From: Jim Pryor Date: Thu, 18 Nov 2010 19:19:01 +0000 (-0500) Subject: assignment7 tweaks X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=commitdiff_plain;h=0548ea20113afa9723949ce0802aea8a874b2cd4 assignment7 tweaks Signed-off-by: Jim Pryor --- diff --git a/hints/assignment_7_hint_3.mdwn b/hints/assignment_7_hint_3.mdwn index 0629c1ba..3fec7e2d 100644 --- a/hints/assignment_7_hint_3.mdwn +++ b/hints/assignment_7_hint_3.mdwn @@ -11,19 +11,19 @@ We're going to keep all of that, except dropping the worlds. And instead of talk we'll just talk about \[[expression]] and let that be a monadic object, implemented in part by a function that takes `(r, g)` as an argument. -More specifically, \[[expression]] will be a set of `'a discourse possibility` monads, where `'a` is the appropriate type for *expression*, and the discourse possibility monads are themselves state monads where `(r, g)` is the state that gets updated. Those are implemented as functions from `(r, g)` to `(a, r', g')`, where `a` is a value of type `'a`, and `r', g'` are possibly altered assignment functions and stores. +More specifically, \[[expression]] will be a set of `'a discourse_possibility` monads, where `'a` is the appropriate type for *expression*, and the discourse possibility monads are themselves state monads where `(r, g)` is the state that gets updated. Those are implemented as functions from `(r, g)` to `(a, r', g')`, where `a` is a value of type `'a`, and `r', g'` are possibly altered assignment functions and stores. * In def 2.7, GS&V talk about an operation that takes an existing set of discourse possibilities, and extends each member in the set by allocating a new location in the store, and assigning a variable `'x'` to that location, which holds some object `d` from the domain. It will be useful to have a shorthand way of referring to this operation: - let newpeg_and_bind (variable : char) (d : entity) = + let newpeg_and_bind (bound_variable : char) (d : entity) = fun ((r, g) : assignment * store) -> let newindex = List.length g (* first we store d at index newindex in g, which is at the very end *) (* the following line achieves that in a simple but very inefficient way *) in let g' = List.append g [d] (* next we assign 'x' to location newindex *) - in let r' = fun variable' -> - if variable' = variable then newindex else r variable' + in let r' = fun v -> + if v = bound_variable then newindex else r v (* the reason for returning a triple with () in first position will emerge *) in ((), r',g')