f# - Use of typeof<_> in active pattern -

given following contrived active pattern:

let (|typedef|_|) (typedef:type) (value:obj) =   if obj.referenceequals(value, null) none   else     let typ = value.gettype()     if typ.isgenerictype && typ.getgenerictypedefinition() = typedef some(typ.getgenericarguments())     else none 

the following:

let dict = system.collections.generic.dictionary<string,obj>() match dict | typedef typedefof<dictionary<_,_>> typeargs -> printfn "%a" typeargs | _ -> () 

gives error:

unexpected type application in pattern matching. expected '->' or other token.

but works:

let typ = typedefof<dictionary<_,_>> match dict | typedef typ typeargs -> printfn "%a" typeargs | _ -> () 

why typedefof (or typeof) not allowed here?

even if you're using parameterized active pattern (where argument expression), compiler parses argument pattern (as opposed expression), syntax more restricted.

i think same problem 1 discussed here: how can pass complex expression parametrized active pattern? (i'm not sure actual compiler implementation, f# specification says should parse pattern).

as workaround, can write expression inside quotation, this:

let undef<'t> : 't = unchecked.defaultof<_>  let (|typedef|) (typeexpr:expr) (value:obj) =   let typedef = typeexpr.type.getgenerictypedefinition()   // ...  let dict = system.collections.generic.dictionary<string,obj>() match dict | typedef <@ undef<dictionary<_,_>> @> typeargs -> printfn "%a" typeargs | _ -> ()