The naive model for begin is to think of it as a function that returns its last argument (assuming left-to-right evaluation). So that doesn’t scale.
The example without begin you mention indeed behaves the same, and that follows from the model of “let has an implicit begin” that I tell them about. So at least to me, the complexity comes from understanding begin properly. Thanks, -- Éric > On Nov 3, 2015, at 11:59 AM, Scott Moore <sdmo...@fas.harvard.edu> wrote: > > I think its more a complexity of “define” than of “begin” (though begin is > certainly tricky be because it can introduce and internal definition context). > > Your student’s program has the same behavior as this program without the > begin, since the let also introduces a definition context: > > (let ([y 5]) > (display y) > (define y 10) > y) > On November 3, 2015 at 8:51:38 AM, Éric Tanter (etan...@dcc.uchile.cl > <mailto:etan...@dcc.uchile.cl>) wrote: > >> Thanks all! This is helpful — the mental model of begin that I presented to >> my students was too simple to account for defines. >> >> -- Éric >> >> >>> On Nov 2, 2015, at 10:00 PM, Scott Moore <sdmo...@fas.harvard.edu >>> <mailto:sdmo...@fas.harvard.edu>> wrote: >>> >>> The relevant part of the reference for these “internal definition contexts” >>> is here: >>> http://docs.racket-lang.org/reference/syntax-model.html#%28part._intdef-body%29 >>> >>> <https://urldefense.proofpoint.com/v2/url?u=http-3A__docs.racket-2Dlang.org_reference_syntax-2Dmodel.html-23-2528part.-5Fintdef-2Dbody-2529&d=CwMFaQ&c=WO-RGvefibhHBZq3fL85hQ&r=OPR-Xys5wfSBIeTkWaH0D_htBR-X7qY24pTHU6ib2iM&m=SB0ys8d9nuzR8hLakyqx5MZ5TQiQW_UpwySsBUxWg30&s=5ElZW1aRhlESddZVvuawVv30hjKxA5PbsHq3AAQgUUU&e=> >>> >>> That’s a very operational description, but amounts to saying “the bindings >>> from a define in an internal definition context are visible to all other >>> definitions/expressions in the same internal definition context." >>> On November 2, 2015 at 7:52:43 PM, Scott Moore (sdmo...@fas.harvard.edu >>> <mailto:sdmo...@fas.harvard.edu>) wrote: >>> >>>> Does it help to think of let and begin as introducing a new scope, and >>>> define as having “block scope” (or whatever they call this sort of thing >>>> in trendy languages like javascript)? >>>> >>>> I’m guessing the reason you found this to be unintuitive is that the ‘y’ >>>> in (display y) refers to something that comes after it in the source, >>>> which would not be the case if the semantics of define were more like a >>>> let: >>>> >>>> (let ([y 5]) >>>> (begin >>>> (display y) >>>> (define y 10) >>>> y)))) >>>> >>>> => >>>> >>>> (let ([y 5]) >>>> (begin >>>> (display y) >>>> (let ([y 10]) >>>> y)))) >>>> >>>> But that ‘y’ appears in the scope of the begin, which is the scope in >>>> which the define inserts it’s bindings. >>>> >>>> Not sure if this was helpful… >>>> On November 2, 2015 at 7:39:16 PM, Robby Findler >>>> (ro...@eecs.northwestern.edu <mailto:ro...@eecs.northwestern.edu>) wrote: >>>> >>>>> Yeah, perhaps I've drunk too much of the koolaid, but I'm not even >>>>> seeing an alternative interpretation that makes any sense! >>>>> >>>>> Does it help to see the arrows in DrRacket? In particular the upward >>>>> pointing one that points at the 'y' in display's argument? >>>>> >>>>> Robby >>>>> >>>>> >>>>> On Mon, Nov 2, 2015 at 6:32 PM, Alex Knauth <alexan...@knauth.org >>>>> <mailto:alexan...@knauth.org>> wrote: >>>>> > This is because begin can have potentially recursive and mutually >>>>> > recursive >>>>> > definitions in it. >>>>> > >>>>> > This does the same thing: >>>>> > (let ([y 5]) >>>>> > (local [(define x y) ; this y should be bound to >>>>> > (define y 10)] ; <- this y, but it is used too early >>>>> > y)) >>>>> > >>>>> > While this slightly different case works fine: >>>>> > (let ([y 5]) >>>>> > (local [(define (x) y) ; this y is bound to >>>>> > (define y 10)] ; <- this y, but it is within a function >>>>> > y)) >>>>> > >>>>> > When the first y in the local is within a function, it isn't evaluated >>>>> > until >>>>> > the function is called, so that's fine. >>>>> > >>>>> > But when the first y in the local is evaluated right away, before it is >>>>> > defined, it raises an error. >>>>> > >>>>> > It doesn't make sense for it to be bound to the outer y, because then it >>>>> > would be inconsistent with the version where it is within a function. >>>>> > >>>>> > >>>>> > On Nov 2, 2015, at 7:13 PM, Éric Tanter <etan...@dcc.uchile.cl >>>>> > <mailto:etan...@dcc.uchile.cl>> wrote: >>>>> > >>>>> > Hi all, >>>>> > >>>>> > Some of my creative students came up with the following: >>>>> > >>>>> > (let ([y 5]) >>>>> > (begin >>>>> > (display y) >>>>> > (define y 10) >>>>> > y))) >>>>> > >>>>> > which raises a mysterious >>>>> > y: undefined; >>>>> > cannot use before initialization >>>>> > >>>>> > I remember earlier discussion on this list about the fact that `define' >>>>> > was >>>>> > somehow “broken” for various cases of nesting, and that it was part of >>>>> > the >>>>> > motivation for coming up with `local’. Sure enough, the following works >>>>> > as >>>>> > expected: >>>>> > >>>>> > (let ([y 5]) >>>>> > (begin >>>>> > (display y) >>>>> > (local [(define y 10)] >>>>> > y))) >>>>> > >>>>> > While I don’t intend to motivate my students to do funky let/define >>>>> > nesting, >>>>> > I would like to be able to explain why the error is raised. Any >>>>> > (sensible) >>>>> > explanation? >>>>> > >>>>> > Thanks! >>>>> > >>>>> > -- Éric >>>>> > >>>>> > -- >>>>> > You received this message because you are subscribed to the Google >>>>> > Groups >>>>> > "Racket Users" group. >>>>> > To unsubscribe from this group and stop receiving emails from it, send >>>>> > an >>>>> > email to racket-users+unsubscr...@googlegroups.com >>>>> > <mailto:racket-users+unsubscr...@googlegroups.com>. >>>>> > For more options, visit https://groups.google.com/d/optout >>>>> > <https://urldefense.proofpoint.com/v2/url?u=https-3A__groups.google.com_d_optout&d=CwMFaQ&c=WO-RGvefibhHBZq3fL85hQ&r=OPR-Xys5wfSBIeTkWaH0D_htBR-X7qY24pTHU6ib2iM&m=SB0ys8d9nuzR8hLakyqx5MZ5TQiQW_UpwySsBUxWg30&s=cyfarVv9aewI9mgi-OMl5YdbYwxsRtTH7z9ktvDxEbI&e=>. >>>>> > >>>>> > >>>>> > -- >>>>> > You received this message because you are subscribed to the Google >>>>> > Groups >>>>> > "Racket Users" group. >>>>> > To unsubscribe from this group and stop receiving emails from it, send >>>>> > an >>>>> > email to racket-users+unsubscr...@googlegroups.com >>>>> > <mailto:racket-users+unsubscr...@googlegroups.com>. >>>>> > For more options, visit https://groups.google.com/d/optout >>>>> > <https://urldefense.proofpoint.com/v2/url?u=https-3A__groups.google.com_d_optout&d=CwMFaQ&c=WO-RGvefibhHBZq3fL85hQ&r=OPR-Xys5wfSBIeTkWaH0D_htBR-X7qY24pTHU6ib2iM&m=SB0ys8d9nuzR8hLakyqx5MZ5TQiQW_UpwySsBUxWg30&s=cyfarVv9aewI9mgi-OMl5YdbYwxsRtTH7z9ktvDxEbI&e=>. >>>>> >>>>> -- >>>>> You received this message because you are subscribed to the Google Groups >>>>> "Racket Users" group. >>>>> To unsubscribe from this group and stop receiving emails from it, send an >>>>> email to racket-users+unsubscr...@googlegroups.com >>>>> <mailto:racket-users+unsubscr...@googlegroups.com>. >>>>> For more options, visit https://groups.google.com/d/optout >>>>> <https://urldefense.proofpoint.com/v2/url?u=https-3A__groups.google.com_d_optout&d=CwMFaQ&c=WO-RGvefibhHBZq3fL85hQ&r=OPR-Xys5wfSBIeTkWaH0D_htBR-X7qY24pTHU6ib2iM&m=SB0ys8d9nuzR8hLakyqx5MZ5TQiQW_UpwySsBUxWg30&s=cyfarVv9aewI9mgi-OMl5YdbYwxsRtTH7z9ktvDxEbI&e=>. >> > > -- > You received this message because you are subscribed to the Google Groups > "Racket Users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to racket-users+unsubscr...@googlegroups.com > <mailto:racket-users+unsubscr...@googlegroups.com>. > For more options, visit https://groups.google.com/d/optout > <https://groups.google.com/d/optout>. -- You received this message because you are subscribed to the Google Groups "Racket Users" group. 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