ezyang’s blog

the arc of software bends towards understanding

August, 2011

8 ways to report errors in Haskell revisited

In 2007, Eric Kidd wrote a quite popular article named 8 ways to report errors in Haskell. However, it has been four years since the original publication of the article. Does this affect the veracity of the original article? Some names have changed, and some of the original advice given may have been a bit... […]

  • August 29, 2011

Joseph and the Amazing Technicolor Box

Consider the following data type in Haskell: data Box a = B a How many computable functions of type Box a -> Box a are there? If we strictly use denotational semantics, there are seven: But if we furthermore distinguish the source of the bottom (a very operational notion), some functions with the same denotation […]

  • August 23, 2011

Diskless Paxos crash recovery

This is an edited version of an email I sent last week. Unfortunately, it does require you to be familiar with the original Paxos correctness proof, so I haven’t even tried to expand it into something appropriate for a lay audience. The algorithm is probably too simple to be in the literature, except maybe informally […]

  • August 14, 2011

First impressions of module programming

During my time at Jane Street, I’ve done a fair bit of programming involving modules. I’ve touched functors, type and module constraints, modules in modules, even first class modules (though only peripherally). Unfortunately, the chapter on modules in Advanced Topics in Types and Programming Languages made my eyes glaze over, so I can’t really call […]

  • August 5, 2011

In-program GC stats redux

Hac Phi was quite productive (since I managed to get two blog posts out of it!) On Saturday I committed a new module GHC.Stats to base which implemented a modified subset of the API I proposed previously. Here is the API; to use it you’ll need to compile GHC from Git. Please test and let […]

  • August 3, 2011

Changes to IntMap

As it stands, it is impossible to define certain value-strict operations on IntMaps with the current containers API. The reader is invited, for example, to try efficiently implementing map :: (a -> b) -> IntMap a -> IntMap b, in such a way that for a non-bottom and non-empty map m, Data.IntMap.map (\_ -> undefined) […]

  • August 1, 2011