Editing Cn/Introduction (section)

===便于理解===
Functional programs are often easier to '''understand'''. You should be able to understand the program without any previous knowledge of either Haskell or quicksort.   The same certainly cannot be said of the C program.  It takes quite a while to understand, and even when you do understand it, it is extremely easy to make a small slip and end up with an incorrect program.  Here is a detailed explanation of the Haskell quicksort:
<haskell>
qsort []     = []
qsort (x:xs) = qsort (filter (< x) xs) ++ [x] ++ qsort (filter (>= x) xs)
</haskell>
The first line reads:
"When you sort an empty list (<tt>[]</tt>), the result is another empty list".  
The second line reads: "To sort a list whose first element is named <tt>x</tt> and
the rest of which is named <tt>xs</tt>, sort the elements of <tt>xs</tt> that are less than <tt>x</tt>, sort the elements of <tt>xs</tt> that are greater than or equal to <tt>x</tt>, and concatenate (<tt>++</tt>) the results, with <tt>x</tt> sandwiched in the middle."