From 2d413395c1eb768b35a9ecf409aa7730ab89d629 Mon Sep 17 00:00:00 2001 From: jim Date: Mon, 6 Apr 2015 17:56:31 -0400 Subject: [PATCH] revisions --- topics/_week9_state_monad_tutorial.mdwn | 28 ++++++++++++++-------------- 1 file changed, 14 insertions(+), 14 deletions(-) diff --git a/topics/_week9_state_monad_tutorial.mdwn b/topics/_week9_state_monad_tutorial.mdwn index e586ae06..14628f7b 100644 --- a/topics/_week9_state_monad_tutorial.mdwn +++ b/topics/_week9_state_monad_tutorial.mdwn @@ -99,9 +99,9 @@ Here is how you'd have to do it using our OCaml/Juli8 monad library: # module S = Monad.State(struct type store = store' end);; # let increment_store'' : 'a S.t = S.(get >>= fun cur -> - let value = cur.total - in let s' = { total = succ cur.total; modifications = succ cur.modifications } - in put s' >> mid value);; + let value = cur.total in + let s' = { total = succ cur.total; modifications = succ cur.modifications } in + put s' >> mid value);; Let's try it out: @@ -111,7 +111,7 @@ Let's try it out: Or if you used the OCaml/Juli8 monad library: - # S.(run increment_store'') s0;; + # S.run increment_store'' s0;; - : int * S.store = (42, {total = 43; modifications = 4}) Great! @@ -120,7 +120,7 @@ Can you write a monadic value that instead of incrementing each of the `total` a What about a value that increments each of `total` and `modifications` twice? Well, you could custom-write that, as with the previous question. But we already have the tools to express it easily, using our existing `increment_store` value: - increment_store >>= fun value -> increment_store >> unit value + increment_store >>= fun value -> increment_store >> mid value That ensures that the value we get at the end is the value returned by the first application of `increment_store`, that is, the contents of the `total` field in the store before we started modifying the store at all. @@ -136,17 +136,17 @@ or, using pattern-matching on the record (you don't have to specify every field But **the point of learning how to do this monadically** is that (1) monads show us how to embed more sophisticated programming techniques, such as imperative state and continuations, into frameworks that don't natively possess them (such as the set-theoretic metalanguage of Groenendijk, Stokhof and Veltman's paper); (2) becoming familiar with monads will enable you to see patterns you'd otherwise miss, and implement some seemingly complex computations using the same simple patterns (same-fringe is an example); and finally, of course (3) monads are delicious. -Keep in mind that the final result of a bind chain doesn't have to be the same type as the starting value: +Keep in mind that the final result of a `mbind` chain doesn't have to be the same type as the starting value: - increment_store >>= fun value -> increment_store >> unit (string_of_int value) + increment_store >>= fun value -> increment_store >> mid (string_of_int value) Or: - unit 1 >> unit "blah" + mid 1 >> mid "blah" The store keeps the same type throughout the computation, but the type of the wrapped value can change. -What are the special-purpose operations that the `State_monad` module defines for us? +What are the special-purpose operations that the `Monad.State` module defines for us? * `get` is a monadic value that passes through the existing store unchanged, and also wraps that same store as its boxed payload. You use it like this: @@ -164,7 +164,7 @@ What are the special-purpose operations that the `State_monad` module defines fo ... >> gets (fun cur -> cur.total) >>= fun total -> ... - For more complex structured stores, consider using the `Ref_monad` version of the State monad in the OCaml library. + For more complex structured stores, consider using the `Monad.Ref` variant of the State monad in the OCaml library. * `put new_store` replaces the existing store with `new_store`. Use it like this: @@ -172,25 +172,25 @@ What are the special-purpose operations that the `State_monad` module defines fo As that code snippet suggests, the boxed payload after the application of `modify new_store` is just `()`. If you want to preserve the existing payload but replace the store, do this: - ... >>= fun value -> put new_store >> unit value >>= ... + ... >>= fun value -> put new_store >> mid value >>= ... * Finally, `modify modifier` applies `modifier` to whatever the existing store is, and substitutes that as the new store. As with `put`, the boxed payload afterwards is `()`. Here's an example from "A State Monad Tutorial": increment_store >> get >>= fun cur -> State (fun s -> ((), { total = s.total / 2; modifications = succ s.modifications })) >> - increment_store >> unit cur.total + increment_store >> mid cur.total Or, as you'd have to write it using our OCaml monad library: increment_store'' >> get >>= fun cur -> put { total = cur.total / 2; modifications = succ cur.modifications } >> - increment_store'' >> unit cur.total + increment_store'' >> mid cur.total The last topic covered in "A State Monad Tutorial" is the use of do-notation to work with monads in Haskell. We discuss that on our [translation page](/translating_between_OCaml_Scheme_and_Haskell). -- 2.11.0