Pugs

Pugs (Perl User's Golfing System) is a Perl 6 compiler written in Haskell. It was started in 2005 by a sole developer and soon attracted more people to the team. Pugs was the most sophisticated compiler of its time. It was difficult and very time consuming to compile the project, the execution speed was low, but the quality of the compilation and the coverage of Perl 6 specification were outstanding.

Pugs main role in Perl 6 as of today is the vast test suite. It was created to test Pugs itself, but turned out to be an official test suite for Perl 6. A compiler that claims to call itself a Perl 6 compiler must pass the test suite tests.

Pugs are not developing anymore, but its source code is available on GitHub at https://github.com/perl6/Pugs.hs.