Trouble with FSharp active patterns

Question

I'm parsing code quotations in FSharp and am building up pattern helpers. All was going well till I tried

let (|BinaryFn|_|) fn (input:Expr) = 
    function
    | SpecificCall fn (_,_,l::r::[]) -> Some(l,r)
    | _ -> None

let (|Multiply|_|) x = 
    function
    | BinaryFn <@ (*) @> (l,r) -> Some(l,r)
    | _ -> None 

The intention is to have a generic binary function matcher that returns the ''left'' and the ''right'' and then create specialised binary matchers such as Multiple, Divide and Add and Subtract.

However I get an error on the second pattern that

Error FS0001: Type mismatch. Expecting a     

'a -> 'b option     

but given a     

'a -> 'c -> (Expr * Expr) option     

The type ''a option' does not match the type 

''b -> (Expr * Expr) option' (FS0001) (Shambolics)

Can somebody please enlighten me on what I should be doing here?

Answer 1

The issue here is that function doesn't only pattern-match the last argument, but also adds an additional argument (function is a combination between of fun and match). Remove the function argument input from the first pattern, and your problem will be solved.