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Question

说我有一个功能：

arbitrary :: String -> String -> Maybe String 
arbitrary st1 st2 | (st1 == st2) = Just "foo" 
        | (arbitrarily_complex_calculation == 7) = Nothing 
        | otherwise = Just $ show arbitrarily_complex_calculation 

我怎么能随意份额在两个后卫块complex_calculation？这可以通过let/where完成，还是必须编写一个辅助函数？

Answer 1

是的，where条款是有效的（而且经常使用）与后卫：

arbitrary :: String -> String -> Maybe String 
arbitrary st1 st2 
    | st1 == st2 = Just "foo" 
    | acc == 7 = Nothing 
    | otherwise = Just $ show acc 
where 
    acc = arbitrarily_complex_calculation