F#: how to elegantly select and group discriminated unions?

Say I have a list of shapes:

type shape = 
| Circle of float
| Rectangle of float * float

let a = [ Circle 5.0; Rectangle (4.0, 6.0)]

How can I then test e.g. a Circle exists in a? I could create a function for each shape

let isCircle s = 
    match s with
    | Circle -> true
    | _ -> false
List.exists isCircle a

but I feel there must be a more elegant way in F#, other than having to define such a function for each shape type. Is there?

Related question is how to group a list of shapes, based on shape types:

a |> seq.groupBy( <shapetype? >)