The monad laws are simply additional rules that instances are expected to follow, beyond what can be expressed in the type system. Insofar as Monad expresses a programming pattern, the laws are part of that pattern. Such laws apply to other type classes as well: Monoid has very similar rules to Monad, and it’s generally … Read more
As mentioned in the comments, it is frequently desirable to have some abstraction between code and database implementation. You can get much of the same abstraction as a free monad by defining a class for your DB Monad (I’ve taken a couple liberties here): class (Monad m) => MonadImageDB m where indexImage :: (ImageId, UTCTime) … Read more
What is the feature that is missing in the Java type system? How do these other languages declare the Monad type? Good question! Eric Lippert refers to this as higher types, but I can’t get my head around them. You are not alone. But they are actually not as crazy as they sound. Let’s answer … Read more
You can easily enough write a function that calls the readFile action, and passes the result to your index function. readAndIndex fileName = do text <- readFile fileName return $ index text However, the IO monad taints everything that uses it, so this function has the type: readAndIndex :: FilePath -> IO [(String, [Integer])]
All monads in Haskell are only monads if you exclude the weird seq combinator. This is also true for IO. Since seq is not actually a regular function but involves magic, you have to exclude it from checking the monad laws for the same reason you have to exclude unsafePerformIO. Using seq you can prove … Read more
Monad is a concept, an abstract interface if you will, that simply defines a way of composing data. Option supports composition via flatMap, and that’s pretty much everything that is needed to wear the “monad badge”. From a theoretical point of view, it should also: support a unit operation (return, in Haskell terms) to create … Read more
As Michael said, the issue is buffering. By default, output is buffered until you print a newline (or until the buffer is full if you have really long lines), so you’ll most often see this issue when trying to do same-line prompts using putStr like you’re doing. I suggest defining a small helper function like … Read more
Take the following mini-language: data Action = Get (Char -> Action) | Put Char Action | End Get f means: read a character c, and perform action f c. Put c a means: write character c, and perform action a. Here’s a program that prints “xy”, then asks for two letters and prints them in … Read more
To see why you get the particular answers that arise, the desugaring explanations are very helpful. Let me supplement them with a little general advice about developing perceptions of Haskell code. Haskell’s type system makes no distinction between two separable “moral” purposes: [x] the type of values which are lists with elements drawn from x … Read more
Kleisli aka ReaderT is from practical point of view #2 (and as I show later #3) – dependency injection of one the same component into several functions. If I have: val makeDB: Config => IO[Database] val makeHttp: Config => IO[HttpClient] val makeCache: Config => IO[RedisClient] then I could combine things as a monad this way: … Read more