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-*- outline -*- |
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This directory holds the Scheme side of a translator for Emacs Lisp. |
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* Usage |
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To load up the base Elisp environment: |
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(use-modules (lang elisp base)) |
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Then you can switch into this module |
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(define-module (lang elisp base)) |
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and start typing away in Elisp, or evaluate an individual Elisp |
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expression from Scheme: |
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(eval EXP (resolve-module '(lang elisp base))) |
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A more convenient, higher-level interface is provided by (lang elisp |
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interface): |
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(use-modules (lang elisp interface)) |
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With this interface, you can evaluate an Elisp expression |
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(eval-elisp EXP) |
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load an Elisp file with no effect on the Scheme world |
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(load-elisp-file "/home/neil/Guile/cvs/guile-core/lang/elisp/example.el") |
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load an Elisp file, automatically importing top level definitions into |
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Scheme |
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(use-elisp-file "/home/neil/Guile/cvs/guile-core/lang/elisp/example.el") |
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export Scheme objects to Elisp |
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(export-to-elisp + - * my-func 'my-var) |
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and try to bootstrap a complete Emacs environment: |
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(load-emacs) |
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* Status |
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Please note that this is work in progress; the translator is |
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incomplete and not yet widely tested. |
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** Trying to load a complete Emacs environment. |
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To try this, type `(use-modules (lang elisp interface))' and then |
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`(load-emacs)'. The following output shows how far I get when I try |
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this. |
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guile> (use-modules (lang elisp interface)) |
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guile> (load-emacs) |
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Calling loadup.el to clothe the bare Emacs... |
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Loading /usr/share/emacs/20.7/lisp/loadup.el... |
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Using load-path ("/usr/share/emacs/20.7/lisp/" "/usr/share/emacs/20.7/lisp/emacs-lisp/") |
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Loading /usr/share/emacs/20.7/lisp/byte-run.el... |
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Loading /usr/share/emacs/20.7/lisp/byte-run.el...done |
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Loading /usr/share/emacs/20.7/lisp/subr.el... |
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Loading /usr/share/emacs/20.7/lisp/subr.el...done |
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Loading /usr/share/emacs/20.7/lisp/version.el... |
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Loading /usr/share/emacs/20.7/lisp/version.el...done |
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Loading /usr/share/emacs/20.7/lisp/map-ynp.el... |
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Loading /usr/share/emacs/20.7/lisp/map-ynp.el...done |
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Loading /usr/share/emacs/20.7/lisp/widget.el... |
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Loading /usr/share/emacs/20.7/lisp/emacs-lisp/cl.el... |
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Loading /usr/share/emacs/20.7/lisp/emacs-lisp/cl.el...done |
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Loading /usr/share/emacs/20.7/lisp/widget.el...done |
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Loading /usr/share/emacs/20.7/lisp/custom.el... |
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Loading /usr/share/emacs/20.7/lisp/custom.el...done |
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Loading /usr/share/emacs/20.7/lisp/cus-start.el... |
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Note, built-in variable `abbrev-all-caps' not bound |
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... [many other variable not bound messages] ... |
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Loading /usr/share/emacs/20.7/lisp/cus-start.el...done |
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Loading /usr/share/emacs/20.7/lisp/international/mule.el... |
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<unnamed port>: In procedure make-char-table in expression (@fop make-char-table (# #)): |
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<unnamed port>: Symbol's function definition is void |
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ABORT: (misc-error) |
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Type "(backtrace)" to get more information or "(debug)" to enter the debugger. |
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guile> |
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That's 3279 lines ("wc -l") of Elisp code already, which isn't bad! |
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I think that progress beyond this point basically means implementing |
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multilingual and multibyte strings properly for Guile. Which is a |
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_lot_ of work and requires IMO a very clear plan for Guile's role with |
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respect to Emacs. |
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* Design |
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When thinking about how to implement an Elisp translator for Guile, it |
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is important to realize that the great power of Emacs does not arise |
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from Elisp (seen as a language in syntactic terms) alone, but from the |
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combination of this language with the collection of primitives |
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provided by the Emacs C source code. Therefore, to be of practical |
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use, an Elisp translator needs to be more than just a transformer that |
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translates sexps to Scheme expressions. |
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The finished translator should consist of several parts... |
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** Syntax transformation |
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Although syntax transformation isn't all we need, we do still need it! |
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This part is implemented by the (lang elisp transform) module; it is |
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close to complete and seems to work pretty reliably. |
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Note that transformed expressions use the `@fop' and `@bind' macros |
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provided by... |
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** C support for transformed expressions |
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For performance and historical reasons (and perhaps necessity - I |
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haven't thought about it enough yet), some of the transformation |
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support is written in C. |
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*** @fop |
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The `@fop' macro is used to dispatch Elisp applications. Its first |
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argument is a symbol, and this symbol's function slot is examined to |
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find a procedure or macro to apply to the remaining arguments. `@fop' |
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also handles aliasing (`defalias'): in this case the function slot |
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contains another symbol. |
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Once `@fop' has found the appropriate procedure or macro to apply, it |
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returns an application expression in which that procedure or macro |
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replaces the `@fop' and the original symbol. Hence no Elisp-specific |
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evaluator support is required to perform the application. |
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*** @bind |
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Currently, Elisp variables are the same as Scheme variables, so |
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variable references are effectively untransformed. |
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The `@bind' macro does Elisp-style dynamic variable binding. |
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Basically, it locates the named top level variables, `set!'s them to |
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new values, evaluates its body, and then uses `set!' again to restore |
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the original values. |
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Because of the body evaluation, `@bind' requires evaluator support. |
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In fact, the `@bind' macro code does little more than replace itself |
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with the memoized SCM_IM_BIND. Most of the work is done by the |
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evaluator when it hits SCM_IM_BIND. |
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One theoretical problem with `@bind' is that any local Scheme variable |
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in the same scope and with the same name as an Elisp variable will |
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shadow the Elisp variable. But in practice it's difficult to set up |
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such a situation; an exception is the translator code itself, so there |
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we mangle the relevant Scheme variable names a bit to avoid the |
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problem. |
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Other possible problems with this approach are that it might not be |
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possible to implement buffer local variables properly, and that |
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`@bind' might become too inefficient when we implement full support |
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for undefining Scheme variables. So we might in future have to |
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transform Elisp variable references after all. |
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*** Truth value stuff |
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Lots of stuff to do with providing the special self-evaluating `nil' |
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and `t' symbols, and macros that convert between Scheme and Elisp |
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truth values, and so on. |
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I'm hoping that most of this will go away, but I need to show that |
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it's feasible first. |
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** Emacs editing primitives |
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Buffers, keymaps, text properties, windows, frames etc. etc. |
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Basically, everything that is implemented as a primitive in the Emacs |
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C code needs to be implemented either in Scheme or in C for Guile. |
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The Scheme files in the primitives subdirectory implement some of |
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these primitives in Scheme. Not because that is the right decision, |
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but because this is a proof of concept and it's quicker to write badly |
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performing code in Scheme. |
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Ultimately, most of these primitive definitions should really come |
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from the Emacs C code itself, translated or preprocessed in a way that |
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makes it compile with Guile. I think this is pretty close to the work |
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that Ken Raeburn has been doing on the Emacs codebase. |
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** Reading and printing support |
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Elisp is close enough to Scheme that it's convenient to coopt the |
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existing Guile reader rather than to write a new one from scratch, but |
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there are a few syntactic differences that will require adding Elisp |
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support to the reader. |
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- Character syntax is `?a' rather than `#\a'. (Not done. More |
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precisely, `?a' in Elisp isn't character syntax but an alternative |
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integer syntax. Note that we could support most of the `?a' syntax |
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simply by doing |
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(define ?a (char->integer #\a) |
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(define ?b (char->integer #\b) |
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and so on.) |
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- `nil' and `t' should be read (I think) as #f and #t. (Done.) |
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- Vector syntax is `[1 2 3]' rather than `#(1 2 3)'. (Not done.) |
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Correspondingly, when printing, #f and '() should be written as |
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`nil'. (Not done.) |
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** The Elisp evaluation module (lang elisp base) |
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Fundamentally, Guile's module system can't be used to package Elisp |
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code in the same way it is used for Scheme code, because Elisp |
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function definitions are stored as symbol properties (in the symbol's |
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"function slot") and so are global. On the other hand, it is useful |
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(necessary?) to associate some particular module with Elisp evaluation |
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because |
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- Elisp variables are currently implemented as Scheme variables and so |
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need to live in some module |
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- a syntax transformer is a property of a module. |
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Therefore we have the (lang elisp base) module, which acts as the |
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repository for all Elisp variables and the site of all Elisp |
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evaluation. |
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The initial environment provided by this module is intended to be a |
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non-Emacs-dependent subset of Elisp. To get the idea, imagine someone |
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who wants to write an extension function for, say Gnucash, and simply |
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prefers to write in Elisp rather than in Scheme. He/she therefore |
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doesn't buffers, keymaps and so on, just the basic language syntax and |
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core data functions like +, *, concat, length etc., plus specific |
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functions made available by Gnucash. |
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(lang elisp base) achieves this by |
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- importing Scheme definitions for some Emacs primitives from the |
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files in the primitives subdirectory |
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- then switching into Elisp syntax. |
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After this point, `(eval XXX (resolve-module '(lang elisp base)))' |
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will evaluate XXX as an Elisp expression in the (lang elisp base) |
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module. (`eval-elisp' in (lang elisp interface) is a more convenient |
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wrapper for this.) |
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** Full Emacs environment |
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The difference between the initial (lang elisp base) environment and a |
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fully loaded Emacs equivalent is |
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- more primitives: buffers, char-tables and many others |
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- the bootstrap Elisp code that an undumped Emacs loads during |
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installation by calling `(load "loadup.el")'. |
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We don't have all the missing primitives, but we can already get |
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through some of loadup.el. The Elisp function `load-emacs' (defined |
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in (lang elisp base) initiates the loading of loadup.el; (lang elisp |
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interface) exports `load-emacs' to Scheme. |
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`load-emacs' loads so much Elisp code that it's an excellent way to |
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test the translator. In current practice, it runs for a while and |
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then fails when it gets to an undefined primitive or a bug in the |
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translator. Eventually, it should go all the way. (And then we can |
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worry about adding unexec support to Guile!) For the output that |
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currently results from calling `(load-emacs)', see above in the Status |
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section. |
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* nil, #f and '() |
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For Jim Blandy's notes on this, see the reference at the bottom of |
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this file. Currently I'm investigating a different approach, which is |
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better IMO than Jim's proposal because it avoids requiring multiple |
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false values in the Scheme world. |
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According to my approach... |
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- `nil' and `t' are read (when in Elisp mode) as #f and #t. |
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- `(if x ...)', `(while x ...)' etc. are translated to something |
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like `(if (and x (not (null? x))) ...)'. |
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- Functions which interpret an argument as a list -- |
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`cons', `setcdr', `memq', etc. -- either convert #f to '(), or |
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handle the #f case specially. |
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- `eq' treats #f and '() as the same. |
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- Optionally, functions which produce '() values -- i.e. the reader |
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and `cdr' -- could convert those immediately to #f. This shouldn't |
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affect the validity of any Elisp code, but it alters the balance of |
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#f and '() values swimming around in that code and so affects what |
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happens if two such values are returned to the Scheme world and then |
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compared. However, since you can never completely solve this |
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problem (unless you are prepared to convert arbitrarily deep |
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structures on entry to the Elisp world, which would kill performance), |
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I'm inclined not to try to solve it at all. |
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* Resources |
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** Ken Raeburn's Guile Emacs page |
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http://www.mit.edu/~raeburn/guilemacs/ |
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** Keisuke Nishida's Gemacs project |
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http://gemacs.sourceforge.net |
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** Jim Blandy's nil/#f/() notes |
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http://sanpietro.red-bean.com/guile/guile/old/3114.html |
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** Mikael Djurfeldt's notes on translation |
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See file guile-cvs/devel/translation/langtools.text in Guile CVS. |