Haskell Walkthrough – I

Learn you a Haskell for great good

Started checking out Haskell recently. I chose the Learn you a Haskell for great good book as my guide.

Language philosophy

Purely functional, which basically means the following:

  • No side effects: Function does one thing only. If you give it the same input it will give you the same output. Function always returns a value
  • Lazy: Haskell wont evaluate functions until it needs to show a result. This allows for infinite data structures since only the parts needed are actually computed
  • Statically typed: Compiler is aware of the types
  • Type inference: No need to explicitly specify each type. Compiler figures it out

Quick Functions

No brackets. Example:
*Main> div 50 10
5
Known as a Prefix functions. Can also be written as:
*Main> 50 `div` 10
5
Known as Infix function.

Quick Lists

Concatenation

Prelude> let a = [1,2,3,4]
Prelude> a ++ [5,6]
[1,2,3,4,5,6]

Cons operator

Prelude>0:a
[0,1,2,3,4]
Note that the value of a is unchanged

Index operator

Prelude> [1,2,3,4] !! 0
1

List comprehension

Inspired by mathematical set comprehension. Reflected in the syntax
{ 2*x | x E N, x <= 5}

Main> [2*x | x ← [1..10], x `mod` 2 == 0]
[4, 8, 12, 16, 18 ,20]

The condition after the comma is a predicate. You can have multiple predicates separated by commas. When drawing values from several lists, every combination of elements from these lists is reflected in the resulting list

Main> [(x,y)|x ← [“Good”,”Bad”], y ← [“Apple”,”Cat”]]
[(“Good”,”Apple”),(“Good”,”Cat”),(“Bad”,”Apple”),(“Bad”,”Cat”)]

Moral of the story

You start with a certain set of candidate solutions, and successively apply transformations and filters to them until you’ve narrowed the possibilities down to the one solution (or several solutions) that you’re after.

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