Add Intercalate

So, we have joinWith that joins the strings of the second argument separated by the first argument. Which only work on array of string. What if i need to work with different container? Can we abstract this? Yes, by constraints it with Foldable and Monoid.

The type signature would be:

intercalate :: (Foldable f, Monoid m) => m -> f m -> m

[safareli]

@davidchambers Reader e a describes computation which produces some value of type a if you give it access to environment of type e. This description looks like a function and it’s actually implemented using a function (type Reader e a = Reader (e -> a)).

Here are some videos about it:

• in this talk(Dead-Simple Dependency Injection) @runarorama starts with moving from Dependency injection to Inversion of control (aka passing an argument) and then gradually implements Reader and finally moves on Free monad.
• and this is small introduction from @drboolean about Reader.

[masaeedu]

Ok. I actually need to add them all at once because intercalate is defined in terms of the other two, but I’ll start by only exposing intercalate.