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+# require.bash
+# Author: Noah Friedman <friedman@prep.ai.mit.edu>
+# Created: 1992-07-08
+# Last modified: 1993-09-29
+# Public domain
+
+# Commentary:
+
+# These functions provide an interface based on the lisp implementation for
+# loading libraries when they are needed and eliminating redundant loading.
+# The basic idea is that each "package" (or set of routines, even if it is
+# only one function) registers itself with a symbol that marks a "feature"
+# as being "provided".  If later you "require" a given feature, you save
+# yourself the trouble of explicitly loading it again.
+# 
+# At the bottom of each package, put a "provide foobar", so when another
+# package has a "require foobar", it gets loaded and registered as a
+# "feature" that won't need to get loaded again.  (See warning below for
+# reasons why provide should be put at the end.)
+#
+# The list of provided features are kept in the `FEATURES' variable, which
+# is not exported.  Care should be taken not to munge this in the shell.
+# The search path comes from a colon-separated `FPATH' variable.  It has no
+# default value and must be set by the user.
+#
+# Require uses `fpath_search', which works by scanning all of FPATH for a
+# file named the same as the required symbol but with a `.bash' appended to
+# the name.  If that is found, it is loaded.  If it is not, FPATH is
+# searched again for a file name the same as the feature (i.e. without any
+# extension).  Fpath_search may be useful for doing library filename
+# lookups in other functions (such as a `load' or `autoload' function).
+#
+# Warning: Because require ultimately uses the builtin `source' command to
+# read in files, it has no way of undoing the commands contained in the
+# file if there is an error or if no provide statement appeared (this
+# differs from the lisp implementation of require, which normally undoes
+# most of the forms that were loaded if the require fails).  Therefore, to
+# minize the number of problems caused by requiring a faulty package (such
+# as syntax errors in the source file) it is better to put the provide at
+# the end of the file, rather than at the beginning.
+
+# Code:
+
+# Exporting this variable would cause considerable lossage, since none of
+# the functions are exported (or at least, they're not guaranteed to be)
+export -n FEATURES
+
+#:docstring :
+# Null function.  Provided only so that one can put page breaks in source
+# files without any ill effects.
+#:end docstring:
+#
+# (\\014 == C-l)
+eval "function $(echo -e \\014) () { : }"
+
+
+#:docstring featurep:
+# Usage: featurep argument
+#
+# Returns 0 (true) if argument is a provided feature.  Returns 1 (false)
+# otherwise. 
+#:end docstring:
+
+###;;;autoload
+function featurep ()
+{
+    local feature="$1"
+
+    case " ${FEATURES} " in
+       *" ${feature} "* ) return 0 ;;
+    esac
+
+    return 1
+}
+
+
+#:docstring provide:
+# Usage: provide symbol ...
+#
+# Register a list of symbols as provided features
+#:end docstring:
+
+###;;;autoload
+function provide ()
+{
+    local feature
+
+    for feature in "$@" ; do
+       if ! featurep "${feature}" ; then
+          FEATURES="${FEATURES} ${feature}"
+       fi
+    done
+
+    return 0
+}
+
+
+#:docstring require:
+# Usage: require feature {file}
+#
+# Load FEATURE if it is not already provided.  Note that require does not
+# call `provide' to register features.  The loaded file must do that
+# itself.  If the package does not explicitly do a `provide' after being
+# loaded, require will complain about the feature not being provided on
+# stderr.
+#
+# Optional argument FILE means to try to load FEATURE from FILE.  If no
+# file argument is given, require searches through FPATH (see fpath_search)
+# for the appropriate file.
+#
+# If the variable REQUIRE_FAILURE_FATAL is set, require will cause the
+# current shell invocation to exit, rather than merely return.  This may be
+# useful for a shell script that vitally depends on a package. 
+#
+#:end docstring:
+
+###;;;autoload
+function require ()
+{
+ local feature="$1"
+ local path="$2"
+ local file
+ 
+   if ! featurep "${feature}" ; then
+      file=$(fpath_search "${feature}" "${path}") && source "${file}"
+
+      if ! featurep "${feature}" ; then
+         echo "require: ${feature}: feature was not provided." 1>&2
+         if [ "${REQUIRE_FAILURE_FATAL+set}" = "set" ]; then
+            exit 1
+         fi
+         return 1
+      fi
+   fi
+
+   return 0
+}
+
+#:docstring fpath_search:
+# Usage: fpath_search filename {path ...}
+#
+# Search $FPATH for `filename' or, if `path' (a list) is specified, search
+# those directories instead of $FPATH.  First the path is searched for an
+# occurrence of `filename.bash, then a second search is made for just
+# `filename'.
+#:end docstring:
+
+###;;;autoload
+function fpath_search ()
+{
+ local name="$1"
+ local path="$2"
+ local suffix=".bash"
+ local file
+
+    if [ -z "${path}" ]; then path="${FPATH}"; fi
+
+    for file in "${name}${suffix}" "${name}" ; do
+       set -- $(IFS=':'
+                 set -- ${path}
+                 for p in "$@" ; do
+                    echo -n "${p:-.} "
+                 done)
+
+       while [ $# -ne 0 ]; do
+          test -f "${1}/${file}" && { file="${1}/${file}"; break 2 }
+          shift
+       done
+    done
+
+    if [ $# -eq 0 ]; then
+       echo "fpath_search: ${name}: file not found in fpath" 1>&2
+       return 1
+    fi
+
+    echo "${file}"
+    return 0
+}
+
+provide require
+
+# require.bash ends here