Nim/changelog.md

v2.2.0 - yyyy-mm-dd

Changes affecting backward compatibility

• -d:nimStrictDelete becomes the default. An index error is produced when the index passed to system.delete was out of bounds. Use -d:nimAuditDelete to mimic the old behavior for backwards compatibility.
• The default user-agent in std/httpclient has been changed to Nim-httpclient/<version> instead of Nim httpclient/<version> which was incorrect according to the HTTP spec.
• Methods now support implementations based on a VTable by using --experimental:vtables. Methods are then confined to be in the same module where their type has been defined.
• With -d:nimPreviewNonVarDestructor, non-var destructors become the default.
• A bug where tuple unpacking assignment with a longer tuple on the RHS than the LHS was allowed has been fixed, i.e. code like: nim var a, b: int (a, b) = (1, 2, 3, 4) will no longer compile.

• internalNew is removed from system, use new instead.

• bindMethod in std/jsffi is deprecated, don't use it with closures.

• JS backend now supports lambda lifting for closures. Use --legacy:jsNoLambdaLifting to emulate old behavior.

• owner in std/macros is deprecated.

• Ambiguous type symbols in generic procs and templates now generate symchoice nodes. Previously; in templates they would error immediately at the template definition, and in generic procs a type symbol would arbitrarily be captured, losing the information of the other symbols. This means that generic code can now give errors for ambiguous type symbols, and macros operating on generic proc AST may encounter symchoice nodes instead of the arbitrarily resolved type symbol nodes.

Standard library additions and changes

• Changed std/osfiles.copyFile to allow to specify bufferSize instead of a hardcoded one.
• Changed std/osfiles.copyFile to use POSIX_FADV_SEQUENTIAL hints for kernel-level aggressive sequential read-aheads.

• std/htmlparser has been moved to a nimble package, use nimble or atlas to install it.

• Added newStringUninit to system, which creates a new string of length len like newString but with uninitialized content.

• Added setLenUninit to system, which doesn't initialize slots when enlarging a sequence.

• Added hasDefaultValue to std/typetraits to check if a type has a valid default value.

• Added Viewport API for the JavaScript targets in the dom module.

• Added toSinglyLinkedRing and toDoublyLinkedRing to std/lists to convert from openArrays.

• ORC: To be enabled via nimOrcStats there is a new API called GC_orcStats that can be used to query how many objects the cyclic collector did free. If the number is zero that is a strong indicator that you can use --mm:arc instead of --mm:orc.

• A $ template is provided for Path in std/paths.

• std/hashes.hash(x:string) changed to produce a 64-bit string Hash (based on Google's Farm Hash) which is also often faster than the present one. Define nimStringHash2 to get the old values back. --jsbigint=off mode always only produces the old values. This may impact your automated tests if they depend on hash order in some obvious or indirect way. Using sorted or OrderedTable is often an easy workaround.

• Deprecates system.newSeqUninitialized, which is replaced by newSeqUninit.

Language changes

• noInit can be used in types and fields to disable member initializers in the C++ backend.
• C++ custom constructors initializers see https://nim-lang.org/docs/manual_experimental.html#constructor-initializer
• member can be used to attach a procedure to a C++ type.

• C++ constructor now reuses result instead creating this.

• Tuple unpacking changes:

  • Tuple unpacking assignment now supports using underscores to discard values. nim var a, c: int (a, _, c) = (1, 2, 3)
  • Tuple unpacking variable declarations now support type annotations, but only for the entire tuple. nim let (a, b): (int, int) = (1, 2) let (a, (b, c)): (byte, (float, cstring)) = (1, (2, "abc"))

• The experimental option --experimental:openSym has been added to allow captured symbols in generic routine and template bodies respectively to be replaced by symbols injected locally by templates/macros at instantiation time. bind may be used to keep the captured symbols over the injected ones regardless of enabling the option, but other methods like renaming the captured symbols should be used instead so that the code is not affected by context changes.

Since this change may affect runtime behavior, the experimental switch openSym, or genericsOpenSym and templateOpenSym for only the respective routines, needs to be enabled; and a warning is given in the case where an injected symbol would replace a captured symbol not bound by bind and the experimental switch isn't enabled.

  const value = "captured"
  template foo(x: int, body: untyped): untyped =
    let value {.inject.} = "injected"
    body

  proc old[T](): string =
    foo(123):
      return value # warning: a new `value` has been injected, use `bind` or turn on `experimental:openSym`
  echo old[int]() # "captured"

  template oldTempl(): string =
    block:
      foo(123):
        value # warning: a new `value` has been injected, use `bind` or turn on `experimental:openSym`
  echo oldTempl() # "captured"

  {.experimental: "openSym".} # or {.experimental: "genericsOpenSym".} for just generic procs

  proc bar[T](): string =
    foo(123):
      return value
  assert bar[int]() == "injected" # previously it would be "captured"

  proc baz[T](): string =
    bind value
    foo(123):
      return value
  assert baz[int]() == "captured"

  # {.experimental: "templateOpenSym".} would be needed here if genericsOpenSym was used

  template barTempl(): string =
    block:
      foo(123):
        value
  assert barTempl() == "injected" # previously it would be "captured"

  template bazTempl(): string =
    bind value
    block:
      foo(123):
        value
  assert bazTempl() == "captured"

This option also generates a new node kind nnkOpenSym which contains exactly 1 nnkSym node. In the future this might be merged with a slightly modified nnkOpenSymChoice node but macros that want to support the experimental feature should still handle nnkOpenSym, as the node kind would simply not be generated as opposed to being removed.

Compiler changes

• --nimcache using a relative path as the argument in a config file is now relative to the config file instead of the current directory.

Tool changes

• koch now allows bootstrapping with -d:nimHasLibFFI, replacing the older option of building the compiler directly w/ the libffi nimble package in tow.