Real World Haskell - 第4章 Exercises Part1

Real World Haskell で Haskell を継続的に勉強中。

Chapter 4. Functional programming
Chapter 4. Functional programming

回答は順次追記していく。

Excercises (Section "Special string-handling functions" の後にあるもの)

1.Write your own “safe” definitions of the standard partial list functions, but make sure that yours never fail. As a hint, you might want to consider using the following types.

-- file: ch04/ch04.exercises.hs
safeHead :: [a] -> Maybe a
safeTail :: [a] -> Maybe [a]
safeLast :: [a] -> Maybe a
safeInit :: [a] -> Maybe [a]

2.Write a function splitWith that acts similarly to words, but takes a predicate and a list of any type, and splits its input list on every element for which the predicate returns False.

-- file: ch04/ch04.exercises.hs
splitWith :: (a -> Bool) -> [a] -> [[a]]

3.Using the command framework from the section called “A simple command line framework”, write a program that prints the first word of each line of its input.

4.Write a program that transposes the text in a file. For instance, it should convert "hello\nworld\n" to "hw\neo\nlr\nll\nod\n".

1

safeHead x | null x = Nothing
           | otherwise = Just (head x)

safeTail x | null x = Nothing
           | otherwise = Just (tail x)

safeLast x | null x = Nothing
           | otherwise = Just (last x)

safeInit x | null x = Nothing
           | otherwise = Just (init x)

これもあり。

safeListFunc func [] = Nothing
safeListFunc func xs = Just (func xs)

safeHead = safeListFunc head
safeTail = safeListFunc tail
safeLast = safeListFunc last
safeInit = safeListFunc init