Bindings

Introduction

This article discusses bindings (similar to variable declarations in imperative languages) in Ela.

Overview

Unlike imperative languages Ela doesn't have a concept of variables. Instead like other functional languages Ela has a concept of binding. Binding basically binds an expression to a given name. Unlike variables which can change their values through the code bindings are always immutable. There is no support for mutable variables in Ela. Also Ela even doesn't have a so called assignment operator. An operator = which you can see in bindings is a special form that is only used to bind an expression to a name:

x = 2
x = 3 //Not a valid Ela code!

Function definitions however can have several bodies, if a function is defined by pattern matching, e.g.:

x /. 0 = 0
x /. y = x `div` y

We have implemented a safe division operator that has a special entry for the case when a second argument is zero.

Another important difference from imperative languages is that the order of bindings in Ela is not significant. This is true for functions and for all other bindings as well. For example, the following code is valid:

fun x = x + y
y = 12

The order in which bindings are evaluated is implementation defined and you shouldn't rely on it in your code. However, top level in Ela can also contain expressions, and expressions are always evaluated after all bindings:

fun x = x + y
fun 4
y = 12

In the program above a result of evaluation fun 4 is not ignored and a number 16 is returned as a result of this program.

Bindings in Ela, like in many other, languages have a lexical scoping. Global bindings in a module are by default included in the export list and can be referenced from other modules.

Another important thing to remember about bindings in Ela is that names used in bindings should never start with a capital letter. The following code is not a name binding:

Foo = 42

This is because identifiers that start with a capital letter are used for constructor symbols.

Global Bindings

Global bindings have the following syntax:

pattern = expr

//Guarded equations
pattern "|" guard  = expr { "|" guard = expr }
        "|" "else" = expr

The most common case of pattern is an identifier, however, it can be any supported pattern:

x = 2
(x::xs) = [1..10]
(Foo x,(1,[y,z])) = (Foo 42,(1,[3,4]))
_ = () //Here we simply ignore the value of right-hand expression

One can also use guards in bindings:

y = 0
x | y > 0 = y
  | else  = 0

This code is equivalent to the following:

x = if y > 0 then y else 0

An else clause is mandatory. The rest of the syntax is pretty similar to the one used in Haskell.

Also bindings (global and local) can have an optional header. A header is used to specify attributes (such as private or qualified) for a binding. The header is only valid for bindings where the head symbol is an identifier:

sum # private
sum x y = x + y

In the example below we have declared a global sum function and marked it as private (so it won't be included in the module export list).

Local Bindings

Local bindings can be declared using two constructs - let/in and where. Binding made using let/in have the following syntax:

"let" pattern = expr "in" expr

//Guarded equations
"let" pattern "|" guard  = expr { "|" guard = expr }
              "|" "else" = expr
 "in" expr

The syntax is similar to the one used for global bindings except of a mandatory let keyword and in clause. The in clause should contain an expression in which (and only in which) the binding will be visible, e.g.:

let x = 2 in x*2
y = x //Error ELA302: Name 'x' is not defined

Bindings done using let/in are effectively expressions and always yield a value - this value is a result of evaluation of an expression inside in clause.

These bindings can appear both at top level and in local levels.

Bindings done using where have the following syntax:

expr
 "where" pattern = expr

//Guarded equations
expr
 "where" pattern "|" guard  = expr { "|" guard = expr }
                 "|" "else" = expr

The syntax is similar to the let bindings, however, where bindings follow mathematical notation and are placed after an expression in which they are used. Also a where clause should be indented further than an expression in which it is used (at least by one space).

These bindings also always yield a value which is a value of evaluation of the expression preceeding where clause.

A simple example of a where binding:

x 
 where x = 2

Which is completely equivalent to:

let x = 2 in x

Mutually Recursive Bindings

As it was mentioned before, all bindings in Ela are mutually recursive - including local bindings and top level bindings. No additional keywords or declaration constructs are required:

take (x::xs) = x :: skip xs
take []      = []

skip (_::xs) = take xs 
skip []      = []

Binding Attributes

Currently Ela supports the following attributes in binding headers:

Attribute	Description
private	A binding is private and not included in the module export list (valid only on global bindings).
qualified	A binding will not be imported automatically and should be always qualified with a module alias.
nowarnings	Suppress all warnings for the function.