Unleashed Pascal

Unleashed Pascal is a community-driven fork of Free Pascal that explores modern language features the official compiler chose not to adopt. Upstream is deliberately conservative - which has its value; this fork is where the experimental ideas get to run. Everything turns on with one line:

{$mode unleashed}

The mode is based on objfpc; every modeswitch-gated feature can also be enabled individually in objfpc / delphi via {$modeswitch X}. Backward compatibility is the one hard rule: everything is opt-in, and existing Pascal code keeps compiling untouched. The compiler always defines UNLEASHED, so a shared source tree can {$ifdef} its way between stock FPC and Unleashed; the mode itself defines FPC_UNLEASHED (and not FPC_OBJFPC).

Already sold, don't care about the tour? Yeah yeah, just scroll me straight down to the install steps.

Sixty seconds of Unleashed Pascal

A complete program. Every commented line is a separate feature, and all of them ship today.

program taste;

{$mode unleashed}

uses Classes, SysUtils;

// statement expression: if as a value
function fib(n: integer): int64;
begin
  result := if n < 2 then n else fib(n-1)+fib(n-2);
end;

// tuple return type
function minMax(const a: array of integer): (lo, hi: integer);
begin
  result := (a[0], a[0]);
  for var v in a do begin // inline loop variable
    if v < result.lo then result.lo := v;
    if v > result.hi then result.hi := v;
  end;
end;

var hits: integer;

begin
  var nums := [34, 7, 23, 32, 5, 62];              // inferred array of LongInt
  var (lo, hi) := minMax(nums);                    // tuple destructuring
  writeln($'min = {lo}, max = {hi}');              // string interpolation

  var f := async fib(32);                          // background thread, one keyword

  for parallel var i := 1 to 1000 do               // loop body across all cores
    if fib(15) = 610 then lock(hits) do hits += 1; // zero-setup critical section

  writeln($'fib(32) = {await f}, hits = {hits}');

  var log := autofree TStringList.Create;          // freed at scope exit, no finally
  log.Add($'finished at {Now:hh:nn:ss}');
  write(log.Text);
  {$ifdef WINDOWS}readln;{$endif}
end.

What is in the box

Every feature has a full reference page in unleashed/docs/ - the complete grammar, semantics, edge cases, IDE notes, and a runnable demo for each.

Concurrency without ceremony

Threads in stock Pascal mean TThread subclasses, hand-managed critical sections, and unit-level threadvar. Each collapses into a keyword.

Feature	One-liner	Docs
async / await	var f := async work(x); ... writeln(await f); - one thread per async, one join per await	async-await.md
for parallel	for parallel var i := 1 to N do ... - loop body across a worker pool, with a barrier	parallelfor.md
lock / trylock	lock(counter) do inc(counter); - hidden critical section, auto init and release	lock.md
threadstatic	threadstatic next := 1000; - a per-thread variable, init once per thread	thread-static.md

Write less, say more

Feature	One-liner	Docs
Inline Variables	var x := 42; at the point of use, with type inference and block scope	inline-vars.md
Statement Expressions	var s := if x > 0 then 'pos' else 'neg'; - if / case / try yield values	statement-expressions.md
match	match cmd of 'go': run; _: skip; end; - first-match dispatch over strings, tuples, conditions	match.md
Tuples	function pair: (integer, integer); then var (a, b) := pair;	tuples.md
String Interpolation	writeln($'Hello {name}, pi = {pi:%.2f}');	string-interpolation.md
for ... step	for var i := 1 to 10 step 2 do ...	forstep.md
Multi-Var Init	var a, b, c: integer = 42; - one initializer, independent copies	multi-var-init.md
SwapValues()	SwapValues(a, b); - bitwise swap, no uses, no refcount churn	swapvalues.md
Type()	var tmp: Type(a[0]); - static type of an expression, operand unevaluated	type-intrinsic.md
128-bit Integers	var x: UInt128 := 340282366920938463463374607431768211455;	int128.md

Records and memory layout

Feature	One-liner	Docs
Composable Records	embed TBase;, union ... end;, bitpacked record of byte, OffsetOf(T.field) - C-struct layout, Pascal safety	composable-records.md
Flexible Array Members	data: array[] of byte; as the last field - C99-style variable tail	flexible-arrays.md
Static Variables	static cnt: integer = 0; - program lifetime, block scope	static-section.md
Scoped Cleanup	defer x.Free;, var x := autofree T.Create;, with var http := autofree ...	autofree.md
zeroinit	procedure foo; zeroinit; - zero every local at entry	zeroinit.md
Aligned Heap	GetMemAligned(size, 64) and friends in system	introduced-functions.md

Classes and generics

Feature	One-liner	Docs
Auto-Properties	property Name: string; synthesizes a backing field - zero overhead	auto-properties.md
Nested Generic Methods	function pair<U>(...) inside TBox<T> - class and method specialize independently	extra-improvements.md
Implicit Generics + Type Helpers	TList<integer> without generic / specialize; type helper for integer; multiple helpers at once	extra-improvements.md

Your binary, your rules

Feature	One-liner	Docs
Strip RTTI	{$modeswitch striprtti} empties type-name strings; expose / --rttiexpose= whitelist	strip-rtti.md
Custom Binary Metadata	fpc --fpcsignature="" --linkerversion=14.39 --osversion=Win11	binary-metadata.md
File Embedding	{$embedbytes preset 'default.bin'} bakes a file into the binary	embed.md

Strings and literals

Feature	One-liner	Docs
Multiline Strings	backtick `...` (byte-exact) and triple-quote '''...''' (auto-trimmed)	multiline-strings.md
Numeric Underscores	1_000_000, $FFFFFFFF_FFFFFFFF	extra-improvements.md
String-to-Ordinal Cast	dword('RIFF') folds to a constant in native endianness	extra-improvements.md

Quality-of-life tweaks

Semantic adjustments and small syntax unlocks - full catalog in tweaks.md and extra-improvements.md.

• Preserved For-Loop Counter - after for i := 1 to N do ... break; the counter keeps its value.
• is not / not in - if obj is not TFoo then, if x not in [a, b] then.
• Compound Assignment - i div= 2;, flags xor= $05; in every mode; += and friends without {$coperators on}; both work on properties, as do inc() / dec().
• Array Size Shorthand - array[10] of T means array[0..9] of T.
• Array Equality - if a = b then compares two arrays element by element.
• Indexed and Lazy Labels - label state[0..4]; goto state[n]; compiles to a case dispatch; goto done; needs no prior declaration.
• goto, Macros, C-Operators without Directives - all implied by the mode.
• DEBUG / RELEASE Defines - set automatically from -g: react to a debug build with {$ifdef DEBUG}, no -dDEBUG needed.

Detailed documentation

Each feature has a dedicated reference page with the full grammar, semantics, edge cases, IDE notes, and a runnable demo. Start at the index: unleashed/docs/README.md.

Installation

Option 1: Official installer (recommended)

Self-contained GUI installer for Windows and Linux: it downloads sources, builds the compiler and the Lazarus IDE into a directory of your choice, optionally installs cross compilers, and drops a desktop shortcut. No PATH changes, no registry side effects, no overwriting of an existing FPC. Run it again any time to add a cross target or pull an update - it applies only that change and leaves the rest of the install alone.

• Repo: unleashedpascal/installer
• Downloads: installer/releases - tagged stable releases plus a rolling nightly that tracks main
  • installer_win64_x86_64.exe - Windows host
  • installer_linux_x86_64 / installer_linux_x86_64.AppImage - Linux host

Run it, pick a directory (default C:\unleashed on Windows, $HOME/unleashed on Linux), tick the cross targets you want, click Install.

Option 2: Fresh install via fpcupdeluxe

1. Download fpcupdeluxe and run it once to generate fpcup.ini.
2. Add to fpcup.ini:

[ALIASfpcURL]
unleashed.git=https://github.com/unleashedpascal/compiler.git

[ALIASlazURL]
unleashed.git=https://github.com/unleashedpascal/ide.git

3. Reopen fpcupdeluxe, uncheck GitLab, and select unleashed.git as both the FPC version and the Lazarus version.
4. In Setup+ tick Docked Lazarus IDE (required - the default window layout is tuned for docked mode).
5. Click Install/update FPC+Lazarus. Cross-compilers are on the Cross tab.

Option 3: Upgrade an existing fpcupdeluxe setup

Delete or rename the fpcsrc folder in your installation directory, then:

git clone https://github.com/unleashedpascal/compiler.git fpcsrc

In fpcupdeluxe: Setup+, check FPC/Laz rebuild only, confirm, then click Only FPC.

Unleashed IDE

For the best experience, use Unleashed Pascal IDE - a fork of the Lazarus IDE that understands unleashed mode end to end. Its parser, code completion, and Code Insight know the new syntax, so inline vars, tuples, match, async / await, and the rest autocomplete and navigate like any built-in construct, with no false-positive error markers in the editor.

Stock Lazarus still builds and runs Unleashed projects fine, but its Code Insight does not recognize the new syntax and will flag it as errors. If you stay on stock Lazarus, enable the mode through -Munleashed in the project's Custom Options rather than {$mode unleashed} in the source - that keeps the editor's background parser quieter about the constructs it cannot model.

Contributing

Unleashed Pascal is a home for innovation. If you have built a language feature that did not find a home upstream, this is where it belongs.

We are looking for new language ideas - propose modeswitches, syntax extensions, or compiler enhancements via GitHub Issues or Discussions; a well-described idea with clear use cases is valuable even without an implementation - and complete, high-quality implementations with production-grade code, test coverage, and documentation.

We are not looking for minor convenience patches, trivial reformats, or tweaks that only scratch a personal itch. Every change to a compiler carries weight; contributed code should be a meaningful feature or fix that benefits the broader community.