Monads

The Ur Basis defines the monad constructor class from Haskell.

$$\begin{array}{l} \mathsf{class} \; \mathsf{monad} :: \mathsf... ...\\ \hspace{.1in} \to \mathsf{monad} \; \mathsf{m} \end{array}$$

The Ur/Web compiler provides syntactic sugar for monads, similar to Haskell's do notation. An expression $x \leftarrow e_1; e_2$ is desugared to $\mathsf{bind} \; e_1 \; (\lambda x \Rightarrow e_2)$ , and an expression $e_1; e_2$ is desugared to $\mathsf{bind} \; e_1 \; (\lambda () \Rightarrow e_2)$ . Note a difference from Haskell: as the $e_1; e_2$ case desugaring involves a function with $()$ as its formal argument, the type of $e_1$ must be of the form $m \; \{\}$ , rather than some arbitrary $m \; t$ .

The syntactic sugar also allows $p \leftarrow e_1; e_2$ for $p$ a pattern. The pattern should be guaranteed to match any value of the corresponding type, or there will be a compile-time error.