与 OR 和 WHEN 子句匹配的模式

Question

我有问题，模式匹配没有按预期编译

当前的简化版本如下所示

match smth with
| pattern1 when isSomething pattern1 ->
   doWork()
| pattern2 when isSomething pattern2 ->
   doWork()
| _ -> () // do nothing

这里的问题是在不同的情况下body是重复的

我试过用OR pattern把它改成这个

match smth with
| pattern1 when isSomething pattern1
| pattern2 when isSomething pattern2 ->
   doWork()
| _ -> () // do nothing

但它给出了 Unexpected symbol '|' in pattern matching. Expected '->' or other token. F# Compiler 10

尝试用括号包围 isSomething 没有帮助。在那种情况下如何避免重复？

原代码

match state.SearchOptions with
| { Pattern = empty } when String.IsNullOrEmpty empty ->
    ValueSome (i, p, state.SelectedConfiguration, state.Solution.Configurations)
| { Pattern = pattern; IsRegex = true; IsCaseSensitive = true } when Regex.isMatch p.Name pattern ->
    ValueSome (i, p, state.SelectedConfiguration, state.Solution.Configurations)
| { Pattern = pattern; IsRegex = true; IsCaseSensitive = false } when Regex.isMatchCaseInsensitive p.Name pattern ->
    ValueSome (i, p, state.SelectedConfiguration, state.Solution.Configurations)
| { Pattern = pattern; IsRegex = false; IsCaseSensitive = true } when p.Name.Contains(pattern, StringComparison.InvariantCulture) ->
    ValueSome (i, p, state.SelectedConfiguration, state.Solution.Configurations)
| { Pattern = pattern; IsRegex = false; IsCaseSensitive = false } when p.Name.Contains(pattern, StringComparison.InvariantCultureIgnoreCase) ->
    ValueSome (i, p, state.SelectedConfiguration, state.Solution.Configurations)
| _ -> ValueNone

Answer 1

如评论中所述，这是无法做到的 - when 子句适用于整个子句。我认为有三种替代方法。

首先，您可以将结果值定义为 match 表达式之前的变量，然后仅 return 来自所有四种情况的变量（如评论中所建议）。

其次，您实际上可以将其写成一个普通的 if 表达式：

let so = state.SearchOptions
if String.IsNullOrEmpty so.Pattern then 
  ValueSome (i, p, state.SelectedConfiguration, state.Solution.Configurations)
elif ( so.IsRegex && so.IsCaseSensitive && 
       Regex.isMatch p.Name so.Pattern )
  || ( so.IsRegex && not so.IsCaseSensitive && 
       Regex.isMatchCaseInsensitive p.Name so.Pattern )
  || ( not so.IsRegex && so.IsCaseSensitive && 
       p.Name.Contains(so.Pattern, StringComparison.InvariantCulture) )
  || ( not so.IsRegex && not so.IsCaseSensitive && 
       p.Name.Contains(so.Pattern, StringComparison.InvariantCultureIgnoreCase) ) then
    ValueSome (i, p, state.SelectedConfiguration, state.Solution.Configurations)
else ValueNone

另一种选择是使用活动模式 - 这样，您就可以将条件集成到模式本身中。这样的事情应该可以解决问题：

let (|RegexMatch|_|) input pattern = 
  if Regex.isMatch input pattern then Some() else None
let (|RegexMatchInsensitive|_|) input pattern = 
  if Regex.isMatchCaseInsensitive input pattern then Some() else None
let (|Contains|_|) (input:string) pattern
  input.Contains(pattern, StringComparison.InvariantCulture)
let (|ContainsInsensitive|_|) (input:string) pattern
  input.Contains(pattern, StringComparison.InvariantCultureIgnoreCase)

match state.SearchOptions with
| { Pattern = empty } when String.IsNullOrEmpty empty ->
    ValueSome (i, p, state.SelectedConfiguration, state.Solution.Configurations)
| { Pattern = Match p.Name; IsRegex = true; IsCaseSensitive = true } 
| { Pattern = RegexMatchInsensitive p.Name; IsRegex = true; IsCaseSensitive = false } 
| { Pattern = Contains p.Name; IsRegex = false; IsCaseSensitive = true } 
| { Pattern = ContainsInsensitive p.Name; IsRegex = false; IsCaseSensitive = false } ->
    ValueSome (i, p, state.SelectedConfiguration, state.Solution.Configurations)
| _ -> ValueNone

如果我只在一个地方这样做，我可能会选择选项一或选项二（取决于您觉得哪个更易读）。如果您正在做更多类似的事情，第三个选项是一个不错的选择。

与 OR 和 WHEN 子句匹配的模式

Pattern matching with OR and WHEN clauses

f#

pattern-matching