我尝试学习haskell并使用函数修复重写标准列表操作(map,foldr,zip,iterate等)。我有重复的例子:
repeat a = fix $ \xs -> a : xs
它进一步简化了
repeat a = fix (a:)
repeat = fix . (:)
任何人都可以帮我地图吗?对不起我的坏人,并提前感谢你。
要使用fix
,需要在表单中编写递归定义
map = .... something involving map ....
然后,我们让
map = fix (\m -> .... something involving m ....)
例如,
map = \f xs -> case xs of
[] -> []
y:ys -> f y : map f ys
所以,
map = fix (\m f xs -> case xs of
[] -> []
y:ys -> f y : m f ys)
或者,由于参数f
对于每个递归调用都是相同的,我们可以让
map f = \xs -> case xs of
[] -> []
y:ys -> f y : map f ys
并获得
map f = fix (\m xs -> case xs of
[] -> []
y:ys -> f y : m ys)