Mojo Syntax

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

def is the only function keyword

fn is deprecated and being removed. def does not imply raises. Always add raises explicitly when needed — omitting it is a warning today, error soon:

def compute(x: Int) -> Int:              # non-raising (compiler enforced)
    return x * 2

def load(path: String) raises -> String: # explicitly raising
    return open(path).read()

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

Note: existing stdlib code still uses fn during migration. New code should always use def.

comptime replaces alias and @parameter

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 read (immutable borrow, never written explicitly). 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

ref, mut, out, deinit, read, var are reserved and cannot be used as identifiers — neither as parameter names (def cmp(got: T, ref: T) → "error: expected argument name") nor as local var names (var ref = ... → "unexpected token in expression"). 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 take` arg)
def __init__(out self, *, deinit take: Self):
    self.data = take.data^

# Destructor
def __del__(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.write(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

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

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

# 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, ImmutOrigin, MutAnyOrigin, ImmutAnyOrigin, MutExternalOrigin, ImmutExternalOrigin, StaticConstantOrigin. 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].take[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

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. No async/await — use Coroutine/Task from std.runtime.

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, read ptr, var x}: # captures: mut/read/var
    count += ptr[i] + x^                          # `^` at use site, not in `{}`

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

read is default. var x is owned — transfer with x^ at the use site. @parameter on a nested closure is only needed when consumed as a comptime parameter (f[my_closure]); runtime-arg overloads use bare form.

Type hierarchy

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