Haskell:if-then-else块和具有不对称构造函数的数据

Question

我有以下数据可以发货或不发货:

data LaserCollisionResult = NoCollision | LaserToLaserCollision Ship | LaserToShipCollision Ship deriving (Eq, Show)

然后,稍后,我试图检查LaserCollisionResult是否为LaserToLaserCollision类型,但是我收到错误.我的[lambda]功能是这样的:

laserPaths' = map (\(p,r) -> if r == LaserToLaserCollision then doSomethingWith p else p) $ zip laserPaths laserCollisionResults

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.

如何检查laserCollisionResults中的LaserCollisionResult是否属于LaserToLaserCollision类型？

Answer 1

替换你的lambda

(\(p,r) -> case r of {LaserToLaserCollision _ -> doSomethingWith p; _ -> p})

顺便说一句,为此您不需要派生Eq实例.