Are you as convinced as you were of the semantics of pattern synonyms? I can see they are the right semantics given a run-time implementation but the alternative compile-time implementation has a lot to recommend it. What exactly is "the alternative compile-time implementation"? Remember, although pattern synonyms are set up to call a matching function, that matching function is often inlined, which reduces the overhead to zero. I say "often" inlined. An INLINE pragma for pattern synonyms would be a good feature. Widening to ghc-devs. Simon From: Matthew Roberts Sent: 11 February 2019 00:10 To: matthew.pickering@cs.ox.ac.uk; gergo@erdi.hu; Simon Peyton Jones ; rae@cs.brynmawr.edu Subject: Performance of pattern synonyms Hi all, I am working with someone on a compile-time pattern matching extension and the most important prior work is pattern synonyms in Haskell. I hope you might indulge a couple of questions I have not been able to answer myself from the literature: * [In my testing](http://pattern-benchmarks.herokuapp.com/posts/2019-02-09-peano.htmlhttps://nam06.safelinks.protection.outlook.com/?url=http%3A%2F%2Fpattern-benchmarks.herokuapp.com%2Fposts%2F2019-02-09-peano.html&data=02%7C01%7Csimonpj%40microsoft.com%7Cf4d58b9dd66c476d591a08d68fb549cd%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636854406149018023&sdata=uehvVhVMCpNiYWp7Lr2pRLGqVSqt3gQTACl2kvS9crg%3D&reserved=0), pattern synonyms have remarkable performance, has anyone ever benchmarked them before? * Do you have - on hand - the hackage data you used in investigating uses of pattern synonyms? I am looking for real-world usage of pattern matching extensions. * Are you as convinced as you were of the semantics of pattern synonyms? I can see they are the right semantics given a run-time implementation but the alternative compile-time implementation has a lot to recommend it. Thanks for your time, Matt Roberts Department of Computing, Macquarie University