In most programming language we need ways to declare our own types. These help with readability, modularity etc. Haskell is no different in offering these capabilities.

We can create a simple data type using the data keyword, followed by either of available values (in C# this would be similar to an enum). So here’s the definition of a Boolean type

data Bool = False | True

The values after the = are the value constructors with the | being like an or, hence this Bool have either False or True value.

Note: The type name and the value constructor must be capital cased.

We can also declare types more like structs/classes in that we can define the field types that make up the type. For example here’s a Point type

data Point = Point Integer Integer

In this example we could create a point as follows

pt = Point 1 2

Accessing these values can be a little verbose (especially when we might have lots of fields) because of the lack of any names, hence we’d use pattern matching, i.e.

xpt :: Point -> Integer
xpt (Point x _) = x

-- example of printing the x value of value pt of type Point
print (xpt pt)

Note: The type annotation is not required and the underscore is a discard, hence is ignored.

We can combine types just as we combined False and True into essentially a union. So for example here we have a data type Shape with constructors for Circle and Triangle.

data Shape = Triangle Int Int Int | Circle Int

triangle = Triangle 1 2 3
circle = Circle 4

It’s not clear what these fields of the constructors mean, so let’s add some names which also allows us to more easily access specific values from the data (i.e. fixes the issue with using pattern matching – which ofcourse is still a valid way of doing things).

data Shape = Triangle { 
   hypotenuse :: Int, 
   opposite :: Int, 
   adjacent :: Int 
} | Circle { 
  radius :: Int 
}

We can declare instances of this type using names or without names, for example

-- not using names
triangle = Triangle 1 2 3
-- with names
triangle1 = Triangle { opposite = 5, hypotenuse = 6, adjacent = 7 }

Instead of creating a function to extract a value (like we did with the xpt function we created) we can now use the following

print (hypotenuse triangle)

and Haskell essentially creates the function for us, i.e. using the REPL along with :t hypotenuse we get the following

hypotenuse :: Shape -> Int

Haskell is immutable, so how do we make changes to an instance of data? Well we “copy and update”. Thankfully Haskell makes this easy (so we don’t have to literally create copies of every field on our data and then change values).

If you’ve used JavaScript it’s like using a spread operator of in F# and with.

newTriangle = triangle { hypotenuse = 10 } 

In this case newTriangle is a copy of the triangle data, with the hypotenuse changed.

Note: I’ve writing these Haskell blog posts whilst learning Haskell, so do not take them as expert guidance. I will amend the post(s) as I learn more.

Before we begin, let’s use cabal init to create a starter project, then we’ll add our code to this project as part of this post. This will create a Main.hs with the classic “Hello World” type of sample along with the Setup.hs and the .cabal file.

Modules

Apart from the simplest applications we would probably want to package functions, types etc. into modules. Modules exist in most languages, such as those in F#, TypeScript or similar to namespaces in C# & Java. In Haskell each module is stored within it’s own file and the file begins with the following declaration

module ModuleName where

Note: module names should be in PascalCase.

Obviously replacing the ModuleName with your module name and, similar to Java, the ModuleName should be made up of the path to the module file, ending with the module name. For example let’s assume we’re going to create a Calculator module and our application folder is HaskellBasics, let’s assume we’ve created a Modules folder off of this and within that a Calculator.hs file, hence the ModuleName should look like this

module Modules.Calculator where

We need to add the module to the .cabal build file using the other-modules key, for example

executable HaskellBasics
  main-is:             Main.hs
  other-modules:       Modules.Calculator

Note: We don’t include the file extension for the Calculator module.

If we add more modules then we simply add as comma separated values in the other-modules field in the .cabal file, i.e.

  other-modules: Modules.Calculator, Modules.Output

To import modules into our Main.hs we use the import keyword, like this (we’re importing two modules here).

import Modules.Output
import Modules.Calculator

There’s a lot more we can do in terms of importing modules, for example importing only some functions, like this

import Modules.Calculator (add, sub)

We can also import all function except for some, essentially hiding the module functions, like this

import Modules.Calculator hiding (sub)

The ability to specify the functions available and those not available is useful to help with name clashes etc. Another way to handle such name clashes is to use the module qualified names, for example

main = print (Modules.Calculator.add 10 6)

We can also enforce qualified module names for each function within a module be importing out module like this

import qualified Modules.Calculator

and hence all uses of the functions within this module must be fully qualified.

This can end up requiring a lot more verbosity than we really want, so we can import a qualified module and alias it like this

import qualified Modules.Calculator as C

meaning we can use the function like this

main = print (C.add 10 6)

Note: The alias for the module name must start with a capital letter.