Basics
Learn the basics of Eta
Learn the basics of Eta
If you've installed using Gradle, you would have already ran your first program and we'll help you understand the project structure in the Gradle Project Structure section.
If you've installed using any other method, please skip to the Etlas Project Structure section.
In the last module, you have cloned the eta-init repository which gives you a template project structure to work with.
The project layout should look like this:
eta-init
- src
- main
- eta
- Main.hs
- Primes.hs
- build.gradle
There will also be a gradle folder and a bunch of gradlew files. All these files allow the Gradle Wrapper script to work so that other developers can reproduce your build easily.
1 2 3 4 5 6 7 8 9 10 11 12 13 | plugins {
id 'com.typelead.eta' version '0.6.0'
id 'application'
}
eta {
version = '0.8.6b4'
etlasVersion = '1.5.0.0'
}
dependencies {
compile eta('base:4.11.1.0')
}
|
The plugins block contains information about which plugin to download from the Gradle Plugin Portal.
You can use the following command to build & run your project:
./gradlew run
You can follow along with the examples in the upcoming modules by updating the Main.hs with your new code and run the ./gradlew run command to get the output of the new program.
For more information about how to use Gradle and the Eta Plugin, you can check out the Gradle Plugin User Guide.
When working on Eta projects, we use the Etlas build tool to manage our eta version and take care of our package management. Etlas also has special features that allow it to patch Haskell packages to be compatible with Eta, an important function we'll discuss later.
We'll show you how to run the eta compiler directly and also how to use etlas to make our lives easier.
Create a file with a simple program.
1 2 3 4 5 6 7 | module Main where
primes = filterPrime [2..]
where filterPrime (p:xs) =
p : filterPrime [x | x <- xs, x `mod` p /= 0]
main = putStrLn $ "The 101st prime is " ++ show (primes !! 100)
|
Compile the program.
$ etlas exec eta -- Main.hs
NOTE: The command above is equivalent to
$ eta Main.hs
if you had a global eta compiler installed.
This will compile the program to a standalone JAR with a Run- prefix added to the module name.
Run the program with java.
$ java -jar RunMain.jar
While you can manage with manually sending flags to the Eta compiler for single-file programs, for large projects, you want a tool that can compile the program taking into account many files and library dependencies.
You can use the Etlas build tool to handle non-trivial projects. You specify configuration for your project by creating a [project-name].cabal file at the root of your project.
Create a new directory called eta-first and enter it.
$ mkdir eta-first
$ cd eta-first
Initialize the project with Etlas.
Note: <[text-here]> means to type [text-here] and <Enter> means to press the Enter key.
$ etlas init
Package name? [default: eta-first] <Enter>
Package version? [default: 0.1.0.0] <Enter>
Please choose a license:
1) GPL-2
2) GPL-3
3) LGPL-2.1
4) LGPL-3
5) AGPL-3
6) BSD2
* 7) BSD3
8) MIT
9) ISC
10) MPL-2.0
11) Apache-2.0
12) PublicDomain
13) AllRightsReserved
14) Other (specify)
Your choice? [default: BSD3] <Enter>
Author name? [default: ...] <Enter>
Maintainer email? [default: ...] <Enter>
Project homepage URL? <Enter>
Project synopsis? <Enter>
Project category:
* 1) (none)
2) Codec
3) Concurrency
4) Control
5) Data
6) Database
7) Development
8) Distribution
9) Game
10) Graphics
11) Language
12) Math
13) Network
14) Sound
15) System
16) Testing
17) Text
18) Web
19) Other (specify)
Your choice? [default: (none)] <Enter>
What does the package build:
1) Library
2) Executable
Your choice? <2>
What is the main module of the executable:
* 1) Main.hs (does not yet exist, but will be created)
2) Main.lhs (does not yet exist, but will be created)
3) Other (specify)
Your choice? [default: Main.hs (does not yet exist, but will be created)] <Enter>
Source directory:
* 1) (none)
2) src
3) Other (specify)
Your choice? [default: (none)] <2>
What base language is the package written in:
* 1) Haskell2010
2) Haskell98
3) Other (specify)
Your choice? [default: Haskell2010] <Enter>
Add informative comments to each field in the cabal file (y/n)? [default: n] <Enter>
Guessing dependencies...
Generating LICENSE...
Generating Setup.eta...
Generating ChangeLog.md...
Generating src/Main.hs...
Generating eta-first.cabal...
The project structure should look like this:
eta-first
- src
- Main.hs
- ChangeLog.md
- LICENSE
- Setup.hs
- eta-first.cabal
Add the files Main.hs and Primes.hs in src/ as shown below.
1 2 3 4 5 | module Main where
import Primes
main = putStrLn $ "The 101st prime is " ++ show (primes !! 100)
|
1 2 3 4 5 | module Primes where
primes = filterPrime [2..]
where filterPrime (p:xs) =
p : filterPrime [x | x <- xs, x `mod` p /= 0]
|
Update eta-first.cabal, adding an other-modules: Primes field.
name: eta-first
version: 0.1.0.0
license: BSD3
license-file: LICENSE
author: [your name]
maintainer: [your email]
build-type: Simple
extra-source-files: ChangeLog.md
cabal-version: >=1.10
executable eta-first
main-is: Main.hs
other-modules: Primes
build-depends: base >=4.8 && <4.9
hs-source-dirs: src
default-language: Haskell2010
Any additional modules you add to the project should be added at the same indentation level as the Primes entry, but below it.
Build & run your program.
$ etlas run
For more information on how to use Etlas, you can visit the Etlas User Guide.