Comments for Martijn's Journal

Comment on Sneeuw at Warande by arne

arne — Tue, 22 Dec 2009 16:26:55 +0000

Awesome! Ik wil ook sneeuw

Comment on Midwinter Fair 2009 by Sjoerd Visscher

Sjoerd Visscher — Sun, 13 Dec 2009 23:05:06 +0000

Foto #13 is mijn favoriet. Erg mooie compositie en betoverende kleuren.

Comment on GADTs in Haskell 98 by Bob Atkey

Bob Atkey — Fri, 13 Nov 2009 12:28:15 +0000

Unfortunately, not all GADTs can be expressed faithfully in this way, due to “leakage” between the types used for representation and the type parameters in the constructors. An example is given in Section 4 of the paper “Unembedding domain-specific languages” (available from my home page), where we called them “exotically typed terms”. The example in the paper involves Higher-Order Abstract Syntax, but it still happens when you have no negative occurrences of the type variable: a simple example is a GADT for representing typed S and K combinators with application. The problem seems to occur when there is a type parameter inside a GADT term that does not appear on the outside of the term.

Fortunately, all is not lost (if you allow yourself Haskell98 + Rank-2 polymorphism + existential types). One can still represent the Equality GADT as a type class (with single method refl :: f a a), and then use Neil Ghani and Patty Johann’s decomposition of GADTs into normal algebraic datatypes plus the equality GADT. Oleg did this in OCaml in a post to the OCaml mailing list. A disadvantage of this approach is that you have to apply the type equalities by hand.

Comment on GADTs in Haskell 98 by Chung-chieh Shan

Chung-chieh Shan — Fri, 13 Nov 2009 08:45:36 +0000

Jacques Carette and Oleg Kiselyov and I wrote up some more things that one can do (and have been done) in this way: “Finally
tagless, partially evaluated: Tagless staged interpreters for simpler
typed languages.” Journal of Functional Programming 19(5):509-543.

Comment on Myst III On Leopard Intel by Gary

Gary — Fri, 13 Nov 2009 02:39:33 +0000

Ray and Martijn, I can confirm that this procedure does not work with the DVD version I have included with Myst IV Revelation, sadly. After dragging the data and M3T folders into the disk image, the Exile app crashes with a JPEG error, suggesting an issue with the latest Quicktime version. I don’t think it will be easily solved.
Even the OSX Exile installer on the DVD does not work

Gary

Comment on GADTs in Haskell 98 by Brent Yorgey

Brent Yorgey — Thu, 12 Nov 2009 17:58:08 +0000

Aha, I never realized you could encode GADTs as type classes like this! Simple once you see it. Thanks for sharing!

Comment on GADTs in Haskell 98 by jj

jj — Thu, 12 Nov 2009 14:08:10 +0000

What I like about GADTs is not what they let us do, but what they stop us from doing. The standard GADT example you show makes it impossible to produce an ill-typed Term (not term, Term) without using error or naughty recursion. Using the type class TermC, you have to either give up pattern matching or type safety.

Comment on Transforming polymorphic values by Martijn’s Journal » GADTs in Haskell 98

Martijn’s Journal » GADTs in Haskell 98 — Thu, 12 Nov 2009 11:27:34 +0000

[...] Transforming polymorphic values [...]

Comment on Transforming polymorphic values by Linktipps #17 :: Blackflash

Linktipps #17 :: Blackflash — Sun, 25 Oct 2009 13:22:20 +0000

[...] [...]

Comment on Transforming polymorphic values by Martijn

Martijn — Tue, 20 Oct 2009 07:24:55 +0000

@Brent thanks for the feedback! Ah, I didn’t know it had a name already. Thanks for that.

@augustss Can you elaborate a bit? I hope this simplification doesn’t hurt in the context of simple arithmetic expressions?