From: Jim Pryor
Date: Tue, 21 Dec 2010 03:58:42 +0000 (-0500)
Subject: week9: {get,set}_state -> _store
X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=commitdiff_plain;h=dafe92a8eec67171182360ed891f0caa18abcc50
week9: {get,set}_state -> _store
Signed-off-by: Jim Pryor
---
diff --git a/week9.mdwn b/week9.mdwn
index 1992e207..9fa96ac1 100644
--- a/week9.mdwn
+++ b/week9.mdwn
@@ -469,29 +469,29 @@ Notice the similarities (and differences) between the implementation of these tw
With the Reader monad, we also had some special-purpose operations, beyond its general monadic operations. These were `lookup` and `shift`. With the State monad, we'll also have some special-purpose operations. We'll consider two basic ones here. One will be to retrieve what is the current store. This is like the Reader monad's `lookup`, except in this simple implementation there's only a single location for a value to be looked up from. Here's how we'll do it:
- let get_state : store state =
+ let get_store : store state =
fun s -> (s, s);;
This passes through the current store unaltered, and also returns a copy of the store as its value. We can use this operation like this:
- some_existing_state_monad_box >>= fun _ -> get_state >>= (fun cur_store -> ...)
+ some_existing_state_monad_box >>= fun _ -> get_store >>= (fun cur_store -> ...)
The `fun _ ->` part here discards the value wrapped by `some_existing_state_monad_box`. We're only going to pass through, unaltered, whatever *store* is generated by that monadic box. We also wrap that store as *our own value*, which can be retrieved by further operations in the `... >>= ...` chain, such as `(fun cur_store -> ...)`.
The other operation for the State monad will be to update the existing store to a new one. This operation looks like this:
- let set_state (new_store : int) : dummy state =
+ let set_store (new_store : int) : dummy state =
fun s -> (dummy, new_store);;
If we want to stick this in a `... >>= ...` chain, we'll need to prefix it with `fun _ ->` too, like this:
- some_existing_state_monad_box >>= fun _ -> set_state 100 >>= ...
+ some_existing_state_monad_box >>= fun _ -> set_store 100 >>= ...
-In this usage, we don't care what value is wrapped by `some_existing_state_monad_box`. We don't even care what store it generates, since we're going to replace that store with our own new store. A more complex kind of `set_state` operation might insert not just some constant value as the new store, but rather the result of applying some function to the existing store. For example, we might want to increment the current store. Here's how we could do that:
+In this usage, we don't care what value is wrapped by `some_existing_state_monad_box`. We don't even care what store it generates, since we're going to replace that store with our own new store. A more complex kind of `set_store` operation might insert not just some constant value as the new store, but rather the result of applying some function to the existing store. For example, we might want to increment the current store. Here's how we could do that:
- some_existing_state_monad_box >>= fun _ -> get_state >>= (fun cur_store -> set_state (cur_store + 1) >>= ...
+ some_existing_state_monad_box >>= fun _ -> get_store >>= (fun cur_store -> set_store (cur_store + 1) >>= ...
-We can of course define more complex functions that perform the `get_state >>= (fun cur_store -> set_state (cur_store + 1)` as a single operation.
+We can of course define more complex functions that perform the `get_store >>= (fun cur_store -> set_store (cur_store + 1)` as a single operation.
In general, a State monadic **box** (type `'a state`, what appears at the start of a `... >>= ... >>= ...` chain) is an operation that accepts some starting store as input---where the store might be simple as it is here, or much more complex---and returns a value plus a possibly modified store. This can be thought of as a static encoding of some computation on a store, which encoding is used as a box wrapped around a value of type `'a`. (And also it's a burrito.)