Type test pattern with array [duplicate]

Question

I have a situation where I am handed an object and need to produce a string. In most cases, I can just call .ToString() and be done with it (works for strings, numbers, dates, etc.)

One situation where this doesn't work is a byte array representing a GUID. In that case, ToString() simply reports "System.Byte[]". I am trying to use pattern matching to catch byte arrays so that I can customize the string-ification. But I can't figure out how to match a byte array (or, really, any array) except via the Array pattern. A GUID is 16 bytes long, and I really don't want to have a pattern with 16 named array elements. Also, using the array pattern causes F# to want an array as the input, rather than an object:

match value (* <-- obj *) with
| [|
    b1;  b2;  b3;  b4;
    b5;  b6;  b7;  b8;
    b9;  b10; b11; b12;
    b13; b14; b15; b16
  |] -> (* do some magic here later... *) b1.ToString()
| x -> x.ToString()

> This expression was expected to have type obj but here has type 'a []

I tried to use Type test pattern, but it results in a compiler error (seems to have a problem with the opening square bracket):

match value (* <-- obj *) with
| :? byte[] as guid -> (* do some magic here later... *) guid.ToString()
| x -> x.ToString()

> Unexpected symbol '[' in pattern matching. Expected '->' or other token.

The F# documentation on Pattern Matching doesn't seem to cover array type tests, and I haven't found anything via Google that suits my needs. What's the syntax here?

Answer 1

You can use:

match o with
| :? array<byte> as guid -> ...

You can also put brackets around the type name:

 match o with
    | :? (byte[]) as guid -> ...

or

 match o with
    | :? (byte array) as guid -> ...