[英]Haskell: Multi-way if-expressions need MultiWayIf turned on
尝试使用“堆栈构建”进行构建:
module Main where
analyzeGold :: Int -> String
analyzeGold standard =
if | standard == 999 -> "Wow! 999 standard!"
| standard == 750 -> "Great! 750 standard."
| standard == 585 -> "Not bad! 585 standard."
| otherwise -> "I don't know such a standard..."
main :: IO ()
main = do
putStrLn (analyzeGold 999)
我得到:
Multi-way if-expressions need MultiWayIf turned on
|
6 | if | standard == 999 -> "Wow! 999 standard!"
| ^^
如何解决?
堆栈1.7.1,GHC 8.2.2
在Haskell中,只有if
- then
- else
子句。 如果要使用这些“ 多if ”语句,请使用guard 。
你的语法已经相当接近保护,但它不具备的if
关键词,而等号( =
)用来表示输出是在这种情况下什么。
因此,您应该将其重写为:
analyzeGold :: Int -> String
analyzeGold standard
| standard == 999 = "Wow! 999 standard!"
| standard == 750 = "Great! 750 standard."
| standard == 585 = "Not bad! 585 standard."
| otherwise = "I don't know such a standard..."
有关防护的语法和使用的某些信息,请参见此处[lyah] 。
由于您的检查每次都会检查整数文字的相等性,因此我们实际上可以将检查从警戒移动到模式,例如:
analyzeGold :: Int -> String
analyzeGold 999 = "Wow! 999 standard!"
analyzeGold 750 = "Great! 750 standard."
analyzeGold 585 = "Not bad! 585 standard."
analyzeGold _ = "I don't know such a standard..."
下划线( _
)用作与所有值匹配的通配符 (以及与先前子句不匹配的所有模式)。
MultiWayIf
扩展 您也可以通过在文件的头部写入一个编译指示或在调用解释器时使用-XMultiWayIf
来启用GHCi扩展来启用此扩展。 所以:
{-# LANGUAGE MultiWayIf #-}
analyzeGold :: Int -> String
analyzeGold standard =
if | standard == 999 -> "Wow! 999 standard!"
| standard == 750 -> "Great! 750 standard."
| standard == 585 -> "Not bad! 585 standard."
| otherwise -> "I don't know such a standard..."
要么:
$ ghci -XMultiWayIf
GHCi, version 8.0.2: http://www.haskell.org/ghc/ :? for help
Prelude> :{
Prelude| analyzeGold :: Int -> String
Prelude| analyzeGold standard =
Prelude| if | standard == 999 -> "Wow! 999 standard!"
Prelude| | standard == 750 -> "Great! 750 standard."
Prelude| | standard == 585 -> "Not bad! 585 standard."
Prelude| | otherwise -> "I don't know such a standard..."
Prelude| :}
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