英文:
How to best handle IO Bool logic?
问题
我经常看到自己使用这个模式：
f x = do
-- predicate :: a -> IO Bool
b <- predicate x
if b then
rest ()
else
return ()
where
rest () = do
-- 执行剩下的 IO 操作
有其他人使用的去除 if then else 语句的模式吗？你如何最好地处理控制流和 IO Bool？
英文:
I see myself using this pattern a lot:
f x = do
-- predicate :: a -> IO Bool
b <- predicate x
if b then
rest ()
else
return ()
where
rest () = do
-- rest of my IO operations
is there a pattern other people use to remove the if then else where clauses? How do you best work with control flow and IO Bool?
答案1
得分: 8
f x = do
b <- predicate x
when b $ do
-- 其余的IO操作
英文:
f x = do
b <- predicate x
when b $ do
-- rest of the IO operation
答案2
得分: 2
除了when之外，还有一件方便的事情，与布尔值无关，可以使用-XLambdaCase和一个开放绑定操作符来避免引入变量：
{-# LANGUAGE LambdaCase #-}
f x = do
predicate >>= \case
True -> rest ()
False -> return ()
where
rest () = do
...
顺便提一下，如果when的参数反转了，那么也可以按照这种方式使用：
thenfore :: m () -> Bool -> m ()
thenfore = flip when
f x = do
predicate >>= thenfore (rest ())
where
rest () = do
...
或者，内联rest，
f x = do
predicate >>= thenfore `id` do
...
关于id作为操作符的使用说明：注意id :: a -> a比函数应用操作符($)更一般，所以可以以相同的方式使用它，但它具有更高的优先级，因此可以像上面所示的那样与>>=结合使用。
英文:
In additional to when, another thing that's handy – and not specific to booleans – is to use -XLambdaCase and an open bind operator to avoid having to introduce a variable:
{-# LANGUAGE LambdaCase #-}
f x = do
predicate >>= \case
True -> rest ()
False -> return ()
where
rest () = do
...
Incidentally, if when had its arguments flipped then this could be used in that style too:
thenfore :: m () -> Bool -> m ()
thenfore = flip when
f x = do
predicate >>= thenfore (rest ())
where
rest () = do
...
...or, inlining rest,
f x = do
predicate >>= thenfore`id`do
...
Explanation on the use of id as an operator: notice that id :: a -> a is strictly more general than the function-application operator ($) :: (a -> b) -> (a -> b), so it can be used in the same way, but it has higher precedence and can thus be combined with >>= as done above.
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