X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=hints%2Fassignment_7_hint_3.mdwn;h=838a2af06c771c5a9a027ec884738cca9ba84aa0;hp=139597f9cb07c5d48bed18984ec4747f4b4f3438;hb=a63e962a82f5136828fa6688a12d259cb09c5a4e;hpb=4e38540a3d46c09c41f6c4591ab9b39b401ce050 diff --git a/hints/assignment_7_hint_3.mdwn b/hints/assignment_7_hint_3.mdwn index 139597f9..838a2af0 100644 --- a/hints/assignment_7_hint_3.mdwn +++ b/hints/assignment_7_hint_3.mdwn @@ -1,2 +1,32 @@ +* In def 2.5, they say the denotation of an e-type constant α wrt a discourse possibility `(r, g, w)` is whatever object the world `w` associates with α. Since we don't have worlds, this will just be an object. + +* They say the denotation of a predicate is whatever extension the world `w` associates with the predicate. Since we don't have worlds, this will just be an extension. + +* 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. + +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) = + 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' + (* the reason for returning a triple with () in first position will emerge *) + in ((), r',g') + +* At the top of p. 13 (this is in between defs 2.8 and 2.9), GS&V give two examples, one for \[[∃xPx]] and the other for \[[Qx]]. In fact it will be easiest for us to break \[[∃xPx]] into two pieces, \[[∃x]] and \[[Px]]. Let's consider expressions like \[[Px]] (or \[[Qx]]) first. +