SICP isn' the bible, especially not on programming language knowledge. I'd recommend checking out relevant literature instead.
On May 18, 2015, at 8:24 AM, Michael Tiedtke <michael.tied...@o2online.de> wrote: > Structure and Interpretation of Computer Programs (sicp2) about sameness: > > https://mitpress.mit.edu/sicp/full-text/book/book-Z-H-16.html#footnote_Temp_230 >> We can consider two symbols to be ``the same'' if they consist of the same >> characters in the same order. Such a definition skirts a deep issue that we >> are not yet ready to address: the meaning of ``sameness'' in a programming >> language. We will return to this in chapter 3 (section 3.1.3). > > https://mitpress.mit.edu/sicp/full-text/book/book-Z-H-20.html#%25_idx_2994 >> A language that supports the concept that ``equals can be substituted for >> equals'' in an expresssion without changing the value of the expression is >> said to be referentially transparent. Referential transparency is violated >> when we include set! in our computer language. This makes it tricky to >> determine when we can simplify expressions by substituting equivalent >> expressions. Consequently, reasoning about programs that use assignment >> becomes drastically more difficult. >> >> Once we forgo referential transparency, the notion of what it means for >> computational objects to be ``the same'' becomes difficult to capture in a >> formal way. Indeed, the meaning of ``same'' in the real world that our >> programs model is hardly clear in itself. In general, we can determine that >> two apparently identical objects are indeed ``the same one'' only by >> modifying one object and then observing whether the other object has changed >> in the same way. But how can we tell if an object has ``changed'' other than >> by observing the ``same'' object twice and seeing whether some property of >> the object differs from one observation to the next? Thus, we cannot >> determine ``change'' without some a priori notion of ``sameness,'' and we >> cannot determine sameness without observing the effects of change. > > Maybe the uncertainities of wether to use eq?, eqv? or equal? have their > roots in the general concept of sameness and the needs of different > implementations. Two symbols with the same sequence of characters should be > eq? and eqv?. > > But eq? is not well defined for numbers: (eq? 2 2) is unspecified but what > about (eq? '2 '2)? > (+ '1 '2) => 3 because the reader treats numerical symbols in another way. > > Anyway (eq? 'l 'l) should always be true even though this morning I read it > as (eq? 1 1) which looks almost the same. > > > Il giorno 18/mag/2015, alle ore 07.28, Alexis King ha scritto: > >> Yes, Scheme (and therefore Racket) has eq?, eqv?, and equal?. I understand >> the desire for eq? and equal?, but I’ve always been skeptical of the >> necessity of eqv?. Either way, Scheme left this behavior unspecified, but I >> believe Racket specifies it (though I could be wrong). >> >> Racket has two kinds of symbols, interned and uninterned. Symbols produced >> by the reader, whether via read or read-syntax, are interned. Additionally, >> string->symbol produces interned symbols. Certain functions such as gensym >> and string->uninterned-symbol produce uninterned symbols, but these are not >> commonly encountered, anyway. >> >> Interned symbols, I believe, are guaranteed to be eq? since they’re, well, >> interned. There is a pool of interned symbols such that all symbols made up >> of the same string are all the same object, so they’re all eq?. >> >> I can’t reproduce the behavior in the original message, and I don’t think it >> should be possible. >> >>> On May 17, 2015, at 22:19, Michael Tiedtke <michael.tied...@o2online.de> >>> wrote: >>> >>> I'm new to Racket but even R5RS is rather clear about this issue: >>> >>> (citation from doc/r5rs/r5rs-std/r5rs-Z-H-9.html) >>>> (eq? 2 2) ===> unspecified >>>> >>>> Rationale: It will usually be possible to implement eq? much more >>>> efficiently than eqv?, for example, as a simple pointer comparison instead >>>> of as some more complicated operation. One reason is that it may not be >>>> possible to compute eqv? of two numbers in constant time, whereas eq? >>>> implemented as pointer comparison will always finish in constant time. Eq? >>>> may be used like eqv? in applications using procedures to implement >>>> objects with state since it obeys the same constraints as eqv?. >>> >>> >>> You should use "=" with numbers as far as I remember. >>> >>> >>> Il giorno 18/mag/2015, alle ore 02.41, George Neuner ha scritto: >>> >>>> Hi, >>>> >>>> On 5/17/2015 5:32 PM, Atticus wrote: >>>>> --------------------------- >>>>> $ racket >>>>> Welcome to Racket v6.1.1. >>>>>> (eq? 'l 'l) >>>>> #f >>>>>> (eq? 'l 'l) >>>>> #t >>>>>> >>>>> >>>>> $ racket --no-jit >>>>> Welcome to Racket v6.1.1. >>>>>> (eq? 'l 'l) >>>>> #f >>>>>> (eq? 'l 'l) >>>>> #t >>>>>> (eq? 'l 'l) >>>>> #f >>>>>> (eq? 'l 'l) >>>>> #t >>>>>> (eq? 'l 'l) >>>>> #t >>>>>> (eq? 'l 'l) >>>>> #t >>>>>> (eq? 'l 'l) >>>>> #f >>>>>> (eq? 'l 'l) >>>>> #f >>>>>> >>>>> >>>>> --------------------------- >>>>> >>>>> How to reproduce this behaviour? Just start racket from the command line >>>>> and type '(eq? 'l 'l), this should return #f (or sometimes #t). The next >>>>> time the same expression returns #t. Whats also interesting is that with >>>>> the command line option --no-jit the return value seems to randomly >>>>> change between #t and #f. ... >>>> >>>> I can't reproduce this. Might there have been some control characters >>>> accidentally in your input? >>>> >>>> I have 6.1.1 32-bit and 64-bit on Windows 7, and 64-bit on Centos 6.6 and >>>> Ubuntu 14.04. Tried them all with and without JIT. Also tried the repl >>>> in DrRacket on Windows (Linux are command line only). All worked as >>>> expected. >>>> >>>> George >>>> >>>> >>>> -- >>>> 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. >>>> For more options, visit https://groups.google.com/d/optout. >>>> >>> >>> -- >>> 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. >>> For more options, visit https://groups.google.com/d/optout. >> >> -- >> 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. >> For more options, visit https://groups.google.com/d/optout. >> > > -- > 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. > For more options, visit 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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