As ever, the terminology people use is not entirely consistent. There’s a variety of inspired-by-monads-but-strictly-speaking-isn’t-quite notions. The term “indexed monad” is one of a number (including “monadish” and “parameterised monad” (Atkey’s name for them)) of terms used to characterize one such notion. (Another such notion, if you’re interested, is Katsumata’s “parametric effect monad”, indexed by … Read more
The package Control.Monad.Writer does not export the data constructor Writer. I guess this was different when LYAH was written. Using the MonadWriter typeclass in ghci Instead, you create writers using the writer function. For example, in a ghci session I can do ghci> import Control.Monad.Writer ghci> let logNumber x = writer (x, [“Got number: ” … Read more
map exists to simplify operations on lists and for historical reasons (see What’s the point of map in Haskell, when there is fmap?). You might ask why we need a separate map function. Why not just do away with the current list-only map function, and rename fmap to map instead? Well, that’s a good question. … Read more
My suggestion is, if you want to learn Scala, to read the book from Paul Chiusano and Runar Bjarnason: http://manning.com/bjarnason/ Part II: Functional design and combinator libraries Making little languages JSON serialization Specification-based testing Parsers Purely functional parallelism Purely functional state Part III: Functional design patterns The case for abstraction Monoids Functors Monads Applicative functors … Read more
As far as possible, don’t use exceptions for control flow or unexceptional circumstances. But to answer your question directly (assuming all the exception-types are the same): Func<double>[] calcs = { calc1, calc2, calc3 }; foreach(var calc in calcs) { try { return calc(); } catch (CalcException){ } } throw new NoCalcsWorkedException();
If we compare the types (<*>) :: Applicative a => a (s -> t) -> a s -> a t (>>=) :: Monad m => m s -> (s -> m t) -> m t we get a clue to what separates the two concepts. That (s -> m t) in the type of (>>=) … Read more
Don’t be scared! The reader monad is actually not so complicated, and has real easy-to-use utility. There are two ways of approaching a monad: we can ask What does the monad do? What operations is it equipped with? What is it good for? How is the monad implemented? From where does it arise? From the … Read more
Suppose a function has side effects. If we take all the effects it produces as the input and output parameters, then the function is pure to the outside world. So, for an impure function f’ :: Int -> Int we add the RealWorld to the consideration f :: Int -> RealWorld -> (Int, RealWorld) — … Read more
Most of what you do in programming all day is combining some functions together to build bigger functions from them. Usually you have not only functions in your toolbox but also other things like operators, variable assignments and the like, but generally your program combines together lots of “computations” to bigger computations that will be … Read more
A type constructor which is not a Functor: newtype T a = T (a -> Int) You can make a contravariant functor out of it, but not a (covariant) functor. Try writing fmap and you’ll fail. Note that the contravariant functor version is reversed: fmap :: Functor f => (a -> b) -> f a … Read more