On Tue, Oct 30, 2018 at 12:55 AM Mark H Weaver <m...@netris.org> wrote:
> However, there's a complication with using '$' in this way. '$' is
> already widely used as part of the syntax for (ice-9 match), to specify
> patterns that match record objects.
Yes, I actually looked at this, but thought that $ would be interpreted as
a literal inside the match expression, but was probably wrong according to
what you write below:
> More precisely, it is a literal
> identifier recognized by 'match' and related macros, in the same sense
> that 'else' and '=>' are literal identifiers recognized by the 'cond'
> macro.
>
> R5RS section 4.3.2 (Pattern language) specifies how these literal
> identifiers are to be compared with identifiers found in each macro use:
>
> Identifiers that appear in <literals> are interpreted as literal
> identifiers to be matched against corresponding subforms of the
> input. A subform in the input matches a literal identifier if and
> only if it is an identifier and either both its occurrence in the
> macro expression and its occurrence in the macro definition have
> the same lexical binding, or the two identifiers are equal and both
> have no lexical binding.
>
> The implication is that these literal identifiers such as 'else', '=>'
> and '$' lose their special meaning in any environment where they are
> bound, unless the same binding is visible in the corresponding macro
> definition environment. R6RS and R7RS also specify this behavior.
>
> For example:
>
> --8<---------------cut here---------------start------------->8---
> mhw@jojen ~$ guile
> GNU Guile 2.2.3
> Copyright (C) 1995-2017 Free Software Foundation, Inc.
>
> Guile comes with ABSOLUTELY NO WARRANTY; for details type `,show w'.
> This program is free software, and you are welcome to redistribute it
> under certain conditions; type `,show c' for details.
>
> Enter `,help' for help.
> scheme@(guile-user)> ,use (ice-9 match)
> scheme@(guile-user)> ,use (srfi srfi-9)
> scheme@(guile-user)> (define-record-type <foo> (make-foo a b) foo? (a
> foo-a) (b foo-b))
> scheme@(guile-user)> (match (make-foo 1 2) (($ <foo> a b) (+ a b)))
> $1 = 3
> scheme@(guile-user)> (define $ 'blah)
> scheme@(guile-user)> (match (make-foo 1 2) (($ <foo> a b) (+ a b)))
> <unnamed port>:6:0: Throw to key `match-error' with args `("match" "no
> matching pattern" #<<foo> a: 1 b: 2>)'.
>
> Entering a new prompt. Type `,bt' for a backtrace or `,q' to continue.
> scheme@(guile-user) [1]>
> --8<---------------cut here---------------end--------------->8---
>
Incidentally, this does *not* throw an error in master (unless I made some
mistake in this late hour), which then is a bug!
>
> To avoid colliding with the popular 'match' syntax, how about making
> '$$' the last value ($$0), and omitting the alias for '$$1'?
>
> What do you think?
>
Not sure. This might be confusing for GDB users... Let's think about it.
