Real World Haskell - 第2章 練習問題

Haskell プログラミング

Real World Haskell を継続的に勉強中。

Exercises

Chapter 2. Types and Functions

Useful composite data types: lists and tuples

What are the types of the following expressions?

* False
* (["foo", "bar"], 'a')
* [(True, []), (False, 'a')]

Prelude> :type False
False :: Bool
Prelude> :type (["foo", "bar"], 'a')
(["foo", "bar"], 'a') :: ([[Char]], Char)
Prelude> :type [(True, []), (False, [['a']])]
[(True, []), (False, [['a']])] :: [(Bool, [[Char]])]
Chapter 2

1.Haskell provides a standard function, last :: [a] -> a, that returns the last element of a list. From reading the type alone, what are the possible valid behaviours (omitting crashes and infinite loops) that this function could have? What are a few things that this function clearly cannot do?

2.Write a function lastButOne, that returns the element before the last.

3.Load your lastButOne function into ghci, and try it out on lists of different lengths. What happens when you pass it a list that's too short?

1
  • the possible valid behaviours
    • リストを受け取り、同じ型の値を 1つ適当に返す
    • リストを受け取り、その要素を 1つ返す
  • this function clearly cannot do
    • it cannot access files; it cannot talk to the network; it cannot tell what time it is.
    • 受け取ったリストを別のリストに変換する
2
lastButOne xs = if length xs <= 2 || null xs
              then take 1 xs
              else lastButOne (tail xs)
3
Prelude> :load ex2-2.hs
[1 of 1] Compiling Main             ( ex2-2.hs, interpreted )
Ok, modules loaded: Main.
 *Main> lastButOne "hoge"
"g"
 *Main> lastButOne "hog"
"o"
 *Main> lastButOne "ho"
"h"
 *Main> lastButOne "h"
"h"
 *Main> lastButOne ""
""
 *Main> lastButOne [1,2,3,4]
[3]
 *Main> lastButOne [1,2,3]
[2]
 *Main> lastButOne [1,2]
[1]
 *Main> lastButOne [1]
[1]
 *Main> lastButOne []
[]