Version 0.18.0 released

The Nim team is happy to announce that the latest release of Nim, version 0.18.0, is now available. Nim is a systems programming language that focuses on performance, portability and expressiveness.

Installing 0.18.0

If you have installed a previous version of Nim using choosenim, getting Nim 0.18.0 is as easy as:

$ choosenim update stable

If you don’t have it already, you can get choosenim by following these instructions or you can install Nim manually by following the instructions on our install page. Note that Windows users might still have an easier time with the unzip+finish.exe method of installing Nim which is described on the install page.

What’s new in 0.18.0

It’s been a while since our last release, but we’ve been busy. At over 1000 commits since the last release, v0.18.0 is one of (if not the) biggest releases of Nim ever.

We have mentioned multiple times that this will be a major release. Primarily because our goal has been to clean up the standard library significantly in preparation for v1.0. We’ve made massive progress in this release to achieve that goal. The downside is that this release has a much larger proportion of breaking changes than normal releases, but it is all in the spirit of a cleaner Nim v1.0.

Breaking changes to be mindful of

You will find that the [] indexing operator which takes a slice now raises an IndexError exception when it is out of bounds. Previously it would have simply returned the part of the string that was captured. You may find that your program hasn’t taken this into account and will now crash with an IndexError exception. To get back the previous behaviour, use the substr procedure. Here is an example of the issue and how to get around it:

var myString = "hello world"
doAssertRaises IndexError:
  discard myString[6 .. 45]
doAssert myString.substr(6, 45) == "world"

The $ operator now handles collections differently. For example:

doAssert $(@["23", "12"]) == "@[\"23\", \"12\"]"

# Whereas in 0.17.2:
doAssert $(@["23", "12"]) == "@[23, 12]"

An array[x, char] can no longer be converted to a cstring type. This has the side-effect of allowing $ to be defined on an array:

var x: array[5, char]

doAssert $x == r"['\x00', '\x00', '\x00', '\x00', '\x00']"

Be sure to check out the changelog below for a comprehensive list of breaking changes. If you run into any strange regressions feel free to pop into IRC, Gitter, Discord or any of the other chat rooms/forums we have available. Links to them all are available in our community page.

New features

Of course, this being a major new release, there are also plenty of new features.

Formatted string literals

The greatest new addition is the strformat module. It implements string formatting that is very similar to Python 3’s f-strings.

Formatted string literals are prefixed with fmt or the & operator:

import strformat

let name = "Fred"
let age = 24
let weight = 94.52

doAssert fmt"My name is {name}. I'm {age} years of age and weigh {weight:.3f}." ==
            "My name is Fred. I'm 24 years of age and weigh 94.520."

For more information, take a look at the strformat documentation.

Testable documentation examples

The documentation generator can now test your examples for you! This is done using the new runnableExamples macro.

proc add*(x, y: int): int =
  ## Adds two numbers together.
  ## Examples:
    doAssert add(5, 5) == 10
    doAssert add(-5, 2) == -3

  x + y

If you save this as addLib.nim and then generate its documentation via nim doc addLib.nim, you should see something like this:

Hint: /tmp/addLib_examples  [Exec]
Hint: operation successful (13129 lines compiled; 0.492 sec total; 19.742MiB peakmem; Debug Build) [SuccessX]

The code under runnableExamples will be embedded in the procedure’s documentation.

If your examples have an error in them then you will see it, together with a stack trace if your error occurs at runtime. This is great for ensuring that your documentation is up-to-date.


This is a new macro that allows you to easily create array and sequence literals.

import sequtils

let x = mapLiterals([12, 34, 15, 1], uint32)
doAssert x is array[4, uint32]

New memory manager algorithm

The new TLSF algorithm has been implemented to reduce memory fragmentation. This has a side effect of making alloc and dealloc O(1) operations.

Async improvements

There have been a number of various improvements to the asynchronous IO modules in the stdlib. There is no longer an “upcoming” and standard asyncdispatch split, the former was merged and is now the default asyncdispatch implementation.

A new getIoHandler procedure is now available, it returns a handle to the underlying IO completion port or epoll/kqueue fd used by asyncdispatch. Main advantage of this is that libraries now have even greater control over the event loop. As an example, it allows much more efficient HTTP server implementations (one such implementation is called httpbeast).

A new implementation of async await has also been added for the JavaScript backend in the asyncjs module.

Nimble v0.8.10

In the release are also updates to the Nim package manager. The latest version of Nimble is included and contains multiple new features and bug fixes.

The major new feature is the support for multiple Nimble packages in a single Git/Hg repository.

Check out the changelog for a list of changes since Nimble’s last release.

Contributors to v0.18.0

Our contributors are amazing, and there is far too many to list here. Big thanks to all of you, we couldn’t have pulled off this release without you!


Changes affecting backwards compatibility

Breaking changes in the standard library

  • The [] proc for strings now raises an IndexError exception when the specified slice is out of bounds. See issue #6223 for more details. You can use substr(str, start, finish) to get the old behaviour back, see this commit for an example.

  • strutils.split and strutils.rsplit with an empty string and a separator now returns that empty string. See issue #4377.

  • Arrays of char cannot be converted to cstring anymore, pointers to arrays of char can! This means $ for arrays can finally exist in system.nim and do the right thing. This means $myArrayOfChar changed its behaviour! Compile with -d:nimNoArrayToString to see where to fix your code.

  • reExtended is no longer default for the re constructor in the re module.

  • The behavior of $ has been changed for all standard library collections. The collection-to-string implementations now perform proper quoting and escaping of strings and chars.

  • newAsyncSocket taking an AsyncFD now runs setBlocking(false) on the fd.

  • mod and bitwise and do not produce range subtypes anymore. This turned out to be more harmful than helpful and the language is simpler without this special typing rule.

  • formatFloat/formatBiggestFloat now support formatting floats with zero precision digits. The previous precision = 0 behavior (default formatting) is now available via precision = -1.

  • Moved from stdlib into Nimble packages:
  • Proc toCountTable now produces a CountTable with values corresponding to the number of occurrences of the key in the input. It used to produce a table with all values set to 1.

    Counting occurrences in a sequence used to be:

    let mySeq = @[1, 2, 1, 3, 1, 4]
    var myCounter = initCountTable[int]()
    for item in mySeq: item

    Now, you can simply do:

      mySeq = @[1, 2, 1, 3, 1, 4]
      myCounter = mySeq.toCountTable()
  • If you use --dynlibOverride:ssl with OpenSSL 1.0.x, you now have to define openssl10 symbol (-d:openssl10). By default OpenSSL 1.1.x is assumed.

  • newNativeSocket is now named createNativeSocket.

  • newAsyncNativeSocket is now named createAsyncNativeSocket and it no longer raises an OS error but returns an osInvalidSocket when creation fails.

  • The securehash module is now deprecated. Instead import std / sha1.

  • The readPasswordFromStdin proc has been moved from the rdstdin to the terminal module, thus it does not depend on linenoise anymore.

Breaking changes in the compiler

  • \n is now only the single line feed character like in most other programming languages. The new platform specific newline escape sequence is written as \p. This change only affects the Windows platform.

  • The overloading rules changed slightly so that constrained generics are preferred over unconstrained generics. (Bug #6526)

  • We changed how array accesses “from backwards” like a[^1] or a[0..^1] are implemented. These are now implemented purely in system.nim without compiler support. There is a new “heterogeneous” slice type system.HSlice that takes 2 generic parameters which can be BackwardsIndex indices. BackwardsIndex is produced by system.^. This means if you overload [] or []= you need to ensure they also work with system.BackwardsIndex (if applicable for the accessors).

  • The parsing rules of if expressions were changed so that multiple statements are allowed in the branches. We found few code examples that now fail because of this change, but here is one:

    t[ti] = if exp_negative: '-' else: '+'; inc(ti)

    This now needs to be written as:

    t[ti] = (if exp_negative: '-' else: '+'); inc(ti)
  • The experimental overloading of the dot . operators now take an untyped parameter as the field name, it used to be a static[string]. You can use when defined(nimNewDot) to make your code work with both old and new Nim versions. See special-operators for more information.

  • yield (or await which is mapped to yield) never worked reliably in an array, seq or object constructor and is now prevented at compile-time.

Library additions

  • Added sequtils.mapLiterals for easier construction of array and tuple literals.

  • Added system.runnableExamples to make examples in Nim’s documentation easier to write and test. The examples are tested as the last step of nim doc.

  • Implemented getIoHandler proc in the asyncdispatch module that allows you to retrieve the underlying IO Completion Port or Selector[AsyncData] object in the specified dispatcher.

  • For string formatting / interpolation a new module called strformat has been added to the stdlib.

  • The ReadyKey type in the selectors module now contains an errorCode field to help distinguish between Event.Error events.

  • Implemented an accept proc that works on a SocketHandle in nativesockets.

  • Added algorithm.rotateLeft.

  • Added typetraits.$ as an alias for

  • Added system.getStackTraceEntries that allows you to access the stack trace in a structured manner without string parsing.

  • Added parseutils.parseSaturatedNatural.

  • Added macros.unpackVarargs.

  • Added support for asynchronous programming for the JavaScript backend using the asyncjs module.

  • Added true color support for some terminals. Example:

    import colors, terminal
    const Nim = "Efficient and expressive programming."
      fg = colYellow
      bg = colBlue
      int = 1.0
    for i in 1..15:
      styledEcho bgColor, bg, fgColor, fg, Nim, resetStyle
      int -= 0.01
      fg = intensity(fg, int)
    setForegroundColor colRed
    setBackgroundColor colGreen
    styledEcho "Red on Green.", resetStyle

Library changes

  • echo now works with strings that contain \0 (the binary zero is not shown) and nil strings are equal to empty strings.

  • JSON: Deprecated getBVal, getFNum, and getNum in favour of getBool, getFloat, getBiggestInt. A new getInt procedure was also added.

  • rationals.toRational now uses an algorithm based on continued fractions. This means its results are more precise and it can’t run into an infinite loop anymore.

  • os.getEnv now takes an optional default parameter that tells getEnv what to return if the environment variable does not exist.

  • The random procs in random.nim have all been deprecated. Instead use the new rand procs. The module now exports the state of the random number generator as type Rand so multiple threads can easily use their own random number generators that do not require locking. For more information about this rename see issue #6934

  • writeStackTrace is now proclaimed to have no IO effect (even though it does) so that it is more useful for debugging purposes.

  • db_mysql module: DbConn is now a distinct type that doesn’t expose the details of the underlying PMySQL type.

  • parseopt2 is now deprecated, use parseopt instead.

Language additions

  • It is now possible to forward declare object types so that mutually recursive types can be created across module boundaries. See package level objects for more information.

  • Added support for casting between integers of same bitsize in VM (compile time and nimscript). This allows to, among other things, reinterpret signed integers as unsigned.

  • Custom pragmas are now supported using pragma pragma, please see language manual for details.

  • Standard library modules can now also be imported via the std pseudo-directory. This is useful in order to distinguish between standard library and nimble package imports:

    import std / [strutils, os, osproc]
    import someNimblePackage / [strutils, os]

Language changes

  • The unary < is now deprecated, for .. < use ..< for other usages use the pred proc.

  • Bodies of for loops now get their own scope:

    # now compiles:
    for i in 0..4:
      let i = i + 1
      echo i
  • To make Nim even more robust the system iterators .. and countup now only accept a single generic type T. This means the following code doesn’t die with an “out of range” error anymore:

    var b = 5.Natural
    var a = -5
    for i in a..b:
      echo i
  • atomic and generic are no longer keywords in Nim. generic used to be an alias for concept, atomic was not used for anything.

  • The memory manager now uses a variant of the TLSF algorithm that has much better memory fragmentation behaviour. According to the maximum fragmentation measured is lower than 25%. As a nice bonus alloc and dealloc became O(1) operations.

  • The compiler is now more consistent in its treatment of ambiguous symbols: Types that shadow procs and vice versa are marked as ambiguous (bug #6693).

  • codegenDecl pragma now works for the JavaScript backend. It returns an empty string for function return type placeholders.

  • Extra semantic checks for procs with noreturn pragma: return type is not allowed, statements after call to noreturn procs are no longer allowed.

  • Noreturn proc calls and raising exceptions branches are now skipped during common type deduction in if and case expressions. The following code snippets now compile:
    import strutils
    let str = "Y"
    let a = case str:
      of "Y": true
      of "N": false
      else: raise newException(ValueError, "Invalid boolean")
    let b = case str:
      of nil, "": raise newException(ValueError, "Invalid boolean")
      elif str.startsWith("Y"): true
      elif str.startsWith("N"): false
      else: false
    let c = if str == "Y": true
      elif str == "N": false
        echo "invalid bool"
        quit("this is the end")
  • Pragmas now support call syntax, for example: {.exportc"myname".} and {.exportc("myname").}

  • The deprecated pragma now supports a user-definable warning message for procs.

    proc bar {.deprecated: "use foo instead".} =

Tool changes

  • The nim doc command is now an alias for nim doc2, the second version of the documentation generator. The old version 1 can still be accessed via the new nim doc0 command.

  • Nim’s rst2html command now supports the testing of code snippets via an RST extension that we called :test:::

    .. code-block:: nim
      # shows how the 'if' statement works
      if true: echo "yes"


  • Fixed “ReraiseError when using try/except within finally block” (#5871)
  • Fixed “Range type inference leads to counter-intuitive behvaiour” (#5854)
  • Fixed “JSON % operator can fail in extern procs with dynamic types” (#6385)
  • Fixed ““intVal is not accessible” in VM” (#6083)
  • Fixed “Add excl for OrderedSet” (#2467)
  • Fixed “newSeqOfCap actually doesn’t reserve memory” (#6403)
  • Fixed “[Regression] Nim segfaults” (#6435)
  • Fixed “Seq assignment is slower than expected” (#6433)
  • Fixed “json module issues with empty dicts and lists” (#6438)
  • Fixed “mingw installed via finish.exe fails to link if Nim located in path with whitespace” (#6452)
  • Fixed “unittest.check does not perform short-circuit evaluation” (#5784)
  • Fixed “Error while concatenating an array of chars.” (#5861)
  • Fixed “range initialization: [ProveInit] hint: Cannot prove that” (#6474)
  • Fixed “scanf can call procs with side-effects multiple times” (#6487)
  • Fixed “gcsafe detection problem” (#5620)
  • Fixed “C++ codegen: mitems generates invalid code.” (#4910)
  • Fixed “strange runtime behavior on macOS” (#6496)
  • Fixed “stdtmpl: invalid indentation after a line ending in question mark” (#5070)
  • Fixed “Windows: NAN troubles on c backend” (#6511)
  • Fixed “lib/nim/system/cellsets.nim(33, 31) Error: type mismatch while attempting to compile for 16bit CPUs” (#3558)
  • Fixed “Can’t compile dynlib with -d:useNimRtl and --threads:on” (#5143)
  • Fixed “var s = @[0,1,2,…] can generate thousand of single assignments in C code” (#5007)
  • Fixed “echo discards everything after a null character” (#1137)
  • Fixed “Turn off reExtended by default” (#5627)
  • Fixed “Bad Links in docs/backends.html” (#5914)
  • Fixed “Index out of bounds error in db_postgres when executing non parameter-substituted queries containing “?”” (#6571)
  • Fixed “Please add pipe2 support to posix stdlib” (#6553)
  • Fixed “Return semantics vary depending on return style” (#6422)
  • Fixed “ reports SIGSEGV when calling ‘open’ on missing file” (#6148)
  • Fixed “VCC: Nim generates non-compilable code for system.nim” (#6606)
  • Fixed “Generic subtype matches worse than a generic” (#6526)
  • Fixed “formatFloat inconsistent scientific notation” (#6589)
  • Fixed “Generated c code calls function twice” (#6292)
  • Fixed “Range type inference leads to counter-intuitive behvaiour” (#5854)
  • Fixed “New backward indexing is too limited” (#6631)
  • Fixed “Table usage in a macro (SIGSEGV: Illegal storage access.)” (#1860)
  • Fixed “Incorrect deprecation error” (#6634)
  • Fixed “Wrong indices in arrays not starting with 0” (#6675)
  • Fixed “if expressions” (#6609)
  • Fixed “BackwardsIndex: converter + [] + unrelated type[^1]: lib/system.nim(3536, 3) Error” (#6692)
  • Fixed “js backend 0.17.3: array bounds check for non zero based arrays is buggy” (#6532)
  • Fixed “HttpClient’s new API doesn’t work through a proxy for https URLs” (#6685)
  • Fixed “isServing isn’t declared and isn’t compiling” (#6707)
  • Fixed “[Regression] value out of range” (#6710)

  • Fixed “Error when using multisync macro” (#6708)

  • Fixed “formatFloat inconsistent scientific notation” (#6589)
  • Fixed “Using : (constructor arguments) for passing values to functions with default arguments causes a compiler crash.” (#6765)
  • Fixed “In-place object initialization leads to vcc incompatible code” (#6757)
  • Fixed “Improve parseCookies doc” (#5721)
  • Fixed “Parser regression with nested do notation inside conditional” (#6166)
  • Fixed “Request for better error message” (#6776)
  • Fixed “Testament tester does not execute test with exitcode only” (#6775)
  • Fixed “JS integer division off by one” (#6753)
  • Fixed “Regression: cannot prove not nil” (#5781)
  • Fixed “SIGSEGV: Illegal storage access. (Attempt to read from nil?) in generic proc” (#6073)
  • Fixed “Request for better error message” (#6776)
  • Fixed “Nim #head: sorting via reference hangs compiler” (#6724)
  • Fixed “Cannot cast pointer to char in cpp” (#5979)
  • Fixed “asynchttpserver replies with several errors on single request” (#6386)
  • Fixed “object variants superclass trigger bad codegen” (#5521)
  • Fixed “JS integer division off by one” (#6753)
  • Fixed “js backend compiler crash with tables indexed by certain types” (#6568)
  • Fixed “Jsgen bug with is” (#6445)
  • Fixed “Subrange definition with ..<” (#6788)
  • Fixed “fields not initialized: array with enum index type as object field.” (#6682)
  • Fixed “Can not delete data in table when table’s data type is kind of “not nil”” (#6555)
  • Fixed “tables.nim: Cannot prove that ‘n’ is initialized” (#6121)
  • Fixed “issues with ‘not nil’ applied to a closure proc” (#6489)
  • Fixed “not nil not working in some cases” (#4686)
  • Fixed “Cannot prove ‘@[v]’ is not nil” (#3993)

  • Fixed “Feature: support TCP_NODELAY in net.sockets” (#6795)
  • Fixed “Code that makes the compiler throw an error message and then hangs” (#6820)
  • Fixed “Code that makes the compiler throw an error message and then hangs” (#6820)
  • Fixed “Inconsistent behavior with sequence and string slicing” (#6223)
  • Fixed “Wrong behavior of “split” (proc and iterator)” (#4377)
  • Fixed “[Documentation] Invalid module name: [foo, bar]” (#6831)
  • Fixed “The destructor is not called for temporary objects” (#4214)
  • Fixed “Destructors does not work with implicit items iterator in for loop” (#985)
  • Fixed “Error in template when using the type of the parameter inside it” (#6756)
  • Fixed “should respect parent attributes?” (#5856)
  • Fixed “json ‘to’ macro can not marshalize into tuples” (#6095)
  • Fixed “ fails with seq[T]” (#6604)
  • Fixed “ is not worth using compared to[T]” (#5848)
  • Fixed “Memory not being released in time, running out of memory” (#6031)
  • Fixed “[Regression] Bad C codegen for generic code” (#6889)
  • Fixed “rollingFileLogger deletes file on every start.” (#6264)
  • Fixed “Remove/deprecate securehash module.” (#6033)
  • Fixed “[bug or not] object construction for seq[T] failed without space after colon” (#5999)
  • Fixed “issues with the random module” (#4726)
  • Fixed “Reassigning local var to seq of objects results in nil element in Object’s seq field” (#668)
  • Fixed “Compilation error with “newseq[string]”” (#6726)
  • Fixed “await inside array/dict literal produces invalid code - Part 2” (#6626)
  • Fixed “terminal.eraseline() gives OverflowError on Windows” (#6931)
  • Fixed “[Regression] sequtils.any conflicts with system.any” (#6932)
  • Fixed “C++ codegen: mitems generates invalid code.” (#4910)
  • Fixed “seq.mitems produces invalid cpp codegen” (#6892)
  • Fixed “Concepts regression” (#6108)
  • Fixed “Generic iterable concept with array crashes compiler” (#6277)
  • Fixed “C code generation “‘a’ is a pointer; did you mean to use ‘->’?”” (#6462)
  • Fixed “--NimblePath fails if a - in path which is not followed by a number” (#6949)
  • Fixed ““not registered in the selector” in asyncfile.close() for something that clearly was registered” (#6906)
  • Fixed “strange frexp behavior” (#6353)

  • Fixed “noreturn branches of case statements shouldn’t contribute to type” (#6885)
  • Fixed “Type inference for ‘if’ statements changed” (#6980)
  • Fixed “newest asyncdispatch recursion” (#6100)
  • Fixed “Ambiguous identifier between set type and proc” (#6965)

  • Fixed “Inconsistent behavior with sequence and string slicing” (#6223)

  • Fixed “Unsupported OpenSSL library imported dynamically” (#5000)
  • Fixed “nim check segfaults” (#6972)
  • Fixed “GC deadlock” (#6988)
  • Fixed “Create a seq without memory initialization” (#6401)
  • Fixed “Fix bug for getch on Windows while using the arrow keys” (#6966)
  • Fixed “runnableExamples doesn’t work in templates” (#7018)
  • Fixed “runnableExamples doesn’t work with untyped statement blocks” (#7019)

  • Fixed “Critical bug in parseBiggestFloat” (#7060)
  • Fixed “[RFC] strformat.% should be gone” (#7078)
  • Fixed “compiler crash on simple macro” (#7093)
  • Fixed “Make newlines sane again” (#7089)
  • Fixed “JS - Unicode enum string representation issue” (#6741)
  • Fixed “Strange behaviour of 0.17.3 (working ok in 0.17.2)” (#6989)
  • Fixed “Strange behaviour of 0.17.3 (working ok in 0.17.2)” (#6989)
  • Fixed “Compiler crash: try expression with infix as” (#7116)
  • Fixed “nimsuggest crash” (#7140)
  • Fixed “[RFC] Reintroduce readChar” (#7072)
  • Fixed “Copyright line needs updating” (#7129)
  • Fixed “-0.0 doesn’t result in negative zero in VM” (#7079)
  • Fixed “Windows large filesize” (#7121)
  • Fixed “Securehash is not parsimonious with MD5 and other hash modules” (#6961)
  • Fixed “os.findExe() shouldn’t look in current directory on posix, unless exe has a /” (#6939)
  • Fixed “compiles(...) with fatal pragma causes compiler to exit early” (#7080)
  • Fixed “NPE when compile macro that returns concrete value” (#5450)
  • Fixed “Using a variable of type int | float causes internal compiler error” (#6946)
  • Fixed “Unsigned integers could not be used as array indexes.” (#7153)
  • Fixed “countdown with uint causes underflow” (#4220)
  • Fixed “Inconsistent method call syntax” (#7200)
  • Fixed “Impossible to create an empty const array” (#6853)
  • Fixed “Strange UINT handling” (#3985)
  • Fixed “Bad codegen when passing arg that is part of return value destination” (#6960)
  • Fixed “No info about gcsafety in error message when global var is accessed in async proc” (#6186)
  • Fixed “Resolving package vs. local import ambiguities” (#2819)
  • Fixed “Internal error with type() operator” (#3711)
  • Fixed “newAsyncSocket should raise an OS error plus other inconsistencies” (#4995)
  • Fixed “mapLiterals fails with negative values” (#7215)
  • Fixed “fmWrite doesn’t truncate file with openAsync, unlike open()” (#5531)
  • Fixed “Move libsvm to an external nimble module” (#5786)
  • Fixed “Prevent acceptAddr gotcha with newSocket” (#7227)
  • Fixed “strtabs.getOrDefault is inconsistent with tables.getOrDefault” (#4265)

  • Fixed “Code falling through into exception handler when no exception thrown.” (#7232)
  • Fixed “the new generic inference rules are broken” (#7247)
  • Fixed “Odd ..< regression” (#6992)
  • Fixed “Different proc type inferred from default parameter” (#4659)
  • Fixed “Different proc type inferred from default parameter” (#4659)
  • Fixed “Testament sometimes ignores test failures” (#7236)
  • Fixed “New Allocator Fails On >=4GB Requests” (#7120)
  • Fixed “User pragmas hide effect specifications from sempass2” (#7216)
  • Fixed “C++: SIGABRT instead of IndexError for out-of-bounds” (#6512)
  • Fixed “An uncaught exception in cpp mode doesn’t show the exception name/msg” (#6431)