Haskell Getting Started

23rd of February, 2020

Haskell is a pure functional language

Haskell

  • Created in 1987 by enthusiasts to help increase the productivity and accuracy
  • Pure Functional Language
    • Functions are first class member, that is functions can be used like any other values.
    • Expressions never have “side-effects”
    • Functions will always return same output for same input.
    • referential transparency - No Mutation ! this comes handy when working with concurrency.
  • Lazy
    • Haskell expressions are not evaluated until their results are needed.
    • Helps in working with infinite data structures
    • wholemeal programming - a more compositional programming style
  • Statically typed
    • Apart from being Statically typed, haskell is an expressive language (that makes it different form java, c++)
  • Concise
    • A 5M lines of code in Java can be written in 1M lines in Haskell
  • Speed
    • As fast as ( sometimes more ) Java, C++
      • directly to native
      • abstraction provides multicore & parallel processing.

Stack

Stack is a cross-platform program for developing Haskell projects. It is aimed at Haskellers both new and experienced.

It features:

  • Installing GHC automatically, in an isolated location.
  • Installing packages needed for your project.
  • Building your project.
  • Testing your project.
  • Benchmarking your project.

Installation

curl -sSL https://get.haskellstack.org/ | sh

Create a new project

  • stack new <project_name> will create a new directory.

stack.yaml

Once a project is created you’ll see stack.yaml file created, and few others like LICENSE, .cabal, Setup.hs. stack.yaml has a field "resolver", it tells you which version of ghci you want to use.

Building

    stack setup
    stack build
    stack exec <project_name_exe>

notice the project_name_exe : this is mentioned in .cabal file as executable.

Project structure

project is built, tested and managed via the .cabal file. Here you can mention the libraries, packages to include.

cabal file

library
  hs-source-dirs:      src
  exposed-modules:
    Lib

Among the configuration files, you will also see a directory structure: directory_struct

This is the default structure of the project, but can be changed with respective changes in cabal file.

Code

Add code to src library ( or the one mentioned in cabal file (Library->hs-source-dirs).

module

modules are like packages in java. or modules in java 9. You can expose them as library for others to use. publish them to hackage too.

module <module_name> (exposed functions) where
    {-- code here --}

function

function, like any expression in haskell, has a signature and a type.

Declaration

func :: Int -> Int

Here func takes an argument of the type Int and returns Int