v0.7.0 (2024-01-25)

⭐️ New

• A new Mojo-native dictionary type, Dict for storing key-value pairs. Dict stores values that conform to the CollectionElement trait. Keys need to conform to the new KeyElement trait, which is not yet implemented by other standard library types. In the short term, you can create your own wrapper types to use as keys. For example, the following sample defines a StringKey type and uses it to create a dictionary that maps strings to Int values:

  from collections.dict import Dict, KeyElement
  
  @value
  struct StringKey(KeyElement):
      var s: String
  
      fn __init__(inout self, owned s: String):
          self.s = s ^
  
      fn __init__(inout self, s: StringLiteral):
          self.s = String(s)
  
      fn __hash__(self) -> Int:
          return hash(self.s)
  
      fn __eq__(self, other: Self) -> Bool:
          return self.s == other.s
  
  def main():
      var d = Dict[StringKey, Int]()
      d["cats"] = 1
      d["dogs"] = 2
      print(len(d))         # prints 2
      print(d["cats"])      # prints 1
      print(d.pop("dogs"))  # prints 2
      print(len(d))         # prints 1

  We plan to add KeyElement conformance to standard library types in subsequent releases.

• Users can opt-in to assertions used in the standard library code by specifying -D MOJO_ENABLE_ASSERTIONS when invoking mojo to compile your source file(s). In the case that an assertion is fired, the assertion message will be printed along with the stack trace before the program exits. By default, assertions are not enabled in the standard library right now for performance reasons.

• The Mojo Language Server now implements the References request. IDEs use this to provide support for Go to References and Find All References. A current limitation is that references outside of the current document are not supported, which will be addressed in the future.

• The sys.info module now includes num_physical_cores(), num_logical_cores(), and num_performance_cores() functions.

• Homogeneous variadic arguments consisting of memory-only types, such as String are more powerful and easier to use. These arguments are projected into a VariadicListMem.

  (Previous releases made it easier to use variadic lists of register-passable types, like Int.)

  Subscripting into a VariadicListMem now returns the element instead of an obscure internal type. In addition, we now support inout and owned variadic arguments:

  fn make_worldly(inout *strs: String):
      # This "just works" as you'd expect!
      for i in range(len(strs)):
          strs[i] += " world"
  fn main():
      var s1: String = "hello"
      var s2: String = "konnichiwa"
      var s3: String = "bonjour"
      make_worldly(s1, s2, s3)
      print(s1)  # hello world
      print(s2)  # konnichiwa world
      print(s3)  # bonjour world

  (Previous releases made it easier to use variadic lists, but subscripting into a VariadicListMem returned a low-level pointer, which required the user to call __get_address_as_lvalue() to access the element.)

  Note that subscripting the variadic list works nicely as above, but iterating over the variadic list directly with a for loop produces a Reference (described below) instead of the desired value, so an extra subscript is required; We intend to fix this in the future.

  fn make_worldly(inout *strs: String):
      # Requires extra [] to dereference the reference for now.
      for i in strs:
          i[] += " world"

  Heterogeneous variadic arguments have not yet been moved to the new model, but will in future updates.

  Note that for variadic arguments of register-passable types like Int, the variadic list contains values, not references, so the dereference operator ([]) is not required. This code continues to work as it did previously:

  fn print_ints(*nums: Int):
      for num in nums:
          print(num)
      print(len(nums))

• Mojo now has a prototype version of a safe Reference type. The compiler's lifetime tracking pass can reason about references to safely extend local variable lifetime, and check indirect access safety. The Reference type is brand new (and currently has no syntactic sugar) so it must be explicitly dereferenced with an empty subscript: ref[] provides access to the underlying value.

  fn main():
      var a: String = "hello"
      var b: String = " references"
  
      var aref = Reference(a)
      aref[] += b
      print(a)  # prints "hello references"
  
      aref[] += b
      # ^last use of b, it is destroyed here.
  
      print(aref[]) # prints "hello references references"
      # ^last use of a, it is destroyed here.

  While the Reference type has the same in-memory representation as a C pointer or the Mojo Pointer type, it also tracks a symbolic "lifetime" value so the compiler can reason about the potentially accessed set of values. This lifetime is part of the static type of the reference, so it propagates through generic algorithms and abstractions built around it.

  The Reference type can form references to both mutable and immutable memory objects, e.g. those on the stack or borrowed/inout/owned function arguments. It is fully parametric over mutability, eliminating the problems with code duplication due to mutability specifiers and provides the base for unified user-level types. For example, it could be used to implement an array slice object that handles both mutable and immutable array slices.

  While this is a major step forward for the lifetimes system in Mojo, it is still very early and awkward to use. Notably, there is no syntactic sugar for using references, such as automatic dereferencing. Several aspects of it need to be more baked. It is getting exercised by variadic memory arguments, which is why they are starting to behave better now.

  Note: the safe Reference type and the unsafe pointer types are defined in the same module, currently named memory.unsafe. We expect to restructure this module in a future release.

• Mojo now allows types to implement __refattr__() and __refitem__() to enable attribute and subscript syntax with computed accessors that return references. For common situations where these address a value in memory this provides a more convenient and significantly more performant alternative to implementing the traditional get/set pairs. Note: this may be changed in the future when references auto-dereference—at that point we may switch to just returning a reference from __getattr__().

• Parametric closures can now capture register passable typed values by copy using the __copy_capture decorator. For example, the following code will print 5, not 2.

  fn foo(x: Int):
      var z = x
  
      @__copy_capture(z)
      @parameter
      fn formatter() -> Int:
          return z
      z = 2
      print(formatter())
  
  fn main():
      foo(5)

• String now implements KeyElement and may be used as a key in Dict.

• More robust support for structs with fields of self referencing types. For example, the following code will work and print 0:

  struct Foo(CollectionElement):
      var vec: DynamicVector[Self]
  
      fn __init__(inout self: Self):
          self.vec = DynamicVector[Self]()
  
      fn __moveinit__(inout self: Self, owned existing: Self):
          self.vec = existing.vec ^
  
      fn __copyinit__(inout self: Self, existing: Self):
          self.vec = existing.vec
  
  fn main():
      var foo = Foo()
      print(len(foo.vec))

❌ Removed

• The __takeinit__ special constructor form has been removed from the language. This "non-destructive move" operation was previously wired into the x^ transfer operator, but had unpredictable behavior that wasn't consistent. Now that Mojo has traits, it is better to model this as an explicit .take() operation on a type, which would transfer out the contents of the type without ending its lifetime. For example, for a type that holds a pointer, take() might return a new instance pointing to the same data, and null out its own internal pointer.

  This change makes it clear when a lifetime is ended versus when the contents of an LValue are explicitly taken.

• The current implementation of autotuning has been deprecated, as Mojo's autotuning implementation is undergoing a redesign. Tutorials around the current implementation have also been removed as they are being rewritten.

  Consequently, the autotune(), autotune_fork(), and search() functions have been removed from the standard library.

• The _OldDynamicVector type that worked only on register passable element types has been removed. Please migrate uses to DynamicVector which works on both register passable and memory types.

• The UnsafeFixedVector in utils.vector has been removed. We recommend using either DynamicVector or InlinedFixedVector instead.

• The @adaptive decorator has been removed from the language. Any uses of the decorator in a non-search context can be replaced with @parameter if. For example:

  @adaptive
  fn foo[a: Bool]():
      constrained[a]()
      body1()
  
  @adaptive
  fn foo[a: Bool]():
      constrained[not a]()
      body2()

  Can be rewritten as:

  fn foo[a: Bool]():
      @parameter
      if a:
          body1()
      else:
          body2()

  Consequently, the special __adaptive_set attribute has been removed as well.

• Result parameters have been removed from Mojo. Result parameter declarations in function parameter lists are no longer allowed, nor are forward alias declarations. This includes removing the param_return statement.

• The @noncapturing and @closure decorators have been removed due to refinements and improvements to the closure model. See below for more details!

🦋 Changed

• The Mojo closure model has been refined to be more straightforward and safe. Mojo has two closure types: parameter closures and runtime closures. Parameter closures can be used in higher-order functions and are the backbone of functions like vectorize and parallelize. They are always denoted by @parameter and have type fn() capturing -> T (where T is the return type).

  On the other hand, runtime closures are always dynamic values, capture values by invoking their copy constructor, and retain ownership of their capture state. You can define a runtime closure by writing a nested function that captures values:

  fn outer(b: Bool, x: String) -> fn() escaping -> None:
      fn closure():
          print(x) # 'x' is captured by calling String.__copyinit__
  
      fn bare_function():
          print("hello") # nothing is captured
  
      if b:
          # closure can be safely returned because it owns its state
          return closure^
  
      # function pointers can be converted to runtime closures
      return bare_function

  The type of runtime closures are of the form fn() escaping -> T. You can pass equivalent function pointers as runtime closures.

  Stay tuned for capture list syntax for move capture and capture by reference, and a more unified closure model!

• The @unroll(n) decorator can now take a parameter expression for the unroll factor, i.e. n can be a parameter expression that is of integer type.

• The cpython module in the python package has been moved to be an internal module, i.e, _cpython.

• AnyType and Destructable have been unified into a single trait, AnyType. Every nominal type (i.e. all structs) now automatically conform to AnyType.

• Previously, the mojo package command would output a Mojo package that included both partly-compiled Mojo code, as well as fully-compiled machine code for a specific computer architecture -- the architecture of the machine being used to invoke the mojo package command.

  Now, mojo package only includes partly-compiled Mojo code. It is only fully compiled for the specific computer architecture being used at the point that the package is first import-ed. As a result, Mojo packages are smaller and more portable.

• The simd_width and dtype parameters of polynomial_evaluate have been switched. Based on the request in #1587, the polynomial_evaluate function has also been extended so that the coefficients parameter can take either a either a StaticTuple or a VariadicList.

• As a tiny step towards removing let declarations, this release removes the warning: 'var' was never mutated, consider switching to a 'let'.

🛠️ Fixed

• #1595 - Improve error message when trying to materialize IntLiteral in runtime code.
• Raising an error from the initializer of a memory-only type now works correctly in the presence of complex control flow. Previously Mojo could run the destructor on self before it was initialized when exiting with an error.
• #1096 - Improve warning messages for dead code in conditionals like or expressions.
• #1419 - Fix assertion failure with uninitialized lattice values.
• #1402 - Fix movable trait not detected on recursive struct implemented with AnyPointer.
• #1399 - Fix parser crash when a parameter type in a struct that implements a trait is misspelled.
• #1152 - Allow mutable self argument when overloading operators using dunder methods.
• #1493 - Fix crash in DynamicVector copy constructor in certain situations.
• #1316 - The benchmark.keep function now properly handles vector types.
• #1505 - The simd.shuffle operation now works on 64 element permutations.
• #1355 - Fix String.find() returning wrong value when starting index is non-zero.
• #1367 - Fix String.replace() returning incorrect results for multi-character search strings.
• #1535 - Invalid error field 'w.x.y' destroyed out of the middle of a value, preventing the overall value from being destroyed.
• #1475 - Assertion failure in nested loop.
• #1591 - Assertion failure when using AnyType struct member.
• #1503 - Rename the mojo build of LLDB to mojo-lldb, to prevent name collisions with the system's LLDB.
• #1542 - @unroll does not accept alias as unroll factor.
• #1443 - Compiler crash on variadic list of traits.
• #1604 - Variable of trivial type not destroyed by transferring ownership.
• #1341 - Segmentation fault when passing closures around.
• #217 - Closure state is stack allocated.