小编Zas*_*ast的帖子

检查字符串是有效数字时如何检查小数点？

我在想的是我使用类似下面的内容,但添加代码来检查小数点!

isNumber :: String -> Bool
isNumber xs = all isDigit xs || add extra code here

如果有效数字在EBNF中定义为:

number -> .digit+ | digit+ [ .digit*]

例如,.5,1.5,1,1都是有效数字.+表示一次或多次出现,*表示零或更多.

如何为三角形添加不同颜色的圆底？

 #box {
    content:"";
    width: 0;
    height: 0;
    border-style: solid;
    border-width: 0 150px 150px 150px;
    border-color: transparent yellow red blue;
    position: relative;
 }

生产:

我需要的是底部的绿色部分,圆角:

我有一个问题,实现一个可能类型列表操作!

我需要做的是:

ljoin [(2,"S"),(1,"J")] [(2,1),(2,2),(3,4)]
     outputs: [(2,"S",Just 1),(2,"S",Just 2),(1,"J",Nothing)]

到目前为止我写的:

ljoin :: Eq a => [(a,b)] -> [(a,c)] -> [(a,b,Maybe c)]
ljoin xs ys = [if xs==ys then (xa, xb, Just yc) else (xa, xb, Nothing) | (xa, xb) <- xs, (ya, yc) <- ys]

哪会产生错误:

Inferred type is not general enough
*** Expression    : ljoin
*** Expected type : Eq a => [(a,b)] -> [(a,c)] -> [(a,b,Maybe c)]
*** Inferred type : Eq a => [(a,b)] -> [(a,b)] -> …