[Ur] typing bind

Thu Aug 18 09:57:37 EDT 2011

> The situation is exactly the same as in Haskell.  The key is that [src <-
> source 0] by itself is not an expression.  Rather, the holistic form [x <-
> e1; e2] is shorthand for [bind e1 (fn x => e2)].

Thanks. Now, keeping track of typing in increment.ur:

sequence seq

fun increment () = nextval seq

fun main () =
    src <- source 0;
    return <xml><body>
      <dyn signal={n <- signal src; return <xml>{[n]}</xml>}/>
      <button value="Update" onclick={n <- rpc (increment ()); set src n}/>
    </body></xml>

src <- source 0 ; return <xml> is a shorthand for

bind (source 0) (fn src => return xml) whose type is

(m t1 -> (t1 -> m t2) -> m t2) (source int) (int -> transaction xml) =
transaction xml

and, src has type int.

But in n <- signal src; return <xml>{[n]}</xml> where

signal : source t -> signal t

the signal function expects a source int, not an int. What is wrong
with this line of thought?

- Gergely