Syntax

stone’s grammar is derived from Python’s, trimmed down and changed so that common code needs fewer Shift presses. The full grammar is docs/grammar/stone.gram.

Compared with Python

Python	stone
if x: / while x: / for i in xs:	if x; / while x; / for i in xs;
def f(a, b):	def f(a, b);
else: / elif x:	else; / elif x;
return x	ret x
continue	cont
True, False, None	true, false, none
'single' or "double" quotes	"double" quotes only
-7 // 2 == -4	-7 / 2 == -3 (truncates)
-7 % 2 == 1	-7 % 2 == -1 (takes the dividend’s sign, like /)
2 ** -1 == 0.5, 2 ** 0.5	2.0 ** -1 == 0.5; the exponent is always an int

Keywords

These names are reserved and cannot be used for variables or functions:

keyword	meaning
def	defines a function
ret	returns from a function, or ends the program at the top level
if, elif, else	conditional blocks
while	loops while a condition holds
for, in	loops over a list or a range
break	leaves the innermost loop
cont	skips to the next iteration of the innermost loop
and, or, not	boolean operators, with short-circuiting and and or
true, false	the bool values
none	the only value of type none

Lines and blocks

A statement ends at the end of its line. A statement that opens a block (def, if, elif, else, while, for) ends in ;, followed either by an indented block on the next lines or by a single simple statement on the same line:

for i in range(3);
    print(i)
while true; break

Indent with spaces or tabs, as long as each block is consistent. Comments start with // outside a string and run to the end of the line, and blank and comment-only lines may sit at any indentation. Comment lines directly above a function or variable definition document it, and editors show them when you hover over its name.

Literals

literal	examples
int	0, 42, 9223372036854775807
float	1.5, 2.5e-3, 1e9, 1E+9
string	"hello", ""
bool and none	true, false, none
list	[], [1, 2, 3], [[1.0], []]

Negative numbers are the unary - applied to a literal, as in -7. Strings use double quotes, must close on the line they open on, and have no escape sequences, so "a\tb" holds a backslash and a t.

Statements

statement	example
expression	print(x)
assignment	x = 1, a = b = 0, items[i] = x, grid[r][c] = 0
function definition	def f(a, b); plus a block
ret	ret, ret x
if / elif / else	if x < 0; … elif x == 0; … else; …
while	while i < 10; plus a block
for	for x in items;, for i in range(n);, for i in range(a, b);
break, cont	inside a loop only

A for loop’s variable must be a plain name. Functions can only be defined at the top level.

Operators

From lowest to highest precedence:

operators	operands
or	int or bool conditions
and	int or bool conditions
not	an int or bool condition
== != < <= > >=	numbers; == and != also strings, bools, and none
+ -	two ints or two floats; + also joins two strings
* / %	two ints or two floats
unary + -	an int or a float
**	an int or float base and an int exponent
calls f(x), method calls xs.len(), indexing xs[i]

Comparisons chain, so a < b <= c means a < b and b <= c with b evaluated once. ** groups right to left and binds tighter than a unary sign on its left, as in Python, so 2 ** 3 ** 2 == 512 and -2 ** 2 == -4, while 2 ** -1 raises to -1. Parentheses group as usual. Operands are evaluated left to right.

A method call such as xs.append(1) evaluates the value before the . first, then the arguments. The builtin methods are len and append, documented in Builtins. A . right after an int starts a method call, so 1. is not a float; write 1.0.