Language overview

Pyking reads like Python. This page covers the main places where it differs. Every example is a whole program: press ▶ to run it, or edit it first. Each one runs on its own, so a name defined in one example is not visible in the next.

Bindings and variables

= binds a name to a value, and that binding never changes. A variable that changes is declared with var and updated with :=.

limit = 10
var total = 0
for i in range(1, limit + 1):
    total := total + i
print("The sum of 1 ..", limit, "is", total)

Updating a name that was bound with = is a compile-time error. This example is meant to be refused:

count = 1
count := count + 1

Functions and types

Annotations are optional. Where a function has none, the checker infers its most general type, so twice below works with any function whose argument and result have the same type.

def twice(f, x): f(f(x))
def inc(n): n + 1
def shout(s): s ++ "!"

print(twice(inc, 5))
print(twice(shout, "hey"))

A function’s value is its last expression, or what it returns. A body of one expression can go on the header’s line, as above.

Control flow

if is a statement, and also an expression written if c then a else b. while loops while its condition holds.

var n = 27
var steps = 0
while n > 1:
    n := if n / 2 * 2 == n then n / 2 else 3 * n + 1
    steps := steps + 1
print("27 reaches 1 in", steps, "steps")

loop carries values from one turn to the next. Each turn ends with the next values, and break stops the loop. The loop’s value is what it carried last.

def factorial(n: Int) -> Int:
    (_, result) = loop k, acc = n, 1:
        if k == 0:
            break
        (k - 1, acc * k)
    return result

print(factorial(10))

Lists, tuples and pattern matching

match takes values apart. A list pattern [first] ++ rest matches a non-empty list, and a tuple is taken apart with =.

def describe(n: Int) -> String:
    match n:
        case 0: "zero"
        case 1: "one"
        case _: "many"

def sum(xs: [Int]) -> Int:
    match xs:
        case []: 0
        case [first] ++ rest: first + sum(rest)

print(describe(0), describe(1), describe(7))
print(sum([1, 2, 3, 4]))

point = (3, 4)
(x, y) = point
print(x * x + y * y)

Recursion

A call in tail position does not use up stack space, so a recursive function can run for as long as a loop.

def count_down(n: Int) -> Int:
    if n == 0:
        return 0
    return count_down(n - 1)

print(count_down(1000000))

Modules and the standard library

A program uses a module with import. The standard library comes with Pyking. This example reads one line of standard input, which is filled in above the code:

import io

name = io.read_line("What is your name? ")
io.write_line("Nice to meet you, " ++ name ++ "!")

Next steps

• Try your own programs in the playground.
• Browse the libraries.
• pyk repl starts an interactive session in a terminal.