RWH CH02 Notes

Haskell is strong and static language.

 

 

*Main> head [1..10]
1
*Main> tail [1..10]
[2,3,4,5,6,7,8,9,10]
*Main> take 3 [1..10]
[1,2,3]
*Main> drop 3 [1..10]
[4,5,6,7,8,9,10]
*Main> last [1..10]
10
*Main> fst ('a' , 3)
'a'
*Main> snd ('a', 3)
3

 

We cannot assign a value to x twice.

 

 

*Main> let x = 10
*Main> x = 11

<interactive>:1:2: parse error on input `='

 

Recall that Haskell is an expression-oriented language. In an imperative language, it can make sense to omit the else branch from an if, because we're working with statements, not expressions. However, when we're working with expressions, an if that was missing an else wouldn't have a result or type if the predicate evaluated to False, so it would be nonsensical.

 

In Haskell, we can easily define a new function that short circuits benefit from haskell's lazy evaluation.

 

Answer to "Write a function lastButOne, that returns the element before the last." 

 

 

lastButOne :: [a] -> a
lastButOne (x:y:xs) = if null xs
                      then x
                      else lastButOne (y:xs)

 

But it might make a mistake for too short list.