Mercurial > urweb
comparison doc/manual.tex @ 2009:799be3911ce3
Monadic bind supports patterns
| author | Adam Chlipala <adam@chlipala.net> |
|---|---|
| date | Fri, 02 May 2014 17:16:02 -0400 |
| parents | 93ff76058825 |
| children | 403f0cc65b9c |
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| 2008:93ff76058825 | 2009:799be3911ce3 |
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| 1440 \hspace{.1in} \to \mt{monad} \; \mt{m} | 1440 \hspace{.1in} \to \mt{monad} \; \mt{m} |
| 1441 \end{array}$$ | 1441 \end{array}$$ |
| 1442 | 1442 |
| 1443 The Ur/Web compiler provides syntactic sugar for monads, similar to Haskell's \cd{do} notation. An expression $x \leftarrow e_1; e_2$ is desugared to $\mt{bind} \; e_1 \; (\lambda x \Rightarrow e_2)$, and an expression $e_1; e_2$ is desugared to $\mt{bind} \; e_1 \; (\lambda () \Rightarrow e_2)$. 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 $m \; t$. | 1443 The Ur/Web compiler provides syntactic sugar for monads, similar to Haskell's \cd{do} notation. An expression $x \leftarrow e_1; e_2$ is desugared to $\mt{bind} \; e_1 \; (\lambda x \Rightarrow e_2)$, and an expression $e_1; e_2$ is desugared to $\mt{bind} \; e_1 \; (\lambda () \Rightarrow e_2)$. 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 $m \; t$. |
| 1444 | 1444 |
| 1445 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. | |
| 1446 | |
| 1445 \subsection{Transactions} | 1447 \subsection{Transactions} |
| 1446 | 1448 |
| 1447 Ur is a pure language; we use Haskell's trick to support controlled side effects. The standard library defines a monad $\mt{transaction}$, meant to stand for actions that may be undone cleanly. By design, no other kinds of actions are supported. | 1449 Ur is a pure language; we use Haskell's trick to support controlled side effects. The standard library defines a monad $\mt{transaction}$, meant to stand for actions that may be undone cleanly. By design, no other kinds of actions are supported. |
| 1448 $$\begin{array}{l} | 1450 $$\begin{array}{l} |
| 1449 \mt{con} \; \mt{transaction} :: \mt{Type} \to \mt{Type} \\ | 1451 \mt{con} \; \mt{transaction} :: \mt{Type} \to \mt{Type} \\ |