Add :@all: to module/expand-path

Allow more easily importing modules from custom directories
without jumping through too many hoops. Technically, this was
possible before but required circumventing the built-in module/paths
and was just a hassle.

Also add entries to module/path (and module/add-path) to allow code
like the following.

(setdyn :my-libs "/home/me/janet-stuff/")

(import @my-libs/toolbox)

Intended for things like test harnesses where code might not
be installed to the usual directories.

CHANGELOG.md

@@ -2,6 +2,10 @@
 All notable changes to this project will be documented in this file.
 
 ## ??? - Unreleased
+- Allow importing modules from custom directories more easily with the `@` prefix
+  to module paths. For example, if there is a dynamic binding :custom-modules that
+  is a file system path to a directory of modules, import from that directory with
+  `(import @custom-modules/mymod)`.
 - Fix error message bug in FFI library.
 
 ## 1.25.1 - 2022-10-29

src/boot/boot.janet

@@ -2642,9 +2642,10 @@
   [image]
   (unmarshal image load-image-dict))
 
+(defn- check-dyn-relative [x] (if (string/has-prefix? "@" x) x))
 (defn- check-relative [x] (if (string/has-prefix? "." x) x))
 (defn- check-not-relative [x] (if-not (string/has-prefix? "." x) x))
-(defn- check-is-dep [x] (unless (or (string/has-prefix? "/" x) (string/has-prefix? "." x)) x))
+(defn- check-is-dep [x] (unless (or (string/has-prefix? "/" x) (string/has-prefix? "@" x) (string/has-prefix? "." x)) x))
 (defn- check-project-relative [x] (if (string/has-prefix? "/" x) x))
 
 (def module/cache
@@ -2678,6 +2679,8 @@
   (defn- find-prefix
     [pre]
     (or (find-index |(and (string? ($ 0)) (string/has-prefix? pre ($ 0))) module/paths) 0))
+  (def dyn-index (find-prefix ":@all:"))
+  (array/insert module/paths dyn-index [(string ":@all:" ext) loader check-dyn-relative])
   (def all-index (find-prefix ".:all:"))
   (array/insert module/paths all-index [(string ".:all:" ext) loader check-project-relative])
   (def sys-index (find-prefix ":sys:"))

src/core/corelib.c

@@ -111,7 +111,10 @@ JANET_CORE_FN(janet_core_expand_path,
               "This takes in a path (the argument to require) and a template string, "
               "to expand the path to a path that can be "
               "used for importing files. The replacements are as follows:\n\n"
-              "* :all: -- the value of path verbatim\n\n"
+              "* :all: -- the value of path verbatim.\n\n"
+              "* :@all: -- Same as :all:, bu if `path` starts with the @ character,\n"
+              "           the first path segment is replaced with a dynamic binding\n"
+              "           `(dyn <first path segment as keyword>)`.\n\n"
               "* :cur: -- the current file, or (dyn :current-file)\n\n"
               "* :dir: -- the directory containing the current file\n\n"
               "* :name: -- the name component of path, with extension if given\n\n"
@@ -157,6 +160,21 @@ JANET_CORE_FN(janet_core_expand_path,
             if (strncmp(template + i, ":all:", 5) == 0) {
                 janet_buffer_push_cstring(out, input);
                 i += 4;
+            } else if (strncmp(template + i, ":@all:", 6) == 0) {
+                if (input[0] == '@') {
+                    const char *p = input;
+                    while (*p && !is_path_sep(*p)) p++;
+                    size_t len = p - input - 1;
+                    char *str = janet_smalloc(len + 1);
+                    memcpy(str, input + 1, len);
+                    str[len] = '\0';
+                    janet_formatb(out, "%V", janet_dyn(str));
+                    janet_sfree(str);
+                    janet_buffer_push_cstring(out, p);
+                } else {
+                    janet_buffer_push_cstring(out, input);
+                }
+                i += 5;
             } else if (strncmp(template + i, ":cur:", 5) == 0) {
                 janet_buffer_push_bytes(out, (const uint8_t *)curdir, curlen);
                 i += 4;