这是我对扩展功能的尝试:
expand :: FileContents -> FileContents -> FileContents
expand textfile infofile
= concat (combine (fst (split separators textfile)) (expand' (snd (split separators textfile)) infofile "") )
where
expand' [] _ stack = []
expand' (t:ts) info stack
|(head t) == '$' = (head (change t info )):expand' ts info stack
|otherwise = t: expand' ts info stack
where
change x y
= lookUp x (getKeywordDefs (snd (split ['\n'] y)))
当我这样做时:
expand "Keywords (e.g. $x, $y, $z...) may appear anwhere, e.g. <$here>." "$x $a\n$y $b\n$z $c\n$here $this-is-one"
我得到了
Exception: Prelude.head: empty list
但我想要
"Keywords (e.g. $a, $b, $c...) may appear anwhere, e.g. <$this-is-one>."
因为我应该
("Keywords (e.g. $x, $y, $z...) may appear anwhere, e.g. <$here>.",
"$x $a\n$y $b\n$z $c\n$here $this-is-one")
==> "Keywords (e.g. $a, $b, $c...) may appear anwhere, e.g. <$this-is-one>."
ADDITION 我自己定义了分裂,即:
split :: [Char] -> String -> (String, [String])
split separators ( x : wordsrest)
| elem x separators = (x : listseparators,"" : word : listwords)
| otherwise = (listseparators, ((x : word) : listwords))
where
(listseparators, word : listwords) = split separators wordsrest
split _ _ = ("" , [""])
例如,如果我输入
split " .," "A comma, then some words."
然后我会得到
(" , .",["A","comma","","then","some","words",""])
答案 0 :(得分:1)
正如您所见in the doc,split
不会将分隔符保留在它输出的块中,因此它们很可能是空的。
因此,在警卫中使用(head t) == '$'
是不安全的,并且可能导致异常被提出。