在Haskell中执行断言

典型的是使用 Maybe 指定“可能失败”的计算,例如:

computeSum :: Int -> Int -> Maybe Int
computeSum x y | result > 0 && result <= 10 = Just result
               | otherwise = Nothing
    where result = x + y

Just result Nothing 以防断言不满足。

有时 Either String a

computeSum :: Int -> Int -> Either String Int
computeSum x y | result > 0 && result <= 10 = Right result
               | otherwise = Left "Result is note between 0 and 10"
    where result = x + y

你也可以提出一个错误,但我个人认为这是不可取的,因为签名并没有“暗示”计算可能失败:

computeSum :: Int -> Int -> Int
computeSum x y | result > 0 && result <= 10 = result
               | otherwise = error "result is not between 0 and 10"
    where result = x + y

是的,胡格尔告诉我们 Control.Exception 提供 assert :: Bool -> a -> a

但你可以自己写:

assert :: Bool -> a -> a
assert False _ = error "Assertion failed!"
assert _     a = a

是的,Haskell有if语句:

function x y =
  let r = x + y
  in if r > 0 && r <= 10
     then r
     else error "I don't know what I'm doing."