[racket] number->string in the HtDP student languages

See "A Note on Numbers" at http://www.htdp.org/2003-09-26/Book/curriculum-Z-H-5.html#node_sec_2.1 and similarly for 2e. 

Numbers in programming languages cannot be the numbers of plain mathematics because we want programs to be reasonably fast. In the end, all languages must compromise between those two concepts and have an obligation to show learners that there is an issue. I consider *SL in a sweet spot, despite the warts that occasionally show up. 

-- Matthias




On Feb 18, 2015, at 8:28 AM, Nadeem Abdul Hamid wrote:

> And you can use 
> 
> > (number->string (exact->inexact 0.75)) 
> 
> to get numbers printed in decimal notation. It's slightly distracting/annoying, but in class I can briefly explain that there is a distinction between exact and inexact numbers in BSL, and that the former print as rationals (fractions), and the latter in decimal notation. 
> 
> --- nadeem
> 
> 
> On Wed, Feb 18, 2015 at 7:35 AM, Robby Findler <robby at eecs.northwestern.edu> wrote:
> In the teaching languages, decimal syntax for numbers does not imply
> inexactness. The only way to get inexact numbers is to call something
> like sqrt or sin. So, there, the number 0.75 and the number 3/4 are
> the same exact number (exact same exact number?).
> 
> The printing code is the same (well, it is the same for exact numbers.
> Inexact numbers print with a #i prefix).
> 
> Robby
> 
> 
> On Wed, Feb 18, 2015 at 2:32 AM, Alexis King <lexi.lambda at gmail.com> wrote:
> > This is interesting behavior. To explain why it happens, this has something
> > to do with how the HtDP languages handle exact/inexact numbers. For example,
> > in plain Racket, (exact? 0.75) is #f, but in BSL, it is #true. In BSL,
> > numbers are exact unless otherwise specified. Note that explicitly using an
> > exactness prefix in plain Racket, #e0.75, produces 3/4.
> >
> > What’s odd is that the convention for printing numbers in BSL is different
> > from the convention used by number->string. BSL, unlike Racket, prints exact
> > numbers as decimals. I’d expect that this is to avoid confusion when
> > students type in decimals only to have them printed in large fractions.
> > Unfortunately, it would seem that number->string simply uses the plain
> > Racket implementation, which lets the odd implementations details “leak
> > through” the abstraction.
> >
> > It does feel like poor behavior to me. I’d expect number->string to follow
> > the same printing conventions BSL uses.
> >
> > On Feb 18, 2015, at 00:17, Breanndán Ó Nualláin <bon at science.uva.nl> wrote:
> >
> > In BSL, I just got caught out by this behaviour:
> >
> > (number->string 0.75)
> >
> >  "3/4"
> >
> > I expected the same behaviour as in the full Racket language:
> >
> > (number->string 0.75)
> >
> >  "0.75"
> >
> > What's the thinking behind this? Wouldn't the decimal notation be more
> > intuitive, particularly in cases like this?
> >
> > (number->string 0.7555555)
> >
> >  "1511111/2000000"
> >
> > And especially in cases like this?
> >
> > (number->string (string->number (number->string pi)))
> >
> >  "3141592653589793/1000000000000000"
> > ____________________
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