Basics

Directory structure

For a given app the directory structure should look like this:

app
- src
  - Main.hs
- java
  - Utils.java
- eta-first.cabal
- LICENSE

Naming Rules

Source file names

In Eta the source file names should be the same as the module name with a .eta extension appended. And the module name always starts with a capital letter.

It is good practice to name the file after the description of what the file does.

Type names

In Eta types start with a capital letter.

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add :: Int -> Int -> Int
add x y = x + y

In the above example, Int is a type defined in Eta's standard library and starts with a capital letter.

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data Pair = Pair Int Int

In the above example, Pair is a user-defined type and starts with a capital letter.

Function Names

In Eta, functions start with a lowercase letter.

Function names with multiple words are written using camelCase convention.

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add :: Int -> Int -> Int
add x y = x +y

In the above example, add is a function.

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data Pair = Pair Int Int

myPair :: Pair
myPair = Pair 1 2

In the above example, myPair is a function written using the camelCase convention.

Choosing Good names

It good practice to use descriptive names. The names should make the purpose of the entity clear. One should avoid using meaningless words like Wrapper etc. in names.

When using an acronym as part of a declaration name, capitalize it when it consists of two letters (IOStream); capitalize only the first letter if it is longer (HttpInputStream).

Formatting

In most cases, Eta follows Haskell and Java conventions.

Use 4 or 8 spaces for indentation. Do not use tabs.

It's good practice to format your code to fit into 80 columns.

Horizontal Whitespace

Operators

Put spaces around operator ( x + y + z ). Don't put space around range operator (1..n).

Functions

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reverse :: [Int] -> [Int]
reverse [] = []
reverse (x:xs) = reverse xs ++ [x]

• :: - should have space around double colon.
• -> - space around it
• reverse [] - single space between function and its argument.

Never put a space after (, [, or before ], ).

Good Coding Conventions

Imports

Standard library modules like Java.Array, Java.IO, and Java.Utils should be imported in alphabetical order.

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import Java.Array
import Java.IO
import Java.Utils

Next Section

We will now proceed with the section on Java Interop.