This is the example that convinced me to learn Haskell (and boy am I glad I did). — program to copy a file — import System.Environment main = do –read command-line arguments [file1, file2] <- getArgs –copy file contents str <- readFile file1 writeFile file2 str OK, it’s a short, readable program. In that sense … Read more
Sorry, I don’t really know my math, so I’m curious how to pronounce the functions in the Applicative typeclass Knowing your math, or not, is largely irrelevant here, I think. As you’re probably aware, Haskell borrows a few bits of terminology from various fields of abstract math, most notably Category Theory, from whence we get … Read more
We know the type of id, id :: a -> a And when we specialize this for id id, the left copy of id has type: id :: (a -> a) -> (a -> a) And then when you specialize this again for the leftmost id in id id id, you get: id :: ((a … Read more
I would like to make an answer to draw attention to augustss’s comment: That’s not actually how it happens. What happened was that the type of map was generalized to cover Functor in Haskell 1.3. I.e., in Haskell 1.3 fmap was called map. This change was then reverted in Haskell 1.4 and fmap was introduced. … Read more
Not the solution to your problem, but a hint to the cause: Haskell seems to be very conservative in memory reuse and when the interpreter sees the potential to reclaim a memory block, it goes for it. Your problem description fits the minor GC behavior described here (bottom) https://wiki.haskell.org/GHC/Memory_Management. New data are allocated in 512kb … Read more
This is a great question! Here’s a first cut. Be able to log at multiple levels (ex: debug, warning, etc.). hslogger is easily the most popular logging framework. Be able to collect and share metrics/statistics about the types of work the program is doing and how long that work is taking. Ideally, the collected metrics … Read more
Edit: As of GHC 7.6, readMaybe is available in the Text.Read module in the base package, along with readEither: http://hackage.haskell.org/packages/archive/base/latest/doc/html/Text-Read.html#v:readMaybe Great question! The type of read itself isn’t changing anytime soon because that would break lots of things. However, there should be a maybeRead function. Why isn’t there? The answer is “inertia”. There was a … Read more
traverse is the same as fmap, except that it also allows you to run effects while you’re rebuilding the data structure. Take a look at the example from the Data.Traversable documentation. data Tree a = Empty | Leaf a | Node (Tree a) a (Tree a) The Functor instance of Tree would be: instance Functor … Read more
In GHCi 7.x or below, you need a let to define things in it. Prelude> let f x = x * 2 Prelude> f 4 8 Starting from GHC 8.0.1, top-level bindings are supported in GHCi, so OP’s code will work without change. GHCi, version 8.0.1.20161213: http://www.haskell.org/ghc/ 😕 for help Prelude> f x = x … Read more
The short answer is, magic :-). This is to say that automatic deriving is baked into the Haskell spec, and every compiler can choose to implement it in its own way. There’s lots of work on how to make it extensible however. Derive is a tool for Haskell to let you write your own deriving … Read more