Transactions

Ur is a pure language; we use Haskell's trick to support controlled side effects. The standard library defines a monad $\mathsf{transaction}$ , meant to stand for actions that may be undone cleanly. By design, no other kinds of actions are supported.

$$\begin{array}{l} \mathsf{con} \; \mathsf{transaction} :: \ma... ...on\_monad} : \mathsf{monad} \; \mathsf{transaction} \end{array}$$

For debugging purposes, a transactional function is provided for outputting a string on the server process' stderr.

$$\begin{array}{l} \mathsf{val} \; \mathsf{debug} : \mathsf{string} \to \mathsf{transaction} \; \mathsf{unit} \end{array}$$