As 24 Days of Hackage comes to a close, I see no better library to review than
base is a library that all Haskell users will be familiar with, as it defines the
Prelude, along with a variety of other useful modules.
base could be considered to be a bare-bones standard library for Haskell, and while the
Prelude is fairly extensive, there’s actually a lot of other useful functionality that is provided.
Some of my favourite constructs lie in the
Control.Concurrent namespace. Concurrency is a pretty hard problem, but provided with good tools it doesn’t have to be a painful experience. The
Control.Concurrent.Chan module provides “unbounded” channels, which give you a tiny little message queue that runs in memory.
echo :: Chan String -> IO () echo c = forever $ readChan c >>= putStrLn main :: IO () main = do c <- newChan forkIO (echo c) forever $ getLine >>= writeChan c
main we create a new channel, then fork a separate thread to consume messages from this channel. Finally, in the original thread we write messages into the channel from user input. This gives us a concurrent application, with two threads communicating almost transparently.
Concurrency is not all
base has to offer, and recently I’ve been increasingly intrested in
Data.Monoid, after reading Brent Yorgey’s excellent Monoids: Themes and Variations paper. Monoids aren’t particularly complicated beasts, they just have an “empty” element, and an operation to combine two values into one larger value. This can be really nice when combined with tuples, as you can have some really expressive transformations. For example, we can use a variety of monoids to find the sum and product of a list of integers:
stats :: [Int] -> (Sum Int, Product Int) stats = mconcat . map (\x -> (Sum x, Product x))
After a while, this has started to feel like a really natural way to aggregate data. Combined with
semigroups, you can be extremely expressive in very little typing.
If you’ve been following these blog posts, then it’ll be no surprise that I love
Data.Foldable are also great modules. They provide a few more abstractions, and a whole bunch of combinators which can make working with these common data structures an absolute breeze. Combinators like
unless frequently appear in my code, and the combinatros in
Data.Traversable are extremely handy. I commonly have
Maybe values that I want to apply an IO action to:
lookupEnv "HOME" >>= traverse putStrLn
traverse is a lot more powerful than this - you only need to look at recent papers, or the
lens library to see what I mean.
All in all,
base is fairly spartan, but still provides a lot of power. It’s not quite the Python standard library, but that was never it’s aim - we have the Haskell platform for that!
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