State of -XImpredicativeTypes

Simon Peyton Jones simonpj at
Wed Feb 19 13:48:24 UTC 2014

ImpredicativeTypes is not properly finished.  When I first implemented it I implemented a fairly ambitious design based on "boxy types" (see paper with that in the title).  But it proved unsustainably complicated, both in the implementation and indeed for programmers to reason about which programs should be accepted and which should not.

So I took most of it out.  There are some vestiges but to a first approximation it isn't really there at all.

My plan is to do something along the lines of QML (again, look at the paper), plus add explicit type application.  But there are always too many other things to do.

This is swampy territory, and I have spent more time on it that I care to tell you without producing a design that I am satisfied with.  So while I would be very happy if someone wants to start helping, you do need a good grasp of type inference first.  It's not a project to learn on.

However the *internal* intermediate language, Core, is fully impredicative and always has been.  No difficulties there.


| -----Original Message-----
| From: Glasgow-haskell-users [mailto:glasgow-haskell-users-
| bounces at] On Behalf Of Merijn Verstraaten
| Sent: 19 February 2014 12:07
| To: glasgow-haskell-users at
| Subject: State of -XImpredicativeTypes
| Lectori salutem,
| What is the actual state of ImpredicativeTypes? It appears documented as
| a "properly" finished GHC extension, but on IRC and other places I keep
| hearing it's poorly tested, buggy or incomplete. Is this true or just
| FUD?
| Cheers,
| Merijn

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