如何将复杂的表达式传递给参数化的活动模式? [英] How can I pass complex expression to parametrized active pattern?

问题描述

我将活动模式"Expression"定义如下:

I defined the active pattern "Expression" as follows:

let (|Expression|_|) expression _ = Some(expression)

现在我正尝试以这种方式使用它:

Now I'm trying to use it in this way:

match () with
| Expression((totalWidth - wLeft - wRight) / (float model.Columns.Count - 0.5)) cw
    when cw <= wLeft * 4. && cw <= wRight * 4. ->
        cw
| Expression((totalWidth - wLeft) / (float model.Columns.Count - .25)) cw
    when cw <= wLeft * 4. && cw > wRight * 4. ->
        cw
| Expression((totalWidth - wRight) / (float model.Columns.Count - .25)) cw
    when cw > wLeft * 4. && cw <= wRight * 4. ->
        cw
| Expression(totalWidth / float model.Columns.Count) cw
    when cw > wLeft * 4. && cw > wRight * 4. ->
        cw
| _ -> System.InvalidProgramException() |> raise

但这会导致错误FS0010:模式中出现意外的符号'-'".那可以解决吗?

But this results in "error FS0010: Unexpected symbol '-' in pattern". Is that fixable?

我想做的是清楚地写出以下方程式的解决方案:

What am I trying to do is to write clearly a solution to the following equation:

max(wl-cw * .25，0)+ max(wr-cw * .25)+ cw * columnCount = ActualWidth

max(wl - cw * .25, 0) + max(wr - cw * .25) + cw * columnCount = ActualWidth

其中cw是唯一变量.

您能提出更好的建议吗?

Can you suggest any better way?

推荐答案

可用作参数化活动模式参数的表达式的语言在某些方面受到限制.据我所知， F#规范并未明确指出，但语法表明必须有可能将自变量表达式解析为pat-param(第90页):

The langauge of expressions that can be used as arguments for parameterized active patterns is limited in some ways. As far as I can tell, the F# specification doesn't say that explicitly, but the grammar suggests that it must be possible to parse the argument expression as pat-param (page 90):

pat-param:=
    | const
    | 长期身份
    | [ pat-param ； ...; pat-param ]
    | ( pat-param ，...， pat-param )
    | 长期身份 pat-param
    | pat-param : type
    | < @ expr @>
    | < @@ expr @@>
    |空

pat-param :=
    | const
    | long-ident
    | [ pat-param ; ... ; pat-param ]
    | ( pat-param, ..., pat-param )
    | long-ident pat-param
    | pat-param : type
    | <@ expr @>
    | <@@ expr @@>
    | null

因此，我认为您需要以不同的方式编写匹配的模式.您可以将表达式转换为match构造的普通参数，并编写如下内容:

So, I think you'll need to write your pattern matching differently. You could turn the expressions into ordinary arguments of the match construct and write something like this:

match 
  (totalWidth - wLeft - wRight) / (float model.Columns.Count - 0.5),
  (totalWidth - wLeft) / (float model.Columns.Count - .25),
  (totalWidth - wRight) / (float model.Columns.Count - .25)
with
| cw1, _, _ when cw1 <= wLeft * 4. && cw1 <= wRight * 4. -> cw1
| _, cw2, _ when cw2 <= wLeft * 4. && cw2 > wRight * 4. -> cw2
| _, _, cw3 when cw3 > wLeft * 4. && cw3 <= wRight * 4. -> cw3
| _ -> totalWidth / float model.Columns.Count

如果表达式中使用的模式始终相同，则还可以使用活动模式，例如:

If the pattern used in the expression is always the same, you could also use active pattern like:

let (|Calculate|) w p _ =
  (totalWidth - w) / (float model.Columns.Count - p)

...然后编写类似的内容:

... and then write something like:

let wDif = wLeft - wRight
match () with
| Calculate wDif 0.5 cw -> cw
| Calculate wLeft 0.25 cw -> cw
// .. etc.

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