[Learning You a Haskell for Great Goods!] chapter 01 starting out

Installation

under CentOS/Fedora

# yum install ghc

 

Version

[root@bogon1 haskell]# ghc -v

Glasgow Haskell Compiler, Version 7.0.4, for Haskell 98, stage 2 booted by GHC version 7.0.4

 

Change prompt

echo :set prompt "ghci> " > ~/.ghci

 

Command

# Boolean

ghci> True && False

False

 

# call function, get successor of 8

ghci> succ 8

9

 

# load scripts

## echo doubleMe x = x + x > baby.hs

 

ghci> :l baby.hs

[1 of 1] Compiling Main             ( baby.hs, interpreted )

Ok, modules loaded: Main.

 

ghci> doubleMe 9

18

 

# Concatenation between list and string

ghci> [1,2,3,4] ++ [9,10,11,12]

[1,2,3,4,9,10,11,12]

 

or

 

ghci > 'A': "Small cat"

"A Small cat"

 

or

 

ghci> 5:[1,2]

[5,1,2]

 

# List operation

 

## get the element that's index = 3

ghci> "123456" !! 3

'4'

 

## comparing

ghci> [3,2,1] > [2,1,0]
True

## head vs tail, init vs last

ghci> head [5,4,3,2,1]
5

ghci> tail [5,4,3,2,1]
[4,3,2,1]

ghci> last [5,4,3,2,1]
1

ghci> init [5,4,3,2,1]
[5,4,3,2]

## length

ghci> length [5,4,3,2,1]
5

## is null or not

ghci> null [1,2,3]
False
ghci> null []
True

## reverse list

ghci> reverse [5,4,3,2,1]
[1,2,3,4,5]

## take the first N elements

ghci> take 3 [5,4,3,2,1]
[5,4,3]
ghci> take 1 [3,9,3]
[3]
ghci> take 5 [1,2]
[1,2]
ghci> take 0 [6,6,6]
[]

## drop the first N elements

ghci> drop 3 [8,4,2,1,5,6]
[1,5,6]
ghci> drop 0 [1,2,3,4]
[1,2,3,4]
ghci> drop 100 [1,2,3,4]
[]

## element in list or not

ghci> 4 `elem` [1 ,4 ]
True

or

ghci> elem 4 [1 , 4]
True

## range

ghci> [1..4]
[1,2,3,4]

 

## replicate

 

ghci> replicate 3 10

[10,10,10]

 

## List Comprehension

 

ghci> [x * 2 | x <- [1..10] ]

[2,4,6,8,10,12,14,16,18,20]

 

ghci> [x*2 | x <- [1..10], x *2 >= 12]

[12,14,16,18,20]

 

## differnces between list and tuple

list must be the same type of element

tuple could be different but if list contains at least one tuple, the number of element in tuple must be fixed

 

## pair

## contains only two element

## the following function work in pair only

 

## take the first element

ghci> fst (1,2)

1

 

## take the second element

ghci> snd (1,2)

2

 

# zip

# combine two list into one

ghci> zip [1,2] ["one","two"]

[(1,"one"),(2,"two")]

 

# commond pattern in functional programming

start with a certain set fo candidate solutions ,and apply transformations and filters to them util narrowed the possibilities down to the one solution

 

Note

doubleSmallNumber' is valid as a method name