X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?a=blobdiff_plain;ds=sidebyside;f=hints%2Fassignment_7_hint_3.mdwn;h=3fec7e2d3289ad3f8b285f72ba350d30b02cd388;hb=0548ea20113afa9723949ce0802aea8a874b2cd4;hp=1cd62133b3d463f4b592214318e3c9e654f44ce2;hpb=46a03071aee813d1ac978b09766f05d06471ecd4;p=lambda.git diff --git a/hints/assignment_7_hint_3.mdwn b/hints/assignment_7_hint_3.mdwn index 1cd62133..3fec7e2d 100644 --- a/hints/assignment_7_hint_3.mdwn +++ b/hints/assignment_7_hint_3.mdwn @@ -5,21 +5,25 @@ * They say the denotation of a variable is the object which the store `g` assigns to the index that the assignment function `r` assigns to the variable. In other words, if the variable is `'x'`, its denotation wrt `(r, g, w)` is `g[r['x']]`. -We're going to keep all of that, except dropping the worlds. And instead of talking about "\[[expression]] in discourse possibility `(r, g, w)`," 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. +We're going to keep all of that, except dropping the worlds. And instead of talking about -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. +> \[[expression]] in possibility `(r, g, w)` + +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. * 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')