Address all coments in @trws's latest comment in PR #869.

I addressed them by factoring the code better to follow the visitor
pattern.  This will allow actions to be easily added in the future.
These may not even be file sytsem views.  One could add actions to
generate shell init scripts, JSON DAG-dumpers, GraphViz DOT file
generators, etc (yes, some of these are alread in there - just to give
the idea).

Also added is a top-level test

 $ source share/spack/setup-env.sh
 $ ./share/spack/examples/test_view.sh

Read the top of that script first.

lib/spack/spack/cmd/view.py

-'''
-Produce a file-system "view" of a Spack DAG.
-
-Concept from Nix, implemented by brett.viren@gmail.com ca 2016.
-'''
 ##############################################################################
 # Copyright (c) 2013, Lawrence Livermore National Security, LLC.
 # Produced at the Lawrence Livermore National Laboratory.
@@ -27,6 +22,45 @@
 # along with this program; if not, write to the Free Software Foundation,
 # Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
+'''Produce a "view" of a Spack DAG.
+
+A "view" is the product of applying a function on a set of package specs.
+
+This set consists of:
+
+- specs resolved from the package names given by the user (the seeds)
+- all depenencies of the seeds unless user specifies `--no-depenencies`
+- less any specs with names matching the regular expressions given by `--exclude`
+
+The `view` command provides a number of functions (the "actions"):
+
+- symlink :: a file system view which is a directory hierarchy that is
+  the union of the hierarchies of the installed packages in the DAG
+  where installed files are referenced via symlinks.  
+
+- hardlink :: like the symlink view but hardlinks are used
+
+- statlink :: a view producing a status report of a symlink or
+  hardlink view.
+
+
+The file system view concept is imspired by Nix, implemented by
+brett.viren@gmail.com ca 2016.
+
+'''
+# Implementation notes:
+#
+# This is implemented as a visitor pattern on the set of package specs.
+#
+# The command line ACTION maps to a visitor_*() function which takes
+# the set of package specs and any args which may be specific to the
+# ACTION.
+#
+# To add a new view:
+# 1. add a new cmd line args sub parser ACTION
+# 2. add any action-specific options/arguments, most likely a list of specs.
+# 3. add a visitor_MYACTION() function
+# 4. add any visitor_MYALIAS assignments to match any command line aliases
 
 import os
 import re
@@ -38,33 +72,48 @@
 
 description = "Produce a single-rooted directory view of a spec."
 
-def setup_parser(subparser):
-    setup_parser.parser = subparser
+def setup_parser(sp):
+    setup_parser.parser = sp
+
+    sp.add_argument('-v','--verbose', action='store_true', default=False,
+                    help="Display verbose output.")
+    sp.add_argument('-e','--exclude', action='append', default=[],
+                    help="Exclude packages with names matching the given regex pattern.")
+    sp.add_argument('-d', '--dependencies', choices=['true','false','yes','no'],
+                    default='true',
+                    help="Follow dependencies.")
 
-    sp = subparser.add_subparsers(metavar='ACTION', dest='action')
+
+    ssp = sp.add_subparsers(metavar='ACTION', dest='action')
 
     # The action parameterizes the command but in keeping with Spack
     # patterns we make it a subcommand.
-    sps = [
-        sp.add_parser('add', aliases=['link'],
-                      help='Add packages to the view, create view if needed.'),
-        sp.add_parser('remove', aliases=['rm'],
-                      help='Remove packages from the view, and view if empty.'),
-        sp.add_parser('status', aliases=['check'],
-                      help='Check status of packages in the view.')
+    file_system_view_actions = [
+        ssp.add_parser('symlink', aliases=['add','soft'],
+                      help='Add package files to a filesystem view via symbolic links.'),
+        ssp.add_parser('hardlink', aliases=['hard'],
+                      help='Add packages files to a filesystem via via hard links.'),
+        ssp.add_parser('remove', aliases=['rm'],
+                      help='Remove packages from a filesystem view.'),
+        ssp.add_parser('statlink', aliases=['status','check'],
+                      help='Check status of packages in a filesystem view.')
     ]
-
     # All these options and arguments are common to every action.
-    for p in sps:
-        p.add_argument('-e','--exclude', action='append', default=[],
-                       help="exclude any packages which the given re pattern")
-        p.add_argument('--no-dependencies', action='store_true', default=False,
-                       help="just operate on named packages and do not follow dependencies")
-        p.add_argument('prefix', nargs=1,
-                       help="Path to a top-level directory to receive the view.")
-        p.add_argument('specs', nargs=argparse.REMAINDER,
-                       help="specs of packages to expose in the view.")
+    for act in file_system_view_actions:
+        act.add_argument('path', nargs=1,
+                         help="Path to file system view directory.")
+        act.add_argument('specs', metavar='spec', nargs='+', 
+                         help="Seed specs of the packages to view.")
+        
+    ## Other VIEW ACTIONS might be added here.
+    ## Some ideas are the following (and some are redundant with existing cmds)
+    ## A JSON view that dumps a DAG to a JSON file
+    ## A DOT view that dumps to a GraphViz file 
+    ## A SHELL INIT view that dumps bash/csh script setting up to use packages in the view
+    return
+
 
+### Util functions
 
 def assuredir(path):
     'Assure path exists as a directory'
@@ -79,7 +128,6 @@ def relative_to(prefix, path):
         reldir = reldir[1:]
     return reldir
 
-    
 def transform_path(spec, path, prefix=None):
     'Return the a relative path corresponding to given path spec.prefix'
     if os.path.isabs(path):
@@ -92,54 +140,87 @@ def transform_path(spec, path, prefix=None):
         path = os.path.join(prefix, path)
     return path
 
-def action_status(spec, prefix):
-    'Check status of view in prefix against spec'
-    dotspack = os.path.join(prefix, '.spack', spec.name)
+def purge_empty_directories(path):
+    'Ascend up from the leaves accessible from `path` and remove empty directories.'
+    for dirpath, subdirs, files in os.walk(path, topdown=False):
+        for sd in subdirs:
+            sdp = os.path.join(dirpath,sd)
+            try:
+                os.rmdir(sdp)
+            except OSError:
+                pass
+
+def filter_exclude(specs, exclude):
+    'Filter specs given sequence of exclude regex'
+    to_exclude = [re.compile(e) for e in exclude]
+    def exclude(spec):
+        for e in to_exclude:
+            if e.match(spec.name):
+                return True
+        return False
+    return [s for s in specs if not exclude(s)]
+
+def flatten(seeds, descend=True):
+    'Normalize and flattend seed specs and descend hiearchy'
+    flat = set()
+    for spec in seeds:
+        if not descend:
+            flat.add(spec)
+            continue
+        flat.update(spec.normalized().traverse())
+    return flat
+
+
+### Action-specific helpers
+
+def check_one(spec, path, verbose=False):
+    'Check status of view in path against spec'
+    dotspack = os.path.join(path, '.spack', spec.name)
     if os.path.exists(os.path.join(dotspack)):
-        tty.info("Package added: %s"%spec.name)
+        tty.info('Package in view: "%s"'%spec.name)
         return
-    tty.info("Package missing: %s"%spec.name)
+    tty.info('Package not in view: "%s"'%spec.name)
     return
 
-def action_remove(spec, prefix):
-    'Remove any files found in spec from prefix and purge empty directories.'
+def remove_one(spec, path, verbose=False):
+    'Remove any files found in `spec` from `path` and purge empty directories.'
 
-    if not os.path.exists(prefix):
-        return
+    if not os.path.exists(path):
+        return                  # done, short circuit
 
-    dotspack = transform_path(spec, '.spack', prefix)
+    dotspack = transform_path(spec, '.spack', path)
     if not os.path.exists(dotspack):
-        tty.info("Skipping nonexistent package %s"%spec.name)
+        if verbose:
+            tty.info('Skipping nonexistent package: "%s"'%spec.name)
         return
 
-    tty.info("remove %s"%spec.name)
+    if verbose:
+        tty.info('Removing package: "%s"'%spec.name)
     for dirpath,dirnames,filenames in os.walk(spec.prefix):
         if not filenames:
             continue
-
-        targdir = transform_path(spec, dirpath, prefix)
+        targdir = transform_path(spec, dirpath, path)
         for fname in filenames:
-            src = os.path.join(dirpath, fname)
             dst = os.path.join(targdir, fname)
             if not os.path.exists(dst):
-                #tty.warn("Skipping nonexistent file for view: %s" % dst)
                 continue
             os.unlink(dst)
 
-def action_link(spec, prefix):
-    'Symlink all files in `spec` into directory `prefix`.'
+def link_one(spec, path, link = os.symlink, verbose=False):
+    'Link all files in `spec` into directory `path`.'
 
-    dotspack = transform_path(spec, '.spack', prefix)
+    dotspack = transform_path(spec, '.spack', path)
     if os.path.exists(dotspack):
-        tty.warn("Skipping previously added package %s"%spec.name)
+        tty.warn('Skipping existing package: "%s"'%spec.name)
         return
 
-    tty.info("link %s" % spec.name)
+    if verbose:
+        tty.info('Linking package: "%s"' % spec.name)
     for dirpath,dirnames,filenames in os.walk(spec.prefix):
         if not filenames:
             continue        # avoid explicitly making empty dirs
 
-        targdir = transform_path(spec, dirpath, prefix)
+        targdir = transform_path(spec, dirpath, path)
         assuredir(targdir)
 
         for fname in filenames:
@@ -148,70 +229,63 @@ def action_link(spec, prefix):
             if os.path.exists(dst):
                 if '.spack' in dst.split(os.path.sep):
                     continue    # silence these
-                tty.warn("Skipping existing file for view: %s" % dst)
+                tty.warn("Skipping existing file: %s" % dst)
                 continue
-            os.symlink(src,dst)
-
-def purge_empty_directories(path):
-    'Ascend up from the leaves accessible from `path` and remove empty directories.'
-    for dirpath, subdirs, files in os.walk(path, topdown=False):
-        for sd in subdirs:
-            sdp = os.path.join(dirpath,sd)
-            try:
-                os.rmdir(sdp)
-            except OSError:
-                #tty.warn("Not removing directory with contents: %s" % sdp)
-                pass
+            link(src,dst)
 
 
+### The canonically named visitor_* functions and their alias maps.
+### One for each action.
 
+def visitor_symlink(specs, args):
+    'Symlink all files found in specs'
+    path = args.path[0]
+    assuredir(path)
+    for spec in specs:
+        link_one(spec, path, verbose=args.verbose)
+visitor_add = visitor_symlink
+visitor_soft = visitor_symlink
+
+def visitor_hardlink(specs, args):
+    'Hardlink all files found in specs'
+    path = args.path[0]
+    assuredir(path)
+    for spec in specs:
+        link_one(spec, path, os.link, verbose=args.verbose)
+visitor_hard = visitor_hardlink
 
-def view_action(action, parser, args):
-    'The view command parameterized by the action.'
-    to_exclude = [re.compile(e) for e in args.exclude]
-    def exclude(spec):
-        for e in to_exclude:
-            if e.match(spec.name):
-                return True
-        return False
-
-    specs = spack.cmd.parse_specs(args.specs, normalize=True, concretize=True)
-    if not specs:
-        parser.print_help()
-        return 1
-
-    prefix = args.prefix[0]
-    assuredir(prefix)
-
-    flat = set()
+def visitor_remove(specs, args):
+    'Remove all files and directories found in specs from args.path'
+    path = args.path[0]
     for spec in specs:
-        if args.no_dependencies:
-            flat.add(spec)
-            continue
-        flat.update(spec.normalized().traverse())
+        remove_one(spec, path, verbose=args.verbose)
+    purge_empty_directories(path)
+visitor_rm = visitor_remove
 
-    for spec in flat:
-        if exclude(spec):
-            tty.info('Skipping excluded package: "%s"' % spec.name)
-            continue
-        if not os.path.exists(spec.prefix):
-            tty.warn('Skipping unknown package: %s in %s' % (spec.name, spec.prefix))
-            continue
-        action(spec, prefix)
+def visitor_statlink(specs, args):
+    'Give status of view in args.path relative to specs'
+    path = args.path[0]
+    for spec in specs:
+        check_one(spec, path, verbose=args.verbose)
+visitor_status = visitor_statlink
+visitor_check = visitor_statlink
 
-    if args.action in ['remove','rm']:
-        purge_empty_directories(prefix)
 
+# Finally, the actual "view" command.  There should be no need to
+# modify anything below when new actions are added.
 
 
 def view(parser, args):
-    'The view command.'
-    action = {
-        'add': action_link,
-        'link': action_link,
-        'remove': action_remove,
-        'rm': action_remove,
-        'status': action_status,
-        'check': action_status
-        }[args.action]
-    view_action(action, parser, args)
+    'Produce a view of a set of packages.'
+
+    # Process common args 
+    seeds = [spack.cmd.disambiguate_spec(s) for s in args.specs]
+    specs = flatten(seeds, args.dependencies.lower() in ['yes','true'])
+    specs = filter_exclude(specs, args.exclude)
+    
+    # Execute the visitation.
+    try:
+        visitor = globals()['visitor_' + args.action]
+    except KeyError:
+        tty.error('Unknown action: "%s"' % args.action)
+    visitor(specs, args)

share/spack/examples/test_view.sh

+#!/bin/bash
+
+# This will install a few bogus/test packages in order to test the
+# `spack view` command.  It assumes you have "spack" in your path.
+
+# It makes sub-directories in your CWD and installs and uninstalls
+# Spack packages named test-*.
+
+set -x
+set -e
+
+view="spack -m view -v"
+for variant in +nom ~nom+var +nom+var
+do
+    spack -m uninstall -f -a -y test-d
+    spack -m install test-d$variant
+    testdir=test_view
+    rm -rf $testdir
+    echo "hardlink may fail if Spack install area and CWD are not same FS"
+    for action in symlink hardlink
+    do
+	$view --dependencies=no   $action $testdir test-d
+	$view -e test-a -e test-b $action $testdir test-d
+	$view                     $action $testdir test-d
+	$view                     status  $testdir test-d
+	$view -d false            remove  $testdir test-a
+	$view                     remove  $testdir test-d
+	rmdir $testdir		# should not fail
+    done
+done
+echo "Warnings about skipping existing in the above are okay"

var/spack/repos/builtin.mock/packages/test-a/package.py

@@ -9,6 +9,7 @@ class TestA(Package):
     """The test-a package"""
 
     url = 'file://'+source
+    homepage = "http://www.example.com/"
 
     version('0.0', '4e823d0af4154fcf52b75dad41b7fd63')
 

var/spack/repos/builtin.mock/packages/test-b/package.py

@@ -9,6 +9,7 @@ class TestB(Package):
     """The test-b package"""
 
     url = 'file://'+source
+    homepage = "http://www.example.com/"
 
     version('0.0', '4e823d0af4154fcf52b75dad41b7fd63')
 

var/spack/repos/builtin.mock/packages/test-c/package.py

@@ -9,6 +9,7 @@ class TestC(Package):
     """The test-c package"""
 
     url = 'file://'+source
+    homepage = "http://www.example.com/"
 
     version('0.0', '4e823d0af4154fcf52b75dad41b7fd63')
 

var/spack/repos/builtin.mock/packages/test-d/package.py

@@ -9,6 +9,7 @@ class TestD(Package):
     """The test-d package"""
 
     url = 'file://'+source
+    homepage = "http://www.example.com/"
 
     version('0.0', '4e823d0af4154fcf52b75dad41b7fd63')