在下面的代码中,我必须为每次迭代重复使用Active Pattern结果三次。即
match tree.Parent, postion with
我发现我可以保存活动模式结果。即
let pos = ((|Root|Nil|Single|First|Inner|Last|Unknown|) (tree.Parent, position))
我无法弄清楚的是,是否可以在匹配语句中使用活动模式结果。即
match pos with
| ??? -> printf "("
问题是可以在匹配语句中使用保存的活动模式结果吗?
如果是这样,怎么样?如果没有,需要解释它,以便逻辑上有意义。
可能为什么不这样做的例子。即语言规范,语法糖, 不应该允许绑定活动模式结果,ExprItems与PatItems
我查看了F#2.0语言规范(2010年4月) http://research.microsoft.com/en-us/um/cambridge/projects/fsharp/manual/spec.html#_Toc270597500
但是我没有认出任何可以确认答案的内容。
修改
如果我将代码更改为
let pos = (|Root|Nil|Single|First|Inner|Last|Unknown|) (tree.Parent, position)
match pos with
| Choice1Of7 (tree.Parent, position) -> printf "("
| _ -> ()
我在Choice1Of7之后得到以下错误(tree.Parent,position):
此表达式应具有类型单位,但此处的类型为“a *”b
正如Brian建议的那样
match pos with
| Choice1Of7 () -> printf "("
| _ -> ()
结束编辑
注意:我在下面尝试了这个代码,但我找到了一个更好的算法来解决它的问题。
// An F# function to print a CommonTree as an s-expression
//
// Note: Because the CommonTree data structure was created allowing null values,
// the null value has to be handled.
//
let commonTreeToSExpression100 (tree : BaseTree) =
// Define Active Pattern to create new view of node, i.e. BaseTree
// for position in list instead of content.
// Note: The name of the active pattern function is "|Root|Nil|Single|First|Inner|Last|Unknown|"
let (|Root|Nil|Single|First|Inner|Last|Unknown|) (tree : ITree , position) =
let parent = tree :?> BaseTree
match parent with
| null -> Root
| _ ->
let list = parent.Children
match obj.ReferenceEquals(list,null) with
| true -> Nil // This should never happen.
| false ->
let count = list.Count
// TODO: Handle count = 0
if (count = 1) then Single
elif (count > 1) && (position = 0) then First
elif (count > 1) && (position = count - 1) then Last
elif (count > 1) && (0 < position) && (position < count - 1) then Inner
else Unknown // This should never happen.
// Define walk/print function
let rec printTree (tree : BaseTree) (position) =
// Start an s-expression
match tree.Parent, position with
| Root | Single | First -> printf "("
| _ -> ()
// Note: Can save the result of an active pattern which is type Choice<uint,uint,uint,uint,uint,uint,uint>.
// The question is can the saved active pattern result be used in a match statement?
// If so, how? If not, need to explain it so that it logicaly makes sense.
// Give examples of possibly why not. i.e. Language Specification, syntactic sugar,
// should not have allowed result to be bound, not in symbol table but other table.
//let pos = ((|Root|Nil|Single|First|Inner|Last|Unknown|) (tree.Parent, position)) // This works / is allowed
// match pos with
// | ??? -> printf "(" // Can this work? What should ??? be?
// | _ -> ()
// Visit the node
printf "%s" tree.Text
// Space out the values
match tree.Parent, position with
| Root | First | Inner -> printf " "
| _ -> ()
// Process the children
// Note: BaseTree holds the ChildIndex, if this property was not available
// then the use of Seq.iter whould have to be changed for a mechanism that
// would allow the calculation of the child index as the list is processed.
match tree.Children with
| null -> ()
| _ ->
tree.Children |> Seq.iter (fun x -> printTree (x :?> BaseTree) x.ChildIndex)
printf " "
// End an s-expression
match tree.Parent, position with
| Root | Single | Last -> printf ")"
| _ -> ()
// Call the walk/print function
printTree tree 0
// Since s-experssions print as single line,
// print a newline so that what is printed after this
// is not on the same line as this s-expression.
printfn ""
答案 0 :(得分:5)
它不是特别优雅,但您可以使用用于表示活动模式的基础区分联合。使用N个选项的活动模式的结果使用类型Choice<'T1, .., 'Tn>
表示,其中包含Choice1OfN
.. ChoiceNOfN
成员。
这是一个简单的例子,只有三种情况:
let (|Pos|Neg|Zero|) n =
if n < 0 then Neg (-n)
elif n = 0 then Zero
else Pos n
let res = (|Pos|Neg|Zero|) 10
match res with
| Choice1Of3 n -> sprintf "pos %d" n
| Choice2Of3 n -> sprintf "neg %d" n
| Choice3Of3 () -> "zero"
在实践中,我可能不会使用这种方法,但我会定义一个自定义区分联盟:
type Number = Pos of int | Neg of int | Zero
let convertNumber n =
if n < 0 then Neg (-n)
elif n = 0 then Zero
else Pos n
let res = convertNumber 10
match res with
| Pos n -> sprintf "pos %d" n
| Neg n -> sprintf "neg %d" n
| Zero -> "zero"
这需要明确定义一个有区别的联合,但它使代码更具可读性。