Haskell在一个条件下定义多个变量？

Question

好吧,我可能做错了,但它让我把头发拉了出来.我找不到任何可以做我想要的东西

拿这个伪代码

my_function left right
    = another_function new_left new_right (fourth_function new_left new_right)
        where new_left = if some_condition then left else third_function left
            new_right = if some_condition then third_function right else right

我怎样才能避免重新检查some_condition？而且我不是在讨论some_condition作为where构造中的另一个变量保存.如果我把它lets放进去,if那么复制一下in another_function new_left new_right.

在命令式语言中,我可以做类似的事情

int new_left;
int new_right;
if (condition) {
    new_left = left;
    new_right = third_function(right);
} else {
    new_left = third_function(left);
    new_right = right;
}
return another_function(new_left, new_right, fourth_function(new_left, new_right));

我知道在函数式语言中你不应该考虑按顺序做事,而是作为表达式的组合,所以我只是想找一种方法来编写原始的伪代码,这样就干了.这似乎是一个简单而相对常见的案例.

编辑

对困惑感到抱歉.我无法内联,third_function left/right因为我需要使用它的值两次(更新的伪代码).并且fourth_function不能移动到里面another_function

Answer 1

怎么样

my_function left right | somecondition = 
                         another_function left (third_function right)
                       | otherwise     =
                         another_function (third_function left) right

随着新的编辑

my_function left right = ...
  where (new_left, new_right) | somecondition = (left, third_function right)
                              | otherwise     = (third_function left, right)