Contents
Monads
Introduction
Overview
Core functions
Monadic classes
Do-notation
Pattern matching in do-notation

Introduction

This article provides an overview of monadic programming in Ela.

Overview

Ela like other functional languages does provide a natural support for monadic programming through lambda. However, Ela provides additional support for monads through both library and language. It includes flexible monadic abstractions (exposed through Ela classes) and special syntax support that can simplify monadic programming (a so called do notation).
The high level of an approach to monadic programming is pretty similar to Haskell; syntax used in do notation effectively mimics the one used in Haskell. The monadic class hierarchy, however, is rather different.

Core functions

All monadic classes are currently are not exposed through prelude module, but are included in a dedicated monad module. This module also defines several useful functions, including the well known "bind" function. In order to write monadic code you first need to learn how to use the following three basic functions.
The fist function is >>= (so called "bind"):
m >>= f 
The semantics of this function is pretty trivial - it takes the contents of its first argument (which should be a monad) and passes it to a second argument (which should be a function) as a parameter.
This function obeys the following law which can also help to understand its mechanics:
return a >>= k == k a
The code above, if evaluated, would return true. This means that the two expressions are equivalent and would always fetch the same value.
The function return, used in our code sample, is the second basic function - it just takes a value and puts it in a monadic context:
open monad
return 2 ::: List //The result is: [2]
As you can see this function should be always executed in a context of a monadic type (and linked lists in Ela are monads). In fact the name return is just an alias for a function point and is defined in monad module like so: return = point.
The third function is actually a redundant one. This a a >>- function (Haskell equivalent is >>). It can be defined in terms of >>= function like so:
xs >>- ys = xs >>= (\_ -> ys)
(And this is in fact the way how this function is actually defined in monad module).
Behavior of this function is pretty similar to the standard seq function - it takes the contents of the first argument and simply ignores it, moving on to the second argument.

Monadic classes

Module monad defines the following core monadic classes: Functor, Union and Pointed (there are other classes but these three are the most important). Class Functor contains a single fmap function which is a generalization of map on lists. Class Union contains a single join function which is a generalization of concat on lists. And class Pointed contains a single point function which puts a value into a monadic contexts (or, in a case of lists, "packs" a value in a list). For clarity - this is how these classes are implemented for linked lists:
instance Functor List where
  fmap = List.map

instance Union List where
  join = List.concat

instance Pointed List where
  point x = [x]
In fact function "bind" discusses in a previous section is not a class function (like in Haskell, where this function is a member of Monad class), but is defined through fmap and join like so:
xs >>= f = join (fmap f xs)
And return, as it was already mentioned, is simply an alias for point.

Do-notation

A so called do notation is a special syntax that was added to Ela to simplify writing monadic code. Heavy usage of "bind" operator can lead to a pretty unreadable code; the do notation can help to remove the clutter.
A do notation is a special type of an expression that can be used in all the contexts where expression is valid. It starts with do keyword. The layout for do expression is pretty similar to the one used for where binding - the first entry can be on the same line with do, but other entries should indented after do:
open core

foo = do  
    x <- Some "Is it "
    y <- Some 42
    z <- Some '?'
    Some (x ++ show y ++ z)  

foo //The result is: Some "Is it 42?"
The monadic notation in Ela mimics the one used in Haskell.
It is also possible to use let bindings in do notation in the following way:
do
  putStrLn "What is your height?"
  x <- readAny
  putStrLn "What is your weight?"
  y <- readAny
  let rec = x / y
  putStr (
    if rec > 2.25 then "You're too thin!"
    else if rec < 2 then "You're too heavy!"
    else "You're OK...")
  putStr x
The example above is written using IO monad and console IO actions available in io module.

Pattern matching in do-notation

It is possible to pattern match in bindings in do-notation as in regular bindings. For example, consider the following function:
res s = do
  (x::_) <- Some s
  return x
We can call it like so:
res "Hello" ::: Maybe //The result is: Some 'H'
We can, hovewer, provide an empty string as an argument as well:
res "" ::: Maybe //The result is: None
When we pass an empty string to a res function, a pattern match (x::_) fails. But instead of raising an exception it calls a failure function which is a member of Failure class:
class Failure a where
  failure _->a
A type Maybe implements this class like so:
instance Failure Maybe where
  failure _ = None
As a result a function res above returns None instead of an error.