[英]Haskell: if-then-else blocks and data with asymmetrical constructors
I have the following data which can have a Ship or not: 我有以下数据可以发货或不发货:
data LaserCollisionResult = NoCollision | LaserToLaserCollision Ship | LaserToShipCollision Ship deriving (Eq, Show)
then, later on, I am trying to check if a LaserCollisionResult is of type LaserToLaserCollision, but I get an error. 然后,稍后,我试图检查LaserCollisionResult是否为LaserToLaserCollision类型,但是我收到错误。 My [lambda] function is this:
我的lambda函数是这样的:
laserPaths' = map (\(p,r) -> if r == LaserToLaserCollision then doSomethingWith p else p) $ zip laserPaths laserCollisionResults
The error I am getting is: 我得到的错误是:
Couldn't match type 'LaserCollisionResult' with 'Ship -> LaserCollisionResult'
Expected type: [Ship -> LaserCollisionResult]
Actual type: [LaserCollisionResult]
In the second argument of 'zip', namely laserCollisionResults.
How can I check whether a LaserCollisionResult in laserCollisionResults is of type LaserToLaserCollision? 如何检查laserCollisionResults中的LaserCollisionResult是否为LaserToLaserCollision类型?
Replace your lambda by 替换你的lambda
(\(p,r) -> case r of {LaserToLaserCollision _ -> doSomethingWith p; _ -> p})
By the way, for this you don't need to derive an Eq
instance. 顺便说一句,为此您不需要派生
Eq
实例。
You need to match on r
eg 你需要匹配
r
例如
laserPaths' = map (\(p,r) -> if isLaserCollision r then doSomethingWith p else p) $ zip laserPaths laserCollisionResults
where isLaserCollision (LaserToLaserCollision _) = True
isLaserCollision _ = False
or you can match inline: 或者你可以匹配内联:
laserPaths' = map (\(p, r) -> case r of { (LaserToLaserCollision _) -> doSomethingWith p ; _ -> p}) $ zip laserPaths laserCollisionResults
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