On Tue, Feb 19, 2013 at 4:41 PM, Mark H Weaver <m...@netris.org> wrote: > Mikael Djurfeldt <mik...@djurfeldt.com> writes: >> On Tue, Feb 19, 2013 at 12:33 AM, Mark H Weaver <m...@netris.org> wrote: >>> Mikael Djurfeldt <mik...@djurfeldt.com> writes: >>>> I propose to simplify this to only two levels: >>>> >>>> 1. %read-hash-procedures >>>> 2. predefined syntax >> >> It turns out that the change I propose above was already implemented >> in read.c. The effect just wasn't visible due to a bug in flush_ws >> which caused all #! to be erroneously removed if they exist as the >> outermost expression. > > I'm not sure that I consider this a bug.
In this reply I've attached a file mit-reader-scm which installs a hash-read-procedure for #\!. What I wanted to say above is that scm_read_sharp (in HEAD) is implemented with the priorities I list above while flush_ws is implemented with other priorities. Here's a demo of the consequences of this bug: scheme@(guile-user)> (load "mit-reader.scm") scheme@(guile-user)> (quote #!optional) ... !# hi) $1 = hi scheme@(guile-user)> '#!optional $2 = #:optional > [...] > I'm uncomfortable with globally overriding standard read syntax. In a > large scheme system such as Guile, there are many modules that use > 'read' and expect it to act in accordance with standard lexical > conventions. Well, in the mit-scheme compatibility module, my intention was to use dynamic-wind to modify #!-syntax while loading mit-scheme-specific files. Note that %read-hash-procedures is a fluid, so this will be absolutely local and won't leak out in any way to the rest of the system. > The problem with this approach is that it does not compose. Let's now patch guile according to the diff I sent... there! scheme@(guile-user)> (load "mit-reader.scm") scheme@(guile-user)> (quote #!optional) $1 = #:optional scheme@(guile-user)> '#!optional $2 = #:optional scheme@(guile-user)> (quote #!hi!# #!optional) $3 = #:optional My take on this is: * The %read-hash-procedures API is not pretty * The suggested change doesn't make things prettier * The suggested change *does* make things conceptually simpler and more flexible (= you can always override hash syntax if you want; compared to the current: you can override #| but not other hash syntax) * The suggested change fixes a bug * The suggested change does compose and different syntax can be confined to a module by using dynamic-wind Best regards, Mikael
mit-reader.scm
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