异常：Prelude.head：空列表

Question

这是我对扩展函数的尝试：

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) == '



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"


我明白了

 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>."


添加
我自己定义了split，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."


那我就

(" , .",["A","comma","","then","some","words",""])
 = (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"

我明白了

 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>."

添加我自己定义了split，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."

那我就

(" , .",["A","comma","","then","some","words",""])

Answer 1

如您所见，in the doc，split 没有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，将 (head t) == '$' 放在守卫中是不安全的，并可能导致引发异常。

异常：Prelude.head：空列表

Exception: Prelude.head: empty list

haskell

empty-list