Here’s an even simpler answer: A Monad is something that “computes” when monadic context is collapsed by join :: m (m a) -> m a (recalling that >>= can be defined as x >>= y = join (fmap y x)). This is how Monads carry context through a sequential chain of computations: because at each … Read more
What are some common uses for this language? Rapid application development. If you want to know “why Haskell?”, then you need to consider advantages of functional programming languages (taken from https://c2.com/cgi/wiki?AdvantagesOfFunctionalProgramming): Functional programs tend to be much more terse than their ImperativeLanguage counterparts. Often this leads to enhanced programmer productivity FP encourages quick prototyping. As … Read more
I talk a bit about this in Engineering Large Projects in Haskell and in the Design and Implementation of XMonad. Engineering in the large is about managing complexity. The primary code structuring mechanisms in Haskell for managing complexity are: The type system Use the type system to enforce abstractions, simplifying interactions. Enforce key invariants via … Read more
I’m going to order this guide by the level of skill you have in Haskell, going from an absolute beginner right up to an expert. Note that this process will take many months (years?), so it is rather long. Absolute Beginner Firstly, Haskell is capable of anything, with enough skill. It is very fast (behind … Read more
The $ operator is for avoiding parentheses. Anything appearing after it will take precedence over anything that comes before. For example, let’s say you’ve got a line that reads: putStrLn (show (1 + 1)) If you want to get rid of those parentheses, any of the following lines would also do the same thing: putStrLn … Read more
That particular phrasing is by James Iry, from his highly entertaining Brief, Incomplete and Mostly Wrong History of Programming Languages, in which he fictionally attributes it to Philip Wadler. The original quote is from Saunders Mac Lane in Categories for the Working Mathematician, one of the foundational texts of Category Theory. Here it is in … Read more
The horizontal bar means that “[above] implies [below]”. If there are multiple expressions in [above], then consider them anded together; all of the [above] must be true in order to guarantee the [below]. : means has type ∈ means is in. (Likewise ∉ means “is not in”.) Γ is usually used to refer to an … Read more
First: The term monad is a bit vacuous if you are not a mathematician. An alternative term is computation builder which is a bit more descriptive of what they are actually useful for. They are a pattern for chaining operations. It looks a bit like method chaining in object-oriented languages, but the mechanism is slightly … Read more