reference.md

Language Reference

This is not comprehensive, but it should be enough to get started.

Start is a beginning programming language loosely inspired by BASIC and LOGO.

Values

The primitive kinds of values in Start are:

• none (the absence of a value)
• true
• false
• numbers (integers, decimals, decimals written scientific notation, and infinity)
• text strings

none
true
false
1
1.23
1.2345e+6
infinity
"Hello, world!"

In addition to the primivite types, the language contains two important composite types: lists and records. A list is an ordered sequence of other values, while a record (some languages refer to them as "maps" or "dictionaries") is an association between string keys and their corresponding values. Notably, lists and records may hold other lists and records as elements, making it possible to express arbitrarily complex structured data.

[10, 20, 30]
{ x = 1, y = 2 }
{
  name = "Fred",
  year = 1973,
  likes = [
    "philosophy",
    "computers"
  ]
}

Expressions

Values can be combined using operators. Operators have precedence: when an expression contains multiple operators, those that are higher in the list will be peformed first. Use parentheses to force lower-precedence subexpressions to be evaluated first.

• unary + and -
• exponentiation ^
• multiplication *, division /, and modulo %
• addition + and subtraction -
• concatenation ::
• comparison =, !=, <, >, <=, >=
• logical and, or, and not

1 + 2 -> 3
1 + 2 * 3 -> 7
1 + 2 * 3 ^ 4 -> 163
(((1 + 2) * 3) ^ 4) -> 6561
[1, 2] :: [3, 4] -> [1, 2, 3, 4]
1 + 2 > 5 -> false
1 + 2 > 5 or 3 + 4 < 10 -> true

Values also result from calling functions, and function expressions can be combined using operators. Start has a number of built-in functions, but you can also define your own (see Functions below).

sqrt(64) -> 8
sin(33) ^ 2 + cos(33) ^ 2 -> 1
rand(1, 100) -> 42 (maybe!)

Variables

The let statement assigns values to variables. The word let is optional.

let x = 1
a = b + c * d

Indexes

A variable can be indexed if its value is a string, list, or record.

a = [10, 20, 30]
a[2] -> 20

crew = ["Kirk", "Spock", "Bones", "Uhura", "Scotty", "Chekov", "Sulu"]
crew[3] -> "Bones"
crew[3][1] -> "B"

Indexes can be other variables or expressions.

i = 3
crew[i] -> "Bones"

Dot-notation can be used when indexes are literal numbers or strings.

crew = ["Kirk", "Spock", "Bones", "Uhura", "Scotty", "Chekov", "Sulu"]
crew.2 -> "Spock"

person = { name = "Lily", age = 17 }
person.name -> "Lily"
person.name.1 -> "L"

Commands

Commands are typically built-in behaviors provided by the system (like printing a value or drawing a shape on the screen), but they can also execute functions you have defined yourself (see Functions below). Depending on the host where the language is running, some commands may or may not be available, or may behave in different ways.

print 1 + 2 * 3
> 6

rect 10, 10, 100, 100 -> draws a 100x100 rectangle on the screen at (10, 10)

Loops

The repeat, while, and for statements execute a statement or code block (called the body) multiple times, based on different criteria:

• repeat executes the body a given number of times, or forever if no value is provided
• while executes the body as long as some condition is true; if the condition is initially false, the body will never be executed
• for works in two different ways:
  • with =, it executes the code for each number in a numeric sequence
  • with in, it executes the code for each member of a list or record

If the loop body is a single short statement, it can be placed on the same line after do. For most cases, the do/end form is preferable.

repeat 10 do print "Hello, world!"

i = 1
repeat 10 do
  print "i is " :: i
  i = i + 1
end

i = 1
while i <= 10 do
  print "i is " :: i
  i = i + 1
end

for i = 1 to 10 by 2 do
  print "i is " :: i
end

crew = ["Kirk", "Spock", "Bones", "Uhura", "Scotty", "Chekov", "Sulu"]
for i in crew do
  print "i is " :: i
end

Branching

The if/then/else statement expresses conditional branching. The first case that evaluates to true is executed. If no cases are true, it executes the else block if one is provided.

if i < 5 then
  print "i is less than 5"
else if i > 5 then
  print "i is greater than 5"
else
  print "i equals 5"
end

Functions

The begin statement defines a new function which executes the statement or code block when called. If parameters are provided, any values that are passed to the function when called, are assiged to those variable names inside the body of the function, in the order that they appear. If a return statement is encountered, the returned value becomes the value of the expression that called the function.

begin hypotenuse(a, b) do
  return sqrt(a ^ 2 + b ^ 2)
end

print hypotenuse(3, 4)
> 5

A function can also be used as a command. In this case, the parentheses can be omitted, and the return value is ignored.

begin greet do
  print "Hello, world!"
  return 123
end

greet
> Hello, world!
(the 123 is ignored)

Within the body of a function, any assigned variable (including parameters) takes on a temporary value for the duration of the function, eclipsing whatever value it might have outside (its global value). This is referred to as the function's scope, and it prevents functions from tampering with outside data, and introducing hard-to-track-down bugs. It also means that you can freely name variables and parameters without fear of overwriting variables on the outside.

a = 10

begin hypotenuse(a, b) do
  return sqrt(a ^ 2 + b ^ 2) <- this a is 3
end

print hypotenuse(3, 4)
> 5

print a <- this a is still 10!
> 10