Syntax

The language follows a clean, restricted subset of Python syntax. Features intentionally omitted or modified from standard Python are summarized in Differences from Python.

Lines and Indentation

• Code blocks are delimited by indentation. Indentation must use multiples of 4 spaces; tab characters are rejected.
• Comments start with # and run to the end of the line. Blank lines and comment-only lines are ignored.
• Each statement occupies a single logical line. Backslash line continuations (\) and semicolons (;) are not supported, though expressions inside parentheses (...) or brackets [...] may span multiple lines.

Identifiers and Keywords

• Identifiers match [A-Za-z_][A-Za-z0-9_]*. Names starting with a double underscore (__) are reserved and forbidden.
• Reserved keywords: def, return, if, elif, else, for, in, break, continue, pass, and, or, not, True, False.
• User-defined names cannot shadow built-in function names, nor can a name switch kinds (between function, variable, constant, or port).
• Top-level assignments whose identifier consists solely of uppercase letters, digits, and underscores (e.g., BLOCK_SIZE = 16) define immutable constants.

Literals

Bytes Literals

• The prefix (h, b, or s) must be attached directly to the opening double quote with no space (h "87" is a syntax error).
• h"...": Contains only hexadecimal digits and optional spaces (spaces are ignored). No 0x prefix is allowed, and the total hex digit count must be even. h"" is the empty byte array.
• b"...": Contains only 0, 1, and optional spaces. Every 8 bits form one byte, so the total bit count must be a multiple of 8.
• s"...": Encodes the string characters as UTF-8 bytes.

Adjacent literals of the same kind inside parentheses are concatenated automatically across lines, allowing multi-line keys or test vectors with per-line comments:

KEY = (h"00 11 22 33 44 55 66 77"   # first 8 bytes
       h"88 99 aa bb cc dd ee ff")   # last 8 bytes
o = KEY

Statements

A variable's static type is fixed at its first assignment. Conditional expressions take the form val1 if cond else val2, where both branches have the same type:

o = "big" if n > 255 else "small"

Functions and Constants

• Functions are defined only at the top level; nested functions are not permitted.
• Every parameter requires a type annotation. Return types are written -> T or -> tuple[T1, T2]. Procedures with no return value omit -> and are called as standalone statements.
• User functions do not support default parameter values, keyword arguments, or variadic arguments.
• A function can access only its own parameters, local variables, functions defined earlier in the file, constants defined earlier, and built-in functions. It cannot access input or output port variables directly, which also prevents recursion and mutual recursion.
• Every execution path in a value-returning function must end with a return statement (or fail).
• Constants are top-level assignments with all-uppercase identifiers ([A-Z0-9_]+). Their initializer may use only literals, earlier constants, and built-in functions. Constants are immutable and visible inside functions.

HEADER_LEN = 1

def split_head(x: bytes) -> tuple[bytes, bytes]:
    return x[:HEADER_LEN], x[HEADER_LEN:]

first, rest = split_head(b)
size = len(b)

Indexing and Slicing (Subscripts)

Unlike Python, out-of-bounds indices or slice ranges do not silently clamp; if 0 ≤ start ≤ end ≤ len does not hold, evaluation fails with BYTES_OUT_OF_RANGE (or SCRIPT_INDEX_OUT_OF_RANGE for lists). In b[o, n], a negative n acts as an end position counted from the back (identical to b[o:n]).

In the table below, a is 00 11 22 33 44 55 66 77 88:

Subscript assignment updates bytes or list elements in place. Slice replacement requires the replacement bytes to have the exact same length as the target slice (otherwise fails with BYTES_LENGTH_MISMATCH):

o = a
o[1, 2] = h"ff ee"

for Loops and range

• for x in xs: iterates over elements of list[T].
• for v in b: iterates over each byte of bytes as an int in 0..255.
• for i in range(n), range(start, stop), range(start, stop, step) iterates over integers. range may only appear as the iterable of a for loop, and step cannot be 0.
• A single loop may iterate at most 1,048,576 times (checked before the loop starts).

total = 0
for v in b:
    if v == 2:
        continue
    if v == 5:
        break
    total += v

String Formatting (f-strings)

f"..." interpolates expressions into a string. Format specifiers follow [[fill]align][sign][#][0][width][type]:

Differences from Python