-- single line comment

            // single line comment

            /* multi-
                     line comment */

// Start programming immediately

    -- write using UTF8; save as: hello.ex
    -- use ? for output

        ? "😍 hello , 😎 world!"
        ? sqrt(2+2)

// Interpret your program
                            /*
       p hello               */

// Compile your program
                            /*
       p -c hello            */

// Coding mistakes receive gentle help messages
                                                          /*
    string line
    line = 5
           ^ type error (storing atom in string)           */

// Every literal value, constant, and variable is an ''object''

    -- a literal object
        ? "hello"
        ? PI
        ? { "hello", PI }

    -- a named variable object
        object X
        X = "hello"
        X = PI
        X = { "hello", PI }

    -- a named constant object
        constant myPI = 22/7

//  Everything is an ''object'', just two fundemental kinds
                                                                /*
            ┌────────────────────▄
          ┌─┤   object           █─┐
          │ └─▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄█ │
          │                        │
       "atom"                   "container"                     */

       number  x = 3.14159      sequence  s   = { "hello", PI }
       integer y = 3            string    txt = "hello"

            -- simplify,
            -- and use only two primitives
            number x1=3.14156, y1=3
            sequence s1={"hello",PI}, txt1="hello"

                -- simplify even more,
                -- and use just one primitive
                object x2=3.14156, y2=3, s2={"hello",PI}, txt2="hello"

// Elegant data-type design

    -- invent your own "type"
    -- organize with "struct" or "class"
                                                                /*
    ╔═══════════════════════════════╗
    ║   ┌─────────────────────────┐ ║
    ║   │      ┌───────────▄      │ ║
    ║   │    ┌─┤ object    █─┐    │ ║
    ║   │    │ └─▄▄▄▄▄▄▄▄▄▄█ │    │ ║
    ║   │    │               │    │ ║
    ║   │  number        sequence │ ║
    ║   │    │               │    │ ║
    ║   │  integer        string  │ ║
    ║   └──────── type ───────────┘ ║
    ║                               ║
    ╚════════ struct                ║
              class ════════════════╝
                                                                */

// Syntax is consistant: "keyword...end keyword"

    -- no invisible syntax or extra rules needed.

    // loop
    -- while ... end while

        integer index = 1
        while index <= 5 do
            ? index
            index += 1
        end while

    // loop
    -- for ... end for

        for i=5 to 1 by -1 do
            ? i
        end for

    //  conditional
    --  if ... end if

        number p = 4
        if p < 1 then
            ? "p is a small number"
        elsif p > 10 then
            ? "p is a large number"
        else
            ? "p is inbetween"
        end if

    // conditional
    -- switch ... end switch

        object ch = prompt_string("enter one character: " )
        switch ch
            case "a": ? "ch is a"
            case "b": ? "ch is b"
            case "c": ? "ch is c"
            default:  ? "ch is something else"
        end switch

// Operators are always consistant; never overloaded.

    -- the + operator ''always adds''
        ? 2+7      --> 9
        ? 'A' + 32 --> 97

    -- the & operator ''always concatenates''
        ? 2 & 7                --> {2,7}
        ? "cat" & " " & "dog"  --> "cat dog"
        ? {1,2,3} & "fish"     --> {1,2,3} & "fish"
        pp( {1,2,3} & "fish" ) --> {1,2,3,102'f',105'i',115's',104'h'}

// Use ''sq_'' functions to span entire containers.

        ? sq_add( {1,2,3}, 10 )     --> {11,12,13}
        ? sq_sqrt( {4,9,16} )       --> {2,3,4}

// Functions must return a value

        function add2( number x, number y )
            number sum = x + y
            return sum
        end function
        ? add2( 4, 9 )

// Procedures do not return a value

        procedure sum_all( sequence lst )
            number sum = 0
            for i=1 to length(lst) do
                sum += lst[i]
            end for
            ? sum
        end procedure
        sum_all( {1,3,9,11} )

// Recursion and mutal recursion are permitted

        function factorial(number n)
            if n == 0 then
                return 1
            end if
            if n<0 then
                return "error, no negative numbers for factorials"
            end if
            return n * factorial(n - 1)
        end function
        ? factorial(5)

// User defined data-types

        -- defined like a function: type ... end type
        -- they are fully programmable; add your own features

        type positive( number x )
            if not integer(x) then
                ? "use integers for factorials"
                return False
            end if
            if x < 0 then
                ? "error, no negative numbers for factorials"
                return False
            end if
            return True
        end type

     -- use them to declare variables and parameters

        function factorial2( positive n )
            if n == 0 then return 1 end if
            return n * factorial2(n-1)
        end function
        ? factorial(5)

    -- to catch errors, and recover, use: try ... end try

       try
          ? factorial2( -5 )
       catch e
          ? "that was a mistake"
       end try

// Sequences are versatile

    -- multiple assignment
            number a, b, c
            {a,b,c} = { -100, -200/-2, -300*3 }
            ? a      --> -100
            ? b      -->  100
            ? c      --> -900

    -- swapping values
                ? a       --> -100
                ? c       --> -900
            {a,c} = {c,a}
                ? a       --> -900
                ? c       --> -100


// Symmetrical one-based indexing does it all

    -- both sequence and string are mutable and work alike

    --              1   2   3   4   5    -- index head to tail
             s = { 10, 20, 30, 40, 50 }
    --             -5  -4  -3  -2  -1    -- index tail to head

        // one item
            ? s[ 2]
            ? s[-4]
                        -- output for both is:
            ----->  20

        // slice with one item
            ? s[ 2.. 2]
            ? s[-4..-4]
                        -- output for both is:
            -----> {20}

        // inclusive slice
            ? s[ 2.. 4]
            ? s[-4..-2]
                        -- output for both is:
            -----> {20,30,40}

        // empty sequence
            ? s[3 .. 2]
            ? s[-3..-4]
                        -- output for both is:
            -----> {}

        // insert
            s[3..2] = {99}
            ? s
            -----> {10,20,99,30,40,50}

        // prepend and append
            s = { 10,20,30,40,50 }

            s[  1..0] = {0}             -- prepend
            s[$+1..$] = {6}             -- append

            ? s
            -----> {0,10,20,99,30,40,50,6}

            s[0..-1] = {9999}           -- append

            ? s
            -----> {0,10,20,99,30,40,50,6,9999}

        // delete
            s = { 10,20,30,40,50 }

            s[2..2] = {}        -- item deleted
            ? s
            -----> {10,30,40,50}

            s[2..3] = {}        -- slice deleted
            ? s
            -----> {10,50}

// Learn and reuse; you keep what you learn.

            s = { 1,3,5,7 }
            txt = "jello"

        -- "find" locates one item in either a sequence or a string
        ? find( 3, s ) --> 2
        ? find( 'e', txt ) --> 2

        -- "match" locates a slice in either a sequence or a string
        ? match( {5,7}, s ) -- > 3
        ? match( "ll", txt ) --> 3

// Look back at the examples, Phix is generic!

// Batteries are installed

        ? sort( {2, 54,6,4, 0} )
        ? upper( "cat" )
        ? log( 10.4 )
        ? trunc(1.4)         --  1
        ? floor(1.4)         --  1
        ? trunc(-1.4)        -- -1
        ? floor(-1.4)        -- -2

// Batteries are included

        include builtins/regex.e

        string str = "say hello and smile"
        str = gsub( `s...e`, str, "😍" )
        ? str   --> "say hello and 😍"

// Yes, sequences are "powerful"

        function odd(integer a) return remainder(a,2)=1 end function
        function even(integer a) return remainder(a,2)=0 end function

        ? tagset(10)                 --> {1,2,3,4,5,6,7,8,9,10}
        ? filter(tagset(10),odd)     --> {1,3,5,7,9}
        ? filter(tagset(10),even)    --> {2,4,6,8,10}

// A ''struct'' provides named fields, type-checking, and dot notation

        struct point
            number x = 0
            number y = 0
        end struct

        procedure show( point q )
            printf(1, "(%g,%g)", { q.x, q.y } )
        end procedure

        point  p1 = new()
        show(p1)
            --> (0,0)

        p1.x = 3
        p1.y = 5
        show( p1 )
            --> (3,5)

// A ''class'' adds methods and scope control

        class pair
            public number x = 0
            public number y = 0

            procedure show( )
                printf(1, "(%g,%g)", { this.x, this.y } )
            end procedure
        end class

        pair  p2 = new()
        p2.show()
            --> (0,0)

        p2.x = 3
        p2.y = 5
        p2.show()
            --> (3,5)

// Inherit and compose

        class Pair -- any 2 objects
            public sequence xy
            public integer x,y
                 function get_x()
                  return xy[1]
                 end function

                 function get_y()
                  return xy[2]
                 end function
        end class

        type pos_seq(sequence x)
            return min(x) >= 0
        end type

        class Point extends Pair
            public pos_seq loc -- any two numbers >= 0

                procedure set_loc(object x)
                    this.xy = {x[1],x[2]}
                end procedure
        end class

        class Rectangle extends Point
            public Point tlc,brc --top_left, bottom_right corners;
            public sequence size

                  function get_size()
                    this.size = {brc.x-tlc.x , brc.y-tlc.y}
                    return this.size
                  end function
        end class

        Point p1a = new() p1a.loc = {50,10}
        Point p2a = new() p2a.loc = {300,200}

        Rectangle r = new()
                  r.tlc = p1a
                  r.brc = p2a
        ? r          -- {"struct","Rectangle",4,1}
        ? r.tlc      -- {"struct","Point",3,3}

        ? r.size       --> {250,190}
        ? r.get_size() --> {250,190}