flat-maybe

2024 UPDATE: this repo is unsafe in the sense that it randomly segfaults under certain GHC versions and conditions that I have not investigated in detail. Use it at your own risk, but in most cases you should just not use it! I'm writing this note in 2024 because I see a few people starring the repo and I'd prefer them to not get random segfaults. Below is the README from 2015.

In the Rust programming language, there is a nice optimization for the Option (the analogue of Maybe in Haskell) type: if the Option holds a reference type, then the Option<A> values are represented as a pointer that's either some null-like value or a pointer to the object itself. In contrast, Haskell's Maybe always has at least two indirections:

ptr    
 |
 |
Just | ptr
       |
       |
      tag | data

This package implements the Rust scheme in an absolutely ungodly way. Basically, a Maybe a is either a pointer to an a object, or a pointer to a dummy "null" value that isn't exported from the library. The representation is newtype Maybe a = Maybe Any. We check "constructors" with reallyUnsafePtrEquality#. The result looks like this:

ptr
 |
 |
 tag | data

One rather nasty thing about this solution is that our Just isn't parametrically polymorphic: Just (Just Nothing) immediately collapses to Nothing for any number of intermediate Just-s, but it works normally for any non-Maybe type. A funny consequence is that we can implement the monadic join as unsafeCoerce.

Nasty this might be, the performance seems to be really good, probably the best I've seen for fast error handling.