Left-fold enumerators: a safe, expressive and efficient I/O interface for Haskell

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Johan Tibell, from Google, visited Galois on Monday 15th September, and gave a tech talk about efficient IO in Haskell, based on left-fold enumerators, along with a demo of his high perf. Haskell webserver, Hyena, based on left-fold IO. (.pdf slides).For more on left-fold, resource-managing IO subsystems, see Oleg Kiselyov’s work on the best collection traversal interface, From enumerators to cursors: turning the left fold inside out and Peng Li’s work on massively threaded Haskell web servers based on epoll. This stuff is quite hot, with Oleg coincidentally talking about left folds for web servers next week, at DEFUN.

Photo of roomLeft-fold IO


I will describe a programming style for I/O operations that is based on left-fold enumerators. This style of programming is more expressive than imperative style I/O represented by the Unix functions read and write, and safer than lazy I/O using streams. Left-fold enumerators offers both high-performance using block based I/O and safety in terms of error handling and resource usage. I will demonstrate Hyena, a web server prototype written in Haskell, as an example of left-fold enumerator style of programming.This talk is intended as a starting point for further discussions on what would be a good interface for I/O rather than a presentation of finished research.

The talk was held at 1pm, on Monday 15th September, at Galois, in Portland.