What is the difference between the generic signifier ' and the symbol ^ In F# method signatures

Question

I understand the tick to signify a generic parameter, as in:

Seq.append : seq<'T> -> seq<'T> -> seq<'T>

but what does the caret signify, as in:

Seq.average : seq<^T> -> ^T

Answer 1

The detailed signature is:

Seq.average : seq<^T> -> ^T (requires ^T with static member (+) and ^T with static member DivideByInt and ^T with static member Zero)

Unlike Seq.append, Seq.average needs some more constraints on type of elements. Particularly:

                                _ DivideByInt (s1 + s2 + ... + sn) n where n <> 0
Seq.average {s1; s2;...; sn} = /
                               \_ ^T.Zero where n = 0

As you can see, both (+), DivideByInt and Zero are required in order that Seq.average<^T> makes sense.

Useful information about generics could be found here^MSDN.