Question

Using the wildcard pattern with use works within a sequence expression, but not otherwise. Is there a reason for this?

let mkDisposable() = 
  { new IDisposable with 
      member __.Dispose() = () }

let mkSeq() =  
  seq {
    use _ = mkDisposable() //OK
    ()
  }

let f() =  
  use _ = mkDisposable() //ERROR: 'use' bindings must be of the form 'use <var> = <expr>'
  ()

Answer 1

I believe that this is a natural (but unexpected) consequence of the desugaring of computation expressions (a sequence expression in this case, but this behavior applies to all computation expressions). As the spec indicates,

use pat = expr
cexpr

is translated to

Using(expr, fun pat -> cepxr)

Because this is a shallow syntactic translation, you can use any pattern that could be used when writing a function, including just _. However, for normal use bindings, the left hand side of the binding must be an identifier, not a pattern (see section 6.6.3 of the spec).