学步园

第四章内容比第三章多了不少，学习进度明显慢了许多。

在Haskell里，遍历元素，并加以操作的方式与命令式编程感觉真是差太多了，很多时候，你明明知道算法该怎么做，但是就是在Haskell里不知道该如何表达。不怕出丑的说，如何求一组数中的最小值，这种简单东西都把我卡了不少时间，最后瞟了一眼Prelude里的minimum函数实现，我擦，这么简单……因为很难从以前的习惯里解脱出来，想不到如何在haskell这个全是递归的环境里设一个中间变量进行保存临时值。

断断续续用业余时间做了几题练习。咳，放上一部分做个记录，之后再补全。

-- 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.
safeHead :: [a] -> Maybe a<br /> safeHead [] = Nothing<br /> safeHead (x:xs) = Just x<br /> safeTail :: [a] -> Maybe [a]<br /> safeTail [] = Nothing<br /> safeTail (x:xs) = Just xs<br /> safeLast :: [a] -> Maybe a<br /> safeLast [] = Nothing<br /> safeLast xs = Just (head (reverse xs))<br /> safeInit :: [a] -> Maybe [a]<br /> safeInit [] = Nothing<br /> safeInit xs = Just (reverse (tail (reverse xs)))<br />

-- 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.
splitWith :: (a -> Bool) -> [a] -> [[a]]<br /> splitWith func xs<br /> | null xs = []<br /> | otherwise = if pos == 0<br /> then splitWith func (tail xs)<br /> else if pos /= length xs<br /> then [take pos xs] ++ (splitWith func (drop (pos+1) xs))<br /> else if func (head xs)<br /> then [xs]<br /> else []<br /> where pos = firstIn xs<br /> where firstIn (d:ds) = if func d<br /> then if null ds<br /> then 1<br /> else 1 + (firstIn ds)<br /> else 0<br />

--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.
import System.Environment (getArgs)<br /> interactWith function inputFile outputFile = do<br /> input <- readFile inputFile<br /> writeFile outputFile (function input)<br /> main = mainWith myFunction<br /> where mainWith function = do<br /> args <- getArgs<br /> case args of<br /> [input, output] -> interactWith function input output<br /> _ -> putStrLn "error: exactly two arguments needed"<br /> -- replace "id" with the name of our function below<br /> myFunction = getFW<br /> getFW input = unlines (getFirstWord (lines input))<br /> where getFirstWord [] = []<br /> getFirstWord (x:xs) = (head (words x)) : getFirstWord xs<br />

--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".
import System.Environment (getArgs)<br /> interactWith function inputFile outputFile = do<br /> input <- readFile inputFile<br /> writeFile outputFile (function input)<br /> main = mainWith myFunction<br /> where mainWith function = do<br /> args <- getArgs<br /> case args of<br /> [input, output] -> interactWith function input output<br /> _ -> putStrLn "error: exactly two arguments needed"<br /> -- replace "id" with the name of our function below<br /> myFunction = transWords<br /> transWords input = unlines (transW (words input))<br /> where transW xs = if (myMinimum xs) == []<br /> then []<br /> else (map head xs) : transW (map tail xs)<br /> where myMinimum [] = []<br /> myMinimum (d:ds) = foldl myMin d ds<br /> where myMin a b = if length a < length b<br /> then a<br /> else b<br />