[gambit-list] API to parse floats in Scheme

Hendrik Boom hendrik at topoi.pooq.com
Wed May 12 18:05:00 EDT 2021

On Wed, May 12, 2021 at 10:43:16PM +0300, Lassi Kortela wrote:
> We are planning a simple SRFI to mae floats from integers (or from strings),
> since that's useful when parsing any kind of text syntax that has real
> numbers. The details are implementation-dependent and tricky to get right.
> Here's the spec we have so far: https://github.com/pre-srfi/make-float
> The API is:
> (inexact-real-from-strings  int frac             precision exp) -> real
> (inexact-real-from-integers int frac frac-length precision exp) -> real
> Example usage:
> (inexact-real-from-strings  "-123" "45"  'e "6")  ; => -123.45e6
> (inexact-real-from-integers  -123   45 2 'e  6)   ; => -123.45e6
> The 2 in the last example means two decimal places. It's needed to
> disambiguate between .6 .06 .006 .0006 etc.
> There are both math and Scheme wizards on this list. Could someone verify
> whether this is a sane design before we submit the SRFI draft? It's been
> ok'd by one mathematician already so I'm hopeful :)
> There's also an inverse API to serve as a backend for string formatting
> packages:
> (strings-from-inexact-real  real frac-length) -> int; frac; exp
> (integers-from-inexact-real real frac-length) -> int; frac; exp

Conversion on a round trip between float and string form should converge 
rapidly.  At the least,
    string to float to string to float
should give the same value as just
    string to float
and similarly for float to string to float to string.

And there's the question of what to do with he various not-a-number
floating point values.

And what if you do not want an exponent in the resulting string?

And finally, there are ties you want it read and written *exactly*, for 
example, using hexadecimal instead of decimal so you get every bit.
(for this you'd want a symbol for the exponent 'e' that's different from 
the digit 'e'.

-- hendrik

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