Skip to content


Switch branches/tags

Latest commit


Git stats


Failed to load latest commit information.
Latest commit message
Commit time

README for fgg

This fgg package is a minimal and basic prototype of Featherweight Go and Featherweight Generic Go, as presented in:

Featherweight Go
Robert Griesemer, Raymond Hu, Wen Kokke, Julien Lange, Ian Lance Taylor,
Bernardo Toninho, Philip Wadler and Nobuko Yoshida

Currently, many aspects of the code are quite primitive, mainly for the convenience of quick experimentation alongside the above paper. For example, types/functions/variables are not well named, except for some correspondence with the formal definitions. The tool is also not particularly user-friendly:

  • it offers only the small (but meaningful) subset of Go as formalised in the paper;
  • it does not support any syntactic sugar -- e.g., empty parentheses and type lists, and various separators (;), all need to be written out explicitly;
  • most type errors are reported as panics, though an error message may be given at the top of the stack trace.

We plan to improve some of this in the near future. Contact Raymond Hu for issues related to this repository.

See this Go blog post for information about the generics design draft by the Go team, and links to their generic-to-ordinary Go translation tool (including an online playground) based on that draft.

  • Currently, the fgg tool supports a few features (within its fragment of Go) that their translation tool does not. These include type parameters for methods (for now, the latter has type parameters for types and functions only), nomono (monomorphisability) checking, and covariant method receiver bounds.

Featherweight-go-gen is tool that enumerates FGG programs and integrates with fgg for testing.


This package includes:

  • An FG static type checker and interpreter.
  • An FGG static type checker and interpreter.
  • An FGG static nomono (i.e., "is/not monomorphisable") checker and FGG-to-FG monomorphiser.

Package organisation:

  • Makefile -- install, and for running tests and examples.
  • main.go -- main file.
  • fg -- FG AST, typing and evaluation.
  • fgg -- FGG AST, typing, evaluation, nomono check, and monomorphisation.
  • examples
    • examples/fg -- FG examples.
      • examples/fg/oopsla20 -- The FG examples from the paper (i.e., Figs. 1, 2 and 7).
    • examples/fgg -- FGG examples.
      • examples/fgg/oopsla20 -- The FGG examples from the paper (i.e., Figs. 3--6).
  • parser -- FG/FGG parsers (generated using ANTLR).
    • parser/FG.g4 -- FG ANTLR grammar.
    • parser/FGG.g4 -- FGG ANTLR grammar.

Use the main branch for the latest working version.

Syntax. The best source would be the formal grammars in the paper, or else see the above ANTLR grammars.


We assume a standard Go set up. That is:

  • Go (version 1.11+);
  • a Go workspace, at $GOPATH;
  • a src directory in the workspace.

You will also need the ANTLR v4 runtime for Go; e.g., see "Installing ANTLR v4" in this tutorial.

Clone the fgg repo into the src directory of your Go workspace, i.e., $GOPATH/src. It should end up located at src/ You can also use go get -- this should fetch ANTLR for you, but will report that it cannot find the parser packages (that's fine, we generate them in the next step).

Next, either copy over the pre-generated parser files and install by

  • make install-pregen-parser
    (generated using ANTLR 4.7.1)

or generate the parsers yourself using ANTLR and install by

  • (assuming some suitable antlr4 command; e.g., java -jar [antlr-4.7.1-complete.jar])
    antlr4 -Dlanguage=Go -o parser/fg parser/FG.g4
    antlr4 -Dlanguage=Go -o parser/fgg parser/FGG.g4

Finally, do:

  • make install

To test the install -- inside the directory, this command should work:

  • go run -eval=-1 -v examples/fg/oopsla20/fig1/functions.go

Afer installing, you can also use the resulting fgg binary directly instead of go run.

This package has been tested using Go version 1.11.5+ on:

  • MacOS Catalina
  • WSL2/Windows 10

A note on syntax.

Two points:

  • FG and FGG always need explicit ; separators between type/method decls, field decls, etc., even across new lines. E.g., in FG

    package main;
    type A struct {};
    type B struct {
      f1 A;
      f2 A
    type IA interface {
      m1() B;
      m2() B
    func (x0 B) foo() B { return x0 };
    func main() { _ = B{A{}, A{}}.foo() }

    A rule of thumb is, write all FG/FGG code as if you were writing Go without line breaks.

  • FGG does not support any syntactic sugar -- this means empty type declarations and type argument lists must always be written out in full. E.g., the FGG equivalent to the above is:

    package main;
    type A(type ) struct {};
    type B(type ) struct {
      f1 A();
      f2 A()
    type IA(type ) interface {
      m1(type )() B();
      m2(type )() B()
    func (x0 B(type )) foo(type )() B() { return x0 };
    func main() { _ = B(){A(){}, A(){}}.foo()() }

FG/FGG program syntax.

This package implements the grammars defined in the paper. E.g., a basic FGG program has the form:

package main;
/* Type and method decls -- semicolon separated */
func main () { _ = /* main has this specific form */ }

For testing purposes, the package supports this additional form:

package main;
import "fmt";  // Only fmt is allowed
/* Type and method decls -- semicolon separated */
func main () { fmt.Printf("%#v", /* This specific Printf, and only in main */ ) }


  • This package additionally supports interface embedding for both FG and FGG. E.g.,
    type A(type a Any()) interface { B(a) } // Any, B are interfaces

  • The var declarations used for readability in some of the examples in the paper are not supported.

Example run commands.

Warning: Type checking and nomono errors raise a panic -- basic error messages can be found at the top of the stack trace.

The following commands can be run from the directory.

  • FG type check and evaluate, with verbose printing.

    go run -eval=-1 -v examples/fg/oopsla20/fig1/functions.go

    • The argument to -eval is the number of steps to execute. -1 means run to termination (either a value, or a panic due to a failed type assertion.)
    • -eval includes a dynamic type preservation check (an error raises a panic).
  • FGG type check and evaluate, with verbose printing. (Note the -fgg flag.)

    go run -fgg -eval=-1 -v examples/fgg/oopsla20/fig4/functions.fgg

  • FGG type check, nonomo check and monomorphisation, with verbose printing.

    go run -fgg -monomc=-- -v examples/fgg/oopsla20/fig4/functions.fgg

    • The argument to -monomc is a file location for the FG output. -- means print the output.
  • Simulate FGG against its FG monomorphisation, with verbose printing.

    go run -test-monom -v examples/fgg/oopsla20/fig4/functions.fgg

    • This includes dynamic checking of type preservation checking at both levels, and of the monomorphisation correspondence at every evaluation step.

Example Makefile tests.

The following commands assume make install, and that the resulting fgg binary is on the $PATH.

Running from the directory:

  • make test-monom-against-go

    • Type checks and evaluates a series of FGG programs;
    • nomono checks and monomorphises each to an FG program;
    • type checks and evaluates the FG program using fgg;
    • compiles and executes the FG program using go;
    • compares the results from fgg and go.
  • make simulate-monom

    • Simulates a series of FGG programs against their FG monomorphisations.
  • make test-all

    • Run all tests.