It’s interesting that so many people independently pick the name IfNotNull, for this in C# – it must be the most sensible name possible! 🙂 Earliest one I’ve found on SO: Possible pitfalls of using this (extension method based) shorthand My one (in ignorance of the above): Pipe forwards in C# Another more recent example: … Read more
This works because using lists as monads in Haskell makes them model indeterminism. Consider: sequence [[1,2],[3,4]] By definition this is the same as: do x <- [1,2] y <- [3,4] return [x,y] Just read it as “First a choice between 1 and 2, then a choice between 3 and 4”. The list monad will now … Read more
A quick disclaimer: I’m a little shaky on category theory in general, while I get the impression you have at least some familiarity with it. Hopefully I won’t make too much of a hash of this… Does anyone know of a tutorial that starts from category theory and explains IO, state, list monads in those … Read more
Use liftIO You’re already very close! Your suggestion newtype Op a = Op {runOp :: ST -> IO (ST, a)} is excellent and the way to go. To be able to execute getLine in an Op context, you need to ‘lift’ the IO operation into the Op monad. You can do this by writing a … Read more
Haskell has immutable variables (variables in the math sense) by default: foo x y = x + y * 2 By default variables are not mutable cells. Haskell also has mutable cells though, but you enable them explicitly: > import Data.IORef (newIORef, readIORef, writeIORef) > v <- newIORef 0 > readIORef v 0 > writeIORef … Read more
If you’re willing to use Scalaz, it has a handful of tools that make this kind of task more convenient, including a new Validation class and some useful right-biased type class instances for plain old scala.Either. I’ll give an example of each here. Accumulating errors with Validation First for our Scalaz imports (note that we … Read more
I think you have a fundamental misunderstanding about IO in Haskell. Particularly, you say this: Maybe there is a function which can convert from ‘IO String’ to [Char]? No, there isn’t1, and the fact that there is no such function is one of the most important things about Haskell. Haskell is a very principled language. … Read more
It’s almost too trivial to mention, but the first problem is that with isn’t a function and doesn’t take a function as an argument. You can easily get around this by writing a function wrapper for with: def withf(context, f): with context as x: f(x) Since this is so trivial, you could not bother to … Read more
The asynchronous programming model in C# is very similar to asynchronous workflows in F#, which are an instance of the general monad pattern. In fact, the C# iterator syntax is also an instance of this pattern, although it needs some additional structure, so it is not just simple monad. Explaining this is well beyond the … Read more
What alternatives are there to monads for I/O in a pure functional language? I’m aware of two alternatives in the literature: One is a so-called linear type system. The idea is that a value of linear type must be used exactly one time: you can’t ignore it, and you can’t use it twice. With this … Read more