Haskell:用|来约束函数模式变量

是的,这恰恰意味着" 这样 ".这些是守卫,在Haskell中非常常见(通常优于等效if then else表达式).

f x
 | x > 2      = a
 | x < -4     = b
 | otherwise  = x

相当于

f x = if x > 2 then a
               else if x < -4 then b
                              else x

国际海事组织,具体的例子实际上最好既if没有保护也没有保护,但是

    takeConstraint' = case n of
        "Unique"  -> (Nothing, Nothing, Just $ takeUniq ps tableName defs rest, Nothing)
        "Foreign" -> (Nothing, Nothing, Nothing, Just $ takeForeign ps tableName defs rest)
        "Primary" -> (Nothing, Just $ takeComposite defs rest, Nothing, Nothing)
        "Id"      -> (Just $ takeId ps tableName (n:rest), Nothing, Nothing, Nothing)
        _         -> (Nothing, Nothing, Just $ takeUniq ps "" defs (n:rest), Nothing)