. E.g.:

    dec = [(),-1] PLUS;
    zero? = [(),0] EQ 0;
    factorial = <
      zero? 1, 
      [dec factorial, ()] TIMES
    >;
    { () x, factorial "x!" }

Another example could be finding the biggest (max) value in a vector:

max = < 
  SNOC             #  [x0, x1, x2, ...] --> {x0 car, [x1, x2, ...] cdr}
  [.car, .cdr max] #  {car:x0, cdr:[x1, x2, ...]} --> [x0, max(x1,x2,...)], recursive call 
  <GT .0, .1>      #  if x0 > max(x1,x2,...) then x0 else max(x1,x2,...)
,                  # when SNOC is not defined, i.e., if the input vector has one element:
  .0               #  [x0] --> x0
>; 
max

Value encodings (codes)

There are three predefined value encodings: int, string, and bool. The language supports code-expressions:

• product, e.g., {int x, int y, bool flag}
• disjoint union, e.g., <{} true, {} false>
• vector, e.g., [ int ] (all elements of the vector use the same encoding)

One can define recursive codes. E.g.:

$tree = <string leaf, {tree left, tree right} tree>;

Each code definition starts with a $. The above example defines new code called tree.

The code can be then used in a k-expression as a filter. A code-expression within k-expression is again prefixed by $.

$ tree = <string leaf, {tree left, tree right} tree>;
inc = [(),1] PLUS;
max = <GT .0, .1>;
height = $ tree <
    .leaf 0,
    .tree [.left height, .right height] max inc
> $ int;
height

---

k (a command-line JSON processor using k syntax)

There is a wrapper, k (./node_modules/.bin/k), which makes it easy to run the language from command line.

> k
   ... errors ...
Usage: ./node_modules/.bin/k ( k-expr | -k k-file) [ -1 ] [ json-file ]
E.g., cat '{"a": 10}' | ./node_modules/.bin/k '[(),()]'

For example:

1. One k-expression with one json-object:

    > echo '{"x": 12, "y": 13}' | k '{ <.x, "no x"> x, () input}' 
     {"x":12,"input":{"x":12,"y":13}}
   

2. By providing only k-expression, the script will compile the k-expression and apply the generated function to the stdin, line by line:

    > k '<["x=",.x," & y=",.y],["only x=",.x],["only y=",.y],["no x nor y"]>{CONCAT "x&y"}' 
    
     {"y": 123, "x": 432,"others": "..."}  --> {"x&y":"x=432 & y=123"} 
     {"x": 987}                            --> {"x&y":"only x=987"} 
     {"z": 123}                            --> {"x&y":"no x nor y"}
     ^D - to interrupt
   

   If the input is a multiline json object, we need to add -1 to the command-line options.

3. If the k-expression is long, it can be put in a file, e.g.:

    > cat test.k
     ---------  comments start by #, --, or // ----------------------------------
     <                          -- merge of 4 partial functions...
       ["x=", .x, " & y=", .y], -- produces a vector of 4 values, if fields 'x' and 'y' are present
       ["only x=", .x],         -- produces a pair '["only x=", "value-of-x"]', for input like {"x":"value-of-x"}
                                -- it is defined only if field 'x' is present
       ["only y=", .y],
       ["no x nor y"]           -- defined for all input, returns always the same one element vector
     > 
     -- one of the string vectors is passed to the following partial function, 
     -- which produces a record (map) with one field "x&y", whose value is the
     -- result of concatenating elements of the passed in vector
     { CONCAT "x&y" } 
     ------------------------------------------------------------------------------
   

   We can use it by:

    > k -k test.k
   

   If we want to read json objects from a file, e.g., my-objects.jsonl, we do

    > k -k test.k my-objects.jsonl
     {"x&y":"x=432 & y=123"} 
     {"x&y":"only x=987"} 
     {"x&y":"no x nor y"}
   

   where:

    > cat my-objects.jsonl 
     ####################################################
     # empty lines and lines starting with # are ignored
     {"y": 123, "x": 432,"others": "..."}
     {"x": 987}
     {"z": 123}
     ####################################################
   

---

Short comparaison with jq tutorial examples: https://stedolan.github.io/jq/tutorial/

---

k-REPL (Read-Evaluate-Print Loop)

Also there is a REPL, k-repl (./node_modules/.bin/k-repl), which acts like a toy shell for the language.

$ k-repl
Very! experimental repl shell for 'k-language'...
  --c            print aliased codes
  --C code       print 'code' definition
  --r            print relations
  --R rel        print 'rel' definition with type patterns
  --p (--pp)     pretty-print last value
  --s (--g) reg  store (get) the current value in (from) register 'reg'
  --regs         print register names
  --l file.k     load 'file.k'
> 

---

Using k from javascript

 import k from "@fraczak/k";

 let k_expression = '()';
 k.run(k_expression,"ANYTHING...");
 // RETURNS: "ANYTHING..."

 k_expression = '{"ala" name, 23 age}';
 k.run(k_expression,"ANYTHING...");
 // RETURNS: {"name":"ala","age":23}

 k_expression = '[.year, .age]';
 k.run(k_expression,{"year":2002,"age":19});
 // RETURNS: [2002,19]

 k_expression = '[(), ()]';
 k.run(k_expression,"duplicate me");
 // RETURNS: ["duplicate me","duplicate me"]

 k_expression = '[[[()]]]';
 k.run(k_expression,"nesting");
 // RETURNS: [[["nesting"]]]

 k_expression = '[[()]] {() nested, .0.0 val}';
 k.run(k_expression,"nesting and accessing");
 // RETURNS: {"nested":[["nesting and accessing"]],"val":"nesting and accessing"}

 k_expression = '0000';
 k.run(k_expression,{"test":"parse integer"});
 // RETURNS: 0

 k_expression = '[.y,.x] PLUS';
 k.run(k_expression,{"x":3,"y":4});
 // RETURNS: 7

 var k_fn = k.compile('{.name nom, <[.age, 18] GT .0, [.age, 12] GT "ado", "enfant"> age}');

 k_fn({"age":23,"name":"Emily"});
 // RETURNS: {"nom":"Emily","age":23}

 k_fn({"age":16,"name":"Katrina"});
 // RETURNS: {"nom":"Katrina","age":"ado"}

 k_fn({"age":2,"name":"Mark"});
 // RETURNS: {"nom":"Mark","age":"enfant"}

 k_fn = k.compile('$t = < i: int, t: [ t ] > ; <$t, $int>');

 k_fn(1);
 // RETURNS: 1

 k_fn({"i":1});
 // RETURNS: {"i":1}

 k_fn([{"i":2},{"i":3},{"t":[]}]);
 // RETURNS: undefined

 k_fn({"t":[{"i":2},{"i":3},{"t":[]}]});
 // RETURNS: {"t":[{"i":2},{"i":3},{"t":[]}]}

 k_fn = k.compile('$ < < [ int ] ints, [ bool ] bools > list, string None>');

 k_fn({"None":"None"});
 // RETURNS: {"None":"None"}

 k_fn({"list":{"ints":[]}});
 // RETURNS: {"list":{"ints":[]}}

 k_fn({"list":{"ints":[1,2,3]}});
 // RETURNS: {"list":{"ints":[1,2,3]}}