X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=hints%2Fassignment_7_hint_1.mdwn;h=a68c826a0f04301343ed32df232aa40470c841a7;hp=f7d5d288fc4a0fe7f9dcb79519cad2b703a59da9;hb=9c7ca26e6dd0c1ce1b6cd653e27a083b7379a5dd;hpb=4e38540a3d46c09c41f6c4591ab9b39b401ce050 diff --git a/hints/assignment_7_hint_1.mdwn b/hints/assignment_7_hint_1.mdwn index f7d5d288..a68c826a 100644 --- a/hints/assignment_7_hint_1.mdwn +++ b/hints/assignment_7_hint_1.mdwn @@ -3,11 +3,13 @@ * Where they represent pegs as natural numbers, that corresponds to our representing locations in a store by their indexes in the store. -* Where they say "reference system," which they use the leter `r` for, that corresponds to what we've been calling "assignments", and use the letter `g` for. +* Where they say "reference system," which they use the letter `r` for, that corresponds to what we've been calling "assignments", and have been using the letter `g` for. -* Where they say `r[x/n]`, that's our `g{x:=n}`. +* Where they say `r[x/n]`, that's our `g{x:=n}`, which we could represent in OCaml as `fun var -> if var = 'x' then n else g var`. (Earlier we represented assignments as lists of pairs, here we're representing them as functions. Either can work.) -* Their function `g`, which assigns objects from the domain to pegs, corresponds to our store function, which assigns entities to indexes. +* Their function `g`, which assigns entities from the domain to pegs, corresponds to a store with several indexes. To avoid confusion, I'll use `r` for assignments, like they do, and avoid using `g` altogether. Instead I'll use `h` for stores. (We can't use `s` because GS&V use that for something else, which they call "information states.") -* [More hints](/hints/assignment_7_hint_2). +* At several places they talk about some things being *real extensions* of other things. This confused me at first, because they don't ever define a notion of "real extension." (They do define what they mean by "extensions.") Eventually, it emerges that what they mean is what I'd call a *proper extension*: an extension which isn't identical to the original. + +* Is that enough? If not, here are some [more hints](/hints/assignment_7_hint_2). But try to get as far as you can on your own.