> It's Ruby

Thanks for clarifying; I've read a bunch of Ruby but never written it before ;)

From a quick Google I see that "select" and "map" do work as I thought:

https://ruby-doc.org/core-2.2.0/Array.html#method-i-select

https://ruby-doc.org/core-2.2.0/Array.html#method-i-map

So we have a value called "list", we're calling its "select" method/function and then calling the "map" method/function of that result. That's just function composition; no monads in sight!

To clarify, we can rewrite your example in the following way:

list.select { |x| x.foo > 10 }.map { |x| x.bar }

# Define the anonymous functions/blocks elsewhere, for clarity
list.select(checkFoo).map(getBar)

# Turn methods into standalone functions
map(select(list, checkFoo), getBar)

# Swap argument positions
map(getBar, select(checkFoo, list))

# Curry "map" and "select"
map(getBar)(select(checkFoo)(list))

# Pull out definitions, for clarity
mapper = map(getBar)
selector = select(checkFoo)
mapper(selector(list))

This is function composition, which we could write:

go = compose(mapper, selector)
go(list)

The above argument is based solely on the structure of the code: it's function composition, regardless of whether we're using "map" and "select", or "plus" and "multiply", or any other functions.

To understand why "map" and "select" don't need monads, see below.

> the list could be an eager iterator, an actual list, a lazy iterator, a Maybe (though it would be clumsy in Ruby), etc.

Yes, that's because all of those things are functors (so we can "map" them) and collections (so we can "select" AKA filter them).

The interface for monad requires a "wrap" method (AKA "return"), which takes a single value and 'wraps it up' (e.g. for lists we return a single-element list). It also requires either a "bind" method ("concatMap" for lists) or, my preference, a "join" method ("concat" for lists).

I can show that your example doesn't involve any monads by defining another type which is not a monad, yet will still work with your example.

I'll call this type a "TaggedList", and it's a pair containing a single value of one type and a list of values of another type. We can implement "map" and "select" by applying them to the list; the single value just gets passed along unchanged. This obeys the functor laws (I encourage you to check this!), and whilst I don't know of any "select laws" I think we can say it behaves in a reasonable way.

In Haskell we'd write something like this (although Haskell uses different names, like "fmap" and "filter"):

data TaggedList t1 t2 = T t1 [t2]

instance Functor (TaggedList t1) where
map f (T x ys) = T x (map f ys)

instance Collection (TaggedList t1) where
select f (T x ys) = T x (select f ys)

In Ruby we'd write something like:

class TaggedList
def initialize(x, ys)
@x = x
@ys = ys
end

def map(f)
TaggedList.new(@x, @ys.map(f))
end

def select(f)
TaggedList.new(@x, @ys.select(f))
end
end

This type will work for your example, e.g. (in pseudo-Ruby, since I'm not so familiar with it):

myTaggedList = TaggedList.new("hello", [{foo: 1, bar: true}, {foo: 20, bar: false}])
result = myTaggedList.select { |x| x.foo > 10 }.map { |x| x.bar }

# This check will return true
result == TaggedList.new("hello", [false])

Yet "TaggedList" cannot be a monad! The reason is simple: there's no way for the "wrap" function (AKA "return") to know which value to pick for "@x"!

We could write a function which took two arguments, used one for "@x" and wrapped the other in a list for "@ys", but that's not what the monad interface requires.

Since Ruby's dynamically typed (AKA "unityped") we could write a function which picked a default value for "@x", like "nil"; yet that would break the monad laws. Specifically:

bind(m, wrap) == m

If "wrap" used a default value like "nil", then "bind(m, wrap)" would replace the "@x" value in "m" with "nil", and this would break the equation in almost all cases (i.e. except when "m" already contained "nil").