Basics

Defining Modules

As a first step, let's understand the Eta modules and packages at a higher-level.

An Eta package consists of modules and each module is defined in a single file with the .eta extension. An Eta module or program is free to import other Eta modules from other packages.

Module specification should be at the top of the source file. Every Eta program is a collection of modules. Each module contains functions and types.

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module Main where

main :: IO ()
main = putStrLn "You declared a main module!"

NOTE: Module names always start with a capital letter.

Bindings

An Eta program consists of bindings. A binding is an association between a name or identifier and a value. Other languages call these variables because you can set new values to the variable over time.

In Eta, bindings are immutable and cannot be set to a new value once they've been defined. Bindings in Eta can be thought of as constant variables from other languages.

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fullName :: String
fullName = "Harry Potter"

• fullName is an identifier of type String
• fullName has the value "Harry Potter"

Functions

Defining a function in Eta is describing what it does using other functions as building blocks.

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double :: Int -> Int -- function declaration
double n = 2*n       -- function definition

• Function declaration specifies the function name and the type of the parameters and the return type.
• Function definition is where you define the function.

Defining functions can be done in multiple lines.

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xor :: Bool -> Bool -> Bool
xor True  True  = False
xor False False = False
xor x     y     = True

NOTE: Function declaration is optional in Eta since it has a global type inference which we are going to cover in a later chapter.

Comments

It is good practice to describe what is going on in your code by using comments. When you declare a part of your program as a comment, the compiler will ignore it.

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-- This is end-of-line comment

{- This is a block comment
   of multiple lines -}

Next Section

We will now proceed with the basic coding conventions.