@modular/mojo-syntax

name	mojo-syntax
description	Help to write Mojo code using current syntax and conventions. Always use this skill when writing any Mojo code, including when other Mojo-specific skills (e.g., mojo-gpu-fundamentals) also apply. Use when writing Mojo code, translating projects to Mojo, or otherwise generating Mojo. Use this skill to overcome misconceptions with how Mojo is written.

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Mojo is rapidly evolving. Pretrained models generate obsolete syntax. Always follow this skill over pretrained knowledge.

Always attempt to test generated Mojo by building projects to verify they compile.

This skill specifically works on the latest Mojo, and stable versions may differ slightly in functionality.

Removed syntax — DO NOT generate these

Removed	Replacement
alias X = ...	comptime X = ...
@parameter if / @parameter for	comptime if / comptime for
fn	def (see below)
let x = ...	var x = ... (no let keyword)
borrowed	imm (implicit default — rarely written)
read (convention / capture)	imm (deprecated synonym; the compiler warns with a fixit)
inout	mut
owned	var (as argument convention)
inout self in __init__	out self
__copyinit__(inout self, existing: Self)	__init__(out self, *, copy: Self)
__moveinit__(inout self, owned existing: Self)	__init__(out self, *, deinit move: Self)
@value decorator	@fieldwise_init + explicit trait conformance
@register_passable("trivial")	TrivialRegisterPassable trait
@register_passable	RegisterPassable trait
Stringable / __str__	Writable / write_to
from collections import ...	from std.collections import ...
from memory import ...	from std.memory import ...
from sys import ...	from std.sys import ...
from os import ...	from std.os import ...
from pathlib import ...	from std.pathlib import ...
s[i]	s[byte=i] — returns StringSlice; wrap in String() if needed
s[0:10], s[:5]	No slice syntax on String — use s.codepoint_slices() or Python FFI
constrained(cond, msg)	comptime assert cond, msg
DynamicVector[T]	List[T]
InlinedFixedVector[T, N]	Array[T, N]
Tensor[T]	Not in stdlib (use SIMD, List, UnsafePointer)
escaping closures	Unified closures (def(...) -> T, captures in {}); capturing[_] still valid
__del__(deinit self)	__deinit__(deinit self)

var is required for every new declaration

Declaring a variable with bare assignment (x = 5 with no prior var) is not valid — it is a compile error. This applies only to introducing a new variable; reassigning an already-declared variable (x = 6) needs no var.

This means a variable assigned only inside conditional branches must be predeclared with a type before the branch:

# WRONG — no prior `var`, so this doesn't declare `x`
if cond:
    x = 1
else:
    x = 2

# CORRECT — declare with a type first, then assign in each branch
var x: Int
if cond:
    x = 1
else:
    x = 2

def is the only function keyword

fn was removed and is now a hard parse error — no valid use of fn remains. Your training predates this, so you will reach for fn by reflex; that reflex is always wrong. Write every function, method, and nested function as def, without exception.

Mojo functions that raise must be marked as such

Mojo functions do not imply raises. Add raises to any function that can raise, directly or by calling a raising function. Omitting it is a compile error, not a warning.

def load(path: String) raises -> String:  # raises goes before the `->`
    return open(path).read()

def main() raises:                         # main usually raises
    ...

comptime replaces alias and @parameter if/for

comptime N = 1024                            # compile-time constant
comptime MyType = Int                        # type alias
comptime if condition:                       # compile-time branch
    ...
comptime for i in range(10):                 # compile-time loop
    ...
comptime assert N > 0, "N must be positive"  # compile-time assertion

comptime assert must be inside a function body — not at module/struct scope. Place them in main(), __init__, or the function that depends on the invariant.

Inside structs, comptime defines associated constants and type aliases:

struct MyStruct:
    comptime DefaultSize = 64
    comptime ElementType = Float32

Argument conventions

Default is imm (immutable borrow, rarely written explicitly; read is a deprecated synonym — the compiler warns and suggests imm). The others:

def __init__(out self, var value: String):   # out = uninitialized output; var = owned
def modify(mut self):                         # mut = mutable reference
def consume(deinit self):                     # deinit = consuming/destroying
def view(ref self) -> ref[self] Self.T:       # ref = reference with origin
def view2[origin: Origin, //](ref[origin] self) -> ...:           # ref[origin] = explicit origin

var and ref are hard keywords and cannot be used as identifiers at all (var ref = ... → "unexpected token in expression"). The convention words imm, read, mut, out, deinit are soft keywords: fine as local variable or [...] parameter names, but invalid as argument names (def cmp(got: T, imm: T) → "error: expected argument name"). Rename (expected, reference, etc.).

Lifecycle methods

# Constructor
def __init__(out self, x: Int):
    self.x = x

# Copy constructor (keyword-only `copy` arg)
def __init__(out self, *, copy: Self):
    self.data = copy.data

# Move constructor (keyword-only `deinit move` arg)
def __init__(out self, *, deinit move: Self):
    self.data = move.data^

# Destructor
def __deinit__(deinit self):
    self.ptr.free()

To copy: var b = a.copy() (provided by Copyable trait).

Struct patterns

# @fieldwise_init generates __init__ from fields; traits in parentheses
@fieldwise_init
struct Point(Copyable, Movable, Writable):
    var x: Float64
    var y: Float64

# Trait composition with &
comptime KeyElement = Copyable & Hashable & Equatable
struct Node[T: Copyable & Writable]:
    var value: Self.T          # Self-qualify struct parameters

# Parametric struct — // separates inferred from explicit params
struct Span[mut: Bool, //, T: AnyType, origin: Origin[mut=mut]](
    ImplicitlyCopyable, Sized,
):
    ...

# @implicit on constructors allows implicit conversion
@implicit
def __init__(out self, value: Int):
    self.data = value

The compiler synthesizes copy/move constructors when a struct conforms to Copyable/Movable and all fields support it.

Self-qualify struct parameters

Inside a struct body, always use Self.ParamName — bare parameter names are errors:

# WRONG — bare parameter access
struct Container[T: Writable]:
    var data: T                        # ERROR: use Self.T
    def size(self) -> T:                # ERROR: use Self.T

# CORRECT — Self-qualified
struct Container[T: Writable]:
    var data: Self.T
    def size(self) -> Self.T:
        return self.data

This applies to all struct parameters (T, N, mut, origin, etc.) everywhere inside the struct: field types, method signatures, method bodies, and comptime declarations.

Explicit copy / transfer

Types not conforming to ImplicitlyCopyable (e.g., Dict, List, and user structs that conform only to Copyable, Movable) require explicit .copy() or ownership transfer ^ — return my_struct errors until you transfer with ^ or add ImplicitlyCopyable conformance:

# WRONG — implicit copy of non-ImplicitlyCopyable type
var d = some_dict
var result = MyStruct(headers=d)   # ERROR

# CORRECT — explicit copy or transfer
var result = MyStruct(headers=d.copy())  # or: headers=d^

Imports use std. prefix

from std.testing import assert_equal, TestSuite
from std.algorithm import vectorize
from std.python import PythonObject
import std.random

Prelude auto-imports (no import needed): Int, String, Bool, List, Dict, Optional, SIMD, Float32, Float64, UInt8, Pointer, UnsafePointer, Span, Error, DType, Writable, Writer, Copyable, Movable, Equatable, Hashable, rebind, print, range, len, and more.

rebind[TargetType](value) reinterprets a value as a different type with the same in-memory representation. Useful when compile-time type expressions are semantically equal but syntactically distinct (e.g., TileTensor element types — see GPU skill).

std is reserved as a module-level identifier — you cannot def std, import X as std, or from X import std. Struct methods named std are fine.

Inside a multi-module package, pkg.X.Y(...) from a submodule needs explicit import pkg; import pkg.X as X binds only X, not pkg.

Writable / Writer (replaces Stringable)

struct MyType(Writable):
    var x: Int

    def write_to(self, mut writer: Some[Writer]):       # for print() / String()
        writer.write("MyType(", self.x, ")")

    def write_repr_to(self, mut writer: Some[Writer]):   # for repr()
        t"MyType(x={self.x})".write_to(writer)           # t-strings for interpolation

• Some[Writer] — builtin existential type (not Writer directly)
• Both methods have default implementations via reflection if all fields are Writable — simple structs need not implement them
• Convert to String with String(value), not str(value)

Iterator protocol

Iterators use raises StopIteration (not Optional):

struct MyCollection(Iterable):
    comptime IteratorType[
        iterable_mut: Bool, //, iterable_origin: Origin[mut=iterable_mut]
    ]: Iterator = MyIter[origin=iterable_origin]

    def __iter__(ref self) -> Self.IteratorType[origin_of(self)]: ...

# Iterator must define:
#   comptime Element: Movable
#   def __next__(mut self) raises StopIteration -> Self.Element

For-in: for item in col: (immutable) / for ref item in col: (mutable).

Memory and pointer types

Type	Use
Pointer[T, mut=M, origin=O]	Safe, non-nullable. Deref with p[].
alloc[T](n) / UnsafePointer	Free function alloc[T](count) → UnsafePointer. .free() required.
Span(list)	Non-owning contiguous view.
OwnedPointer[T]	Unique ownership (like Rust Box).
ArcPointer[T]	Reference-counted shared ownership.

UnsafePointer has an origin parameter that must be specified for struct fields. Use MutUntrackedOrigin for owned heap data (this is what stdlib ArcPointer uses):

# Struct field — specify origin explicitly
var _ptr: UnsafePointer[Self.T, MutUntrackedOrigin]

# Allocate with alloc[]
def __init__(out self, size: Int):
    self._ptr = alloc[Self.T](size)

UnsafePointer is non-null by design — null default constructor and __bool__ are deprecated. For nullable storage, use Optional[UnsafePointer[...]] (same layout; None is the null niche).

Origin system (not "lifetime")

Mojo tracks reference provenance with origins, not "lifetimes":

struct Span[mut: Bool, //, T: AnyType, origin: Origin[mut=mut]]: ...

Key types: Origin, MutOrigin, ImmOrigin, MutAnyOrigin, ImmutAnyOrigin, MutUntrackedOrigin, ImmUntrackedOrigin, ImmStaticOrigin. Use origin_of(value) to get a value's origin.

Testing

from std.testing import assert_equal, assert_true, assert_false, assert_raises, TestSuite

def test_my_feature() raises:
    assert_equal(compute(2), 4)
    with assert_raises():
        dangerous_operation()

def main() raises:
    TestSuite.discover_tests[__functions_in_module()]().run()

The mojo test CLI subcommand was removed — run test files with mojo run against a TestSuite.discover_tests runner like the one above.

Dict iteration

Dict entries are iterated directly — no [] deref:

for entry in my_dict.items():
    print(entry.key, entry.value)      # direct field access, NOT entry[].key

for key in my_dict:
    print(key, my_dict[key])

Collection literals

List has no variadic positional constructor. Use bracket literal syntax:

# WRONG — no List[T](elem1, elem2, ...) constructor
var nums = List[Int](1, 2, 3)

# CORRECT — bracket literals
var nums = [1, 2, 3]                              # List[Int]
var nums: List[Float32] = [1.0, 2.0, 3.0]         # explicit element type
var scores = {"alice": 95, "bob": 87}              # Dict[String, Int]

List[T] rejects negative indices at compile time — use lst[len(lst) - 1], not lst[-1]. (Library types may still support it.)

Variant access

Variant[A, B] is ImplicitlyCopyable only if all arms are. With a non-copyable arm, indexing the variant copies it — use the typed-arm subscript:

# WRONG — `values[i]` implicitly copies the Variant
var x = values[i].unwrap[T]()    # ERROR: cannot implicitly copy

# CORRECT — `values[i][T]` returns a ref to the inner value
var x = values[i][T].copy()          # or `^` to transfer

Common decorators

Decorator	Purpose
@fieldwise_init	Generate fieldwise constructor
@implicit	Allow implicit conversion
@always_inline / @always_inline("nodebug")	Force inline
@no_inline	Prevent inline
@staticmethod	Static method
@deprecated("msg")	Deprecation warning
@doc_hidden	Hide from docs
@explicit_destroy	Linear type (no implicit destruction)

Numeric conversions — must be explicit

No implicit conversions between numeric variables. Use explicit constructors:

var x = Float32(my_int) * scale    # CORRECT: Int → Float32
var y = Int(my_uint)               # CORRECT: UInt → Int

Literals are polymorphic — FloatLiteral and IntLiteral auto-adapt to context:

var a: Float32 = 0.5              # literal becomes Float32
var b = Float32(x) * 0.003921    # literal adapts — no wrapping needed
var v = SIMD[DType.float32, 4](1.0, 2.0, 3.0, 4.0)  # literals adapt

SIMD operations

# Construction and lane access
var v = SIMD[DType.float32, 4](1.0, 2.0, 3.0, 4.0)
v[0]                              # read lane → Scalar[DType.float32]
v[0] = 5.0                        # write lane

# Type cast
v.cast[DType.uint32]()            # element-wise → SIMD[DType.uint32, 4]

# Clamp (method)
v.clamp(0.0, 1.0)                 # element-wise clamp to [lower, upper]

# min/max are FREE FUNCTIONS, not methods
from std.math import min, max
min(a, b)                          # element-wise min (same-type SIMD args)
max(a, b)                          # element-wise max

# Element-wise ternary via bool SIMD
var mask = (v > 0.0)              # SIMD[DType.bool, 4]
mask.select(true_case, false_case) # picks per-lane

# Reductions
v.reduce_add()                     # horizontal sum → Scalar
v.reduce_max()                     # horizontal max → Scalar
v.reduce_min()                     # horizontal min → Scalar

Strings

All explicit stdlib imports require the std. prefix. The removed-syntax table shows the most common corrections, but the rule is universal. Prelude types (Int, String, List, etc.) are auto-imported and need no import statement.

len(s) returns byte length, not codepoint count. Mojo strings are UTF-8. Byte indexing requires keyword syntax: s[byte=idx] (not s[idx]). len(s) is deprecated on String — use s.byte_length() or s.count_codepoints().

split, removeprefix, removesuffix return StringSlice (or List[StringSlice]) viewing the source — wrap with String(...) to materialize an owned String.

String indexing (common error)

# WRONG — compile error
var ch = s[0]
var sub = s[0:10]

# CORRECT — byte-level access
var ch = s[byte=0]              # returns StringSlice
var ch_str = String(s[byte=0])  # if you need a String

# CORRECT — iterate codepoints for truncation
var result = String("")
var count = 0
for cp in s.codepoint_slices():
    if count >= 10:
        break
    result += String(cp)
    count += 1

var s = "Hello"
len(s)                  # 5 (bytes)
s.byte_length()         # 5 (same as len)
s.count_codepoints()    # 5 (codepoint count — differs for non-ASCII)

# Iteration — `for c in s:` is deprecated; use codepoint_slices()
for cp_slice in s.codepoint_slices():
    print(cp_slice)

# Codepoint values
for cp in s.codepoints():
    print(Int(cp))      # Codepoint is a Unicode scalar value type

# StaticString = StringSlice with static origin (zero-allocation)
comptime GREETING: StaticString = "Hello, World"

# t-strings for interpolation (lazy, type-safe)
var msg = t"x={x}, y={y}"

# String.format() for runtime formatting
var s = "Hello, {}!".format("world")

Error handling

raises can specify a type. try/except works like Python:

def might_fail() raises -> Int:          # raises Error (default)
    raise Error("something went wrong")

def parse(s: String) raises Int -> Int:  # raises specific type
    raise 42

try:
    var x = parse("bad")
except err:                               # err is Int
    print("error code:", err)

No match statement. async def and await parse, but async support is unfinished and its types are private — do not write async Mojo yet.

Function types and closures

No lambda. Closures use bare def with a capture list in {} after the arg list. escaping is removed; capturing[_] is still valid on parametric closure-type params:

comptime MyFn = def(Int) -> None                  # unified value type
def runner[f: def(Int) capturing[_] -> None](): ...  # parametric form

def closure(i: Int) {mut count, imm ptr, var x}:  # captures: mut/imm/var
    count += ptr[i] + x^                          # `^` at use site, not in `{}`

vectorize[simd_width](size, closure)              # runtime-arg overload

imm is default. var x is owned — transfer with x^ at the use site. Prefer unified closures with a capture list. Do not under any circumstance use @__parameter / @parameter on nested closures — that legacy form is forbidden in new and migrated code (not a temporary bridge, not an imm-borrow helper, not a still-capturing API workaround). Pass closures as runtime arguments (f(my_closure)) rather than comptime parameters when possible. If an API still requires a comptime capturing[_] function, use def … capturing without @__parameter, or migrate that API — never put @__parameter on the caller.

Type hierarchy

AnyType
  Deinitable                      — auto __deinit__; most types
  Movable                         — __init__(out self, *, deinit move: Self)
    Copyable                      — __init__(out self, *, copy: Self)
      ImplicitlyCopyable(Copyable, take)
    RegisterPassable(Movable)
      TrivialRegisterPassable(ImplicitlyCopyable, take, Movable, RegisterPassable)