Introduce get_sequence and sequence_at/mapping_At

19 files changed, 118 insertions, 62 deletions

diff --git a/src/buildstream/_basecache.py b/src/buildstream/_basecache.py
index 5cfd7e552..e8923c7b1 100644
--- a/src/buildstream/_basecache.py
+++ b/src/buildstream/_basecache.py
@@ -77,7 +77,7 @@ class BaseCache():
             artifacts = [config_node.get_mapping(cls.config_node_name)]
         except LoadError:
             try:
-                artifacts = _yaml.node_get(config_node, list, cls.config_node_name, default_value=[])
+                artifacts = config_node.get_sequence(cls.config_node_name, default=[])
             except LoadError:
                 provenance = _yaml.node_get_provenance(config_node, key=cls.config_node_name)
                 raise _yaml.LoadError(_yaml.LoadErrorReason.INVALID_DATA,
diff --git a/src/buildstream/_gitsourcebase.py b/src/buildstream/_gitsourcebase.py
index 0b88ebd37..6ba6aa50d 100644
--- a/src/buildstream/_gitsourcebase.py
+++ b/src/buildstream/_gitsourcebase.py
@@ -383,7 +383,7 @@ class _GitSourceBase(Source):
                        'track-tags', 'tags']
         self.node_validate(node, config_keys + Source.COMMON_CONFIG_KEYS)
 
-        tags_node = self.node_get_member(node, list, 'tags', [])
+        tags_node = node.get_sequence('tags', [])
         for tag_node in tags_node:
             self.node_validate(tag_node, ['tag', 'commit', 'annotated'])
 
@@ -663,7 +663,7 @@ class _GitSourceBase(Source):
 
     def _load_tags(self, node):
         tags = []
-        tags_node = self.node_get_member(node, list, 'tags', [])
+        tags_node = node.get_sequence('tags', [])
         for tag_node in tags_node:
             tag = tag_node.get_scalar('tag').as_str()
             commit_ref = tag_node.get_scalar('commit').as_str()
diff --git a/src/buildstream/_includes.py b/src/buildstream/_includes.py
index e29e769cf..65d941c44 100644
--- a/src/buildstream/_includes.py
+++ b/src/buildstream/_includes.py
@@ -41,7 +41,7 @@ class Includes:
                 includes = [includes]
         except LoadError:
             try:
-                includes = _yaml.node_get(node, list, '(@)')
+                includes = node.get_sequence('(@)').as_str_list()
             except LoadError:
                 provenance = _yaml.node_get_provenance(node, key='(@)')
                 raise LoadError(LoadErrorReason.INVALID_DATA,
diff --git a/src/buildstream/_loader/loader.py b/src/buildstream/_loader/loader.py
index 791a21332..a455d5935 100644
--- a/src/buildstream/_loader/loader.py
+++ b/src/buildstream/_loader/loader.py
@@ -469,13 +469,12 @@ class Loader():
         elt_provenance = _yaml.node_get_provenance(node)
         meta_sources = []
 
-        sources = _yaml.node_get(node, list, Symbol.SOURCES, default_value=[])
+        sources = node.get_sequence(Symbol.SOURCES, default=[])
         element_kind = node.get_scalar(Symbol.KIND).as_str()
 
         # Safe loop calling into _yaml.node_get() for each element ensures
         # we have good error reporting
-        for i in range(len(sources)):
-            source = _yaml.node_get(node, dict, Symbol.SOURCES, indices=[i])
+        for index, source in enumerate(sources):
             kind = source.get_scalar(Symbol.KIND).as_str()
             _yaml.node_del(source, Symbol.KIND)
 
@@ -484,7 +483,6 @@ class Loader():
             if directory:
                 _yaml.node_del(source, Symbol.DIRECTORY)
 
-            index = sources.index(source)
             meta_source = MetaSource(element.name, index, element_kind, kind, source, directory)
             meta_sources.append(meta_source)
 
@@ -493,7 +491,7 @@ class Loader():
                                    node.get_mapping(Symbol.CONFIG, default={}),
                                    node.get_mapping(Symbol.VARIABLES, default={}),
                                    node.get_mapping(Symbol.ENVIRONMENT, default={}),
-                                   _yaml.node_get(node, list, Symbol.ENV_NOCACHE, default_value=[]),
+                                   node.get_sequence(Symbol.ENV_NOCACHE, default=[]).as_str_list(),
                                    node.get_mapping(Symbol.PUBLIC, default={}),
                                    node.get_mapping(Symbol.SANDBOX, default={}),
                                    element_kind == 'junction')
diff --git a/src/buildstream/_loader/types.pyx b/src/buildstream/_loader/types.pyx
index 14ebeab7a..b24f6893d 100644
--- a/src/buildstream/_loader/types.pyx
+++ b/src/buildstream/_loader/types.pyx
@@ -72,8 +72,8 @@ cdef class Dependency:
 
         self.provenance = provenance
 
-        if type(dep) is str:
-            self.name = <str> dep
+        if type(dep) is _yaml.ScalarNode:
+            self.name = dep.as_str()
             self.dep_type = default_dep_type
             self.junction = None
 
@@ -138,15 +138,15 @@ cdef class Dependency:
 #    acc (list): a list in which to add the loaded dependencies
 #
 cdef void _extract_depends_from_node(_yaml.Node node, str key, str default_dep_type, list acc) except *:
-    cdef list depends = <list> _yaml.node_get(node, list, key, None, [])
-    cdef int index
+    cdef _yaml.SequenceNode depends = node.get_sequence(key, [])
+    cdef _yaml.Node dep_node
     cdef _yaml.ProvenanceInformation dep_provenance
 
-    for index in range(len(depends)):
+    for dep_node in depends:
         # FIXME: the provenance information would be obtainable from the Node directly if we stop
         #        stripping provenance and have proper nodes for str elements
-        dep_provenance = <_yaml.ProvenanceInformation> _yaml.node_get_provenance(node, key=key, indices=[index])
-        dependency = Dependency(depends[index], dep_provenance, default_dep_type=default_dep_type)
+        dep_provenance = <_yaml.ProvenanceInformation> _yaml.node_get_provenance(dep_node)
+        dependency = Dependency(dep_node, dep_provenance, default_dep_type=default_dep_type)
         acc.append(dependency)
 
     # Now delete the field, we dont want it anymore
diff --git a/src/buildstream/_options/optionenum.py b/src/buildstream/_options/optionenum.py
index f8daa82b1..ec63d5d32 100644
--- a/src/buildstream/_options/optionenum.py
+++ b/src/buildstream/_options/optionenum.py
@@ -39,7 +39,7 @@ class OptionEnum(Option):
 
         _yaml.node_validate(node, valid_symbols)
 
-        self.values = _yaml.node_get(node, list, 'values', default_value=[])
+        self.values = node.get_sequence('values', default=[]).as_str_list()
         if not self.values:
             raise LoadError(LoadErrorReason.INVALID_DATA,
                             "{}: No values specified for {} option '{}'"
diff --git a/src/buildstream/_options/optionflags.py b/src/buildstream/_options/optionflags.py
index ba16244ba..a7dd536f6 100644
--- a/src/buildstream/_options/optionflags.py
+++ b/src/buildstream/_options/optionflags.py
@@ -46,11 +46,11 @@ class OptionFlags(Option):
                             "{}: No values specified for {} option '{}'"
                             .format(_yaml.node_get_provenance(node), self.OPTION_TYPE, self.name))
 
-        self.value = _yaml.node_get(node, list, 'default', default_value=[])
+        self.value = node.get_sequence('default', default=[]).as_str_list()
         self.validate(self.value, _yaml.node_get_provenance(node, 'default'))
 
     def load_value(self, node, *, transform=None):
-        self.value = _yaml.node_get(node, list, self.name)
+        self.value = node.get_sequence(self.name).as_str_list()
         if transform:
             self.value = [transform(x) for x in self.value]
         self.value = sorted(self.value)
@@ -83,4 +83,4 @@ class OptionFlags(Option):
     def load_valid_values(self, node):
         # Allow the more descriptive error to raise when no values
         # exist rather than bailing out here (by specifying default_value)
-        return _yaml.node_get(node, list, 'values', default_value=[])
+        return node.get_sequence('values', default=[]).as_str_list()
diff --git a/src/buildstream/_options/optionpool.py b/src/buildstream/_options/optionpool.py
index c6d23f59e..1b8683186 100644
--- a/src/buildstream/_options/optionpool.py
+++ b/src/buildstream/_options/optionpool.py
@@ -248,7 +248,7 @@ class OptionPool():
     # Return true if a conditional was processed.
     #
     def _process_one_node(self, node):
-        conditions = _yaml.node_get(node, list, '(?)', default_value=None)
+        conditions = node.get_sequence('(?)', default=None)
         assertion = node.get_scalar('(!)', default=None).as_str()
 
         # Process assersions first, we want to abort on the first encountered
diff --git a/src/buildstream/_plugincontext.py b/src/buildstream/_plugincontext.py
index c4132c3f8..a9e69531b 100644
--- a/src/buildstream/_plugincontext.py
+++ b/src/buildstream/_plugincontext.py
@@ -138,7 +138,7 @@ class PluginContext():
             loaded_dependency = False
 
             for origin in self._plugin_origins:
-                if kind not in _yaml.node_get(origin, list, 'plugins'):
+                if kind not in origin.get_sequence('plugins').as_str_list():
                     continue
 
                 if origin.get_scalar('origin').as_str() == 'local':
diff --git a/src/buildstream/_project.py b/src/buildstream/_project.py
index b7cdeea7d..7b7f69ac4 100644
--- a/src/buildstream/_project.py
+++ b/src/buildstream/_project.py
@@ -607,10 +607,10 @@ class Project():
 
         defaults = pre_config_node.get_mapping('defaults')
         _yaml.node_validate(defaults, ['targets'])
-        self._default_targets = _yaml.node_get(defaults, list, "targets")
+        self._default_targets = defaults.get_sequence("targets").as_str_list()
 
         # Fatal warnings
-        self._fatal_warnings = _yaml.node_get(pre_config_node, list, 'fatal-warnings', default_value=[])
+        self._fatal_warnings = pre_config_node.get_sequence('fatal-warnings', default=[]).as_str_list()
 
         self.loader = Loader(self._context, self,
                              parent=parent_loader)
@@ -683,7 +683,7 @@ class Project():
 
         # Load sandbox environment variables
         self.base_environment = config.get_mapping('environment')
-        self.base_env_nocache = _yaml.node_get(config, list, 'environment-nocache')
+        self.base_env_nocache = config.get_sequence('environment-nocache').as_str_list()
 
         # Load sandbox configuration
         self._sandbox = config.get_mapping('sandbox')
@@ -715,7 +715,7 @@ class Project():
         # Parse shell options
         shell_options = config.get_mapping('shell')
         _yaml.node_validate(shell_options, ['command', 'environment', 'host-files'])
-        self._shell_command = _yaml.node_get(shell_options, list, 'command')
+        self._shell_command = shell_options.get_sequence('command')
 
         # Perform environment expansion right away
         shell_environment = shell_options.get_mapping('environment', default={})
@@ -724,7 +724,7 @@ class Project():
             self._shell_environment[key] = os.path.expandvars(value)
 
         # Host files is parsed as a list for convenience
-        host_files = _yaml.node_get(shell_options, list, 'host-files', default_value=[])
+        host_files = shell_options.get_sequence('host-files', default=[])
         for host_file in host_files:
             if isinstance(host_file, str):
                 mount = HostMount(host_file)
@@ -814,7 +814,7 @@ class Project():
         output.default_mirror = self._default_mirror or overrides.get_scalar(
             'default-mirror', default=None).as_str()
 
-        mirrors = _yaml.node_get(config, list, 'mirrors', default_value=[])
+        mirrors = config.get_sequence('mirrors', default=[])
         for mirror in mirrors:
             allowed_mirror_fields = [
                 'name', 'aliases'
@@ -874,7 +874,7 @@ class Project():
         plugin_element_origins = []  # Origins of custom elements
 
         # Plugin origins and versions
-        origins = _yaml.node_get(config, list, 'plugins', default_value=[])
+        origins = config.get_sequence('plugins', default=[])
         source_format_versions = {}
         element_format_versions = {}
         for origin in origins:
diff --git a/src/buildstream/_projectrefs.py b/src/buildstream/_projectrefs.py
index 847935c0b..f296858cf 100644
--- a/src/buildstream/_projectrefs.py
+++ b/src/buildstream/_projectrefs.py
@@ -133,16 +133,16 @@ class ProjectRefs():
 
         # Fetch the element
         try:
-            element_list = _yaml.node_get(project_node, list, element)
+            element_list = project_node.get_sequence(element)
         except LoadError:
             if not ensure:
                 return None
-            element_list = []
+            element_list = _yaml.new_empty_list_node()
             _yaml.node_set(project_node, element, element_list)
 
         # Fetch the source index
         try:
-            node = element_list[source_index]
+            node = element_list.mapping_at(source_index)
         except IndexError:
             if not ensure:
                 return None
diff --git a/src/buildstream/_yaml.pxd b/src/buildstream/_yaml.pxd
index 11271610e..15428b775 100644
--- a/src/buildstream/_yaml.pxd
+++ b/src/buildstream/_yaml.pxd
@@ -32,6 +32,7 @@ cdef class MappingNode(Node):
     cdef Node get(self, str key, default, default_constructor)
     cpdef MappingNode get_mapping(self, str key, default=*)
     cpdef ScalarNode get_scalar(self, str key, default=*)
+    cpdef SequenceNode get_sequence(Self, str key, default=*)
 
 
 cdef class ScalarNode(Node):
@@ -41,6 +42,12 @@ cdef class ScalarNode(Node):
     cpdef bint is_none(self)
 
 
+cdef class SequenceNode(Node):
+    cpdef MappingNode mapping_at(self, int index)
+    cpdef SequenceNode sequence_at(self, int index)
+    cpdef list as_str_list(self)
+
+
 cdef class ProvenanceInformation:
 
     cdef public Node node
diff --git a/src/buildstream/_yaml.pyx b/src/buildstream/_yaml.pyx
index 85b8098c4..626f75d64 100644
--- a/src/buildstream/_yaml.pyx
+++ b/src/buildstream/_yaml.pyx
@@ -167,14 +167,60 @@ cdef class MappingNode(Node):
 
         return value
 
+    cpdef SequenceNode get_sequence(self, str key, default=_sentinel):
+        value = self.get(key, default, SequenceNode)
 
-class SequenceNode(Node):
+        if type(value) is not SequenceNode and value is not None:
+            provenance = node_get_provenance(self)
+            raise LoadError(LoadErrorReason.INVALID_DATA,
+                            "{}: Value of '{}' is not of the expected type '{}'"
+                            .format(provenance, key, ScalarNode.__name__))
+
+        return value
+
+
+cdef class SequenceNode(Node):
     def __init__(self, list value, int file_index, int line, int column):
         self.value = value
         self.file_index = file_index
         self.line = line
         self.column = column
 
+    cpdef MappingNode mapping_at(self, int index):
+        value = self.value[index]
+
+        if type(value) is not MappingNode:
+            provenance = node_get_provenance(self)
+            path = ["[{}]".format(p) for p in node_find_target(provenance, self)] + ["[{}]".format(index)]
+            raise LoadError(LoadErrorReason.INVALID_DATA,
+                            "{}: Value of '{}' is not of the expected type '{}'"
+                            .format(provenance, path, MappingNode.__name__))
+        return value
+
+    cpdef SequenceNode sequence_at(self, int index):
+        value = self.value[index]
+
+        if type(value) is not SequenceNode:
+            provenance = node_get_provenance(self)
+            path = ["[{}]".format(p) for p in node_find_target(provenance, self)] + ["[{}]".format(index)]
+            raise LoadError(LoadErrorReason.INVALID_DATA,
+                            "{}: Value of '{}' is not of the expected type '{}'"
+                            .format(provenance, path, SequenceNode.__name__))
+
+        return value
+
+    cpdef list as_str_list(self):
+        return [node.as_str() for node in self.value]
+
+    def __iter__(self):
+        return iter(self.value)
+
+    def __len__(self):
+        return len(self.value)
+
+    def __reversed__(self):
+        return reversed(self.value)
+
 
 # Metadata container for a yaml toplevel node.
 #
@@ -920,6 +966,11 @@ def new_empty_node(Node ref_node=None):
         return MappingNode({}, _SYNTHETIC_FILE_INDEX, 0, 0)
 
 
+# FIXME: we should never need that
+def new_empty_list_node():
+    return SequenceNode([], _SYNTHETIC_FILE_INDEX, 0, 0)
+
+
 # new_node_from_dict()
 #
 # Args:
diff --git a/src/buildstream/buildelement.py b/src/buildstream/buildelement.py
index 158f5fc11..0c8205bd6 100644
--- a/src/buildstream/buildelement.py
+++ b/src/buildstream/buildelement.py
@@ -281,14 +281,11 @@ class BuildElement(Element):
     #                   Private Local Methods                   #
     #############################################################
     def __get_commands(self, node, name):
-        list_node = self.node_get_member(node, list, name, [])
-        commands = []
-
-        for i in range(len(list_node)):
-            command = self.node_subst_list_element(node, name, [i])
-            commands.append(command)
-
-        return commands
+        list_node = node.get_sequence(name, [])
+        return [
+            self.substitute_variables(node.as_str())
+            for node in list_node
+        ]
 
     def __run_command(self, sandbox, cmd):
         # Note the -e switch to 'sh' means to exit with an error
diff --git a/src/buildstream/element.py b/src/buildstream/element.py
index 7f824fc95..33a00b197 100644
--- a/src/buildstream/element.py
+++ b/src/buildstream/element.py
@@ -482,6 +482,9 @@ class Element(Plugin):
 
         return None
 
+    def substitute_variables(self, value):
+        return self.__variables.subst(value)
+
     def node_subst_member(self, node, member_name, default=_yaml._sentinel):
         """Fetch the value of a string node member, substituting any variables
         in the loaded value with the element contextual variables.
@@ -851,9 +854,9 @@ class Element(Plugin):
 
         if bstdata is not None:
             with sandbox.batch(SandboxFlags.NONE):
-                commands = self.node_get_member(bstdata, list, 'integration-commands', [])
-                for i in range(len(commands)):
-                    cmd = self.node_subst_list_element(bstdata, 'integration-commands', [i])
+                commands = bstdata.get_sequence('integration-commands', [])
+                for command in commands:
+                    cmd = self.substitute_variables(command)
 
                     sandbox.run(['sh', '-e', '-c', cmd], 0, env=environment, cwd='/',
                                 label=cmd)
@@ -2605,7 +2608,7 @@ class Element(Plugin):
         else:
             project_nocache = project.base_env_nocache
 
-        default_nocache = _yaml.node_get(cls.__defaults, list, 'environment-nocache', default_value=[])
+        default_nocache = cls.__defaults.get_sequence('environment-nocache', default=[]).as_str_list()
         element_nocache = meta.env_nocache
 
         # Accumulate values from the element default, the project and the element
diff --git a/tests/format/include.py b/tests/format/include.py
index 0bfc2b342..59e530166 100644
--- a/tests/format/include.py
+++ b/tests/format/include.py
@@ -197,7 +197,7 @@ def test_include_element_overrides_composition(cli, datafiles):
         'element.bst'])
     result.assert_success()
     loaded = _yaml.load_data(result.output)
-    assert _yaml.node_get(loaded, list, 'build-commands') == ['first', 'second']
+    assert loaded.get_sequence('build-commands').as_str_list() == ['first', 'second']
 
 
 @pytest.mark.datafiles(DATA_DIR)
@@ -215,7 +215,7 @@ def test_list_overide_does_not_fail_upon_first_composition(cli, datafiles):
     # Assert that the explicitly overwritten public data is present
     bst = loaded.get_mapping('bst')
     assert 'foo-commands' in bst
-    assert _yaml.node_get(bst, list, 'foo-commands') == ['need', 'this']
+    assert bst.get_sequence('foo-commands').as_str_list() == ['need', 'this']
 
 
 @pytest.mark.datafiles(DATA_DIR)
diff --git a/tests/format/include_composition.py b/tests/format/include_composition.py
index 562f7b002..c306dad5c 100644
--- a/tests/format/include_composition.py
+++ b/tests/format/include_composition.py
@@ -14,7 +14,7 @@ def make_includes(basedir):
     return Includes(loader)
 
 
-def test_main_has_prority(tmpdir):
+def test_main_has_priority(tmpdir):
     includes = make_includes(str(tmpdir))
 
     _yaml.dump({'(@)': ['a.yml'],
@@ -28,7 +28,7 @@ def test_main_has_prority(tmpdir):
 
     includes.process(main)
 
-    assert _yaml.node_get(main, list, 'test') == ['main']
+    assert main.get_sequence('test').as_str_list() == ['main']
 
 
 def test_include_cannot_append(tmpdir):
@@ -44,7 +44,7 @@ def test_include_cannot_append(tmpdir):
 
     includes.process(main)
 
-    assert _yaml.node_get(main, list, 'test') == ['main']
+    assert main.get_sequence('test').as_str_list() == ['main']
 
 
 def test_main_can_append(tmpdir):
@@ -60,7 +60,7 @@ def test_main_can_append(tmpdir):
 
     includes.process(main)
 
-    assert _yaml.node_get(main, list, 'test') == ['a', 'main']
+    assert main.get_sequence('test').as_str_list() == ['a', 'main']
 
 
 def test_sibling_cannot_append_backward(tmpdir):
@@ -77,7 +77,7 @@ def test_sibling_cannot_append_backward(tmpdir):
 
     includes.process(main)
 
-    assert _yaml.node_get(main, list, 'test') == ['b']
+    assert main.get_sequence('test').as_str_list() == ['b']
 
 
 def test_sibling_can_append_forward(tmpdir):
@@ -94,7 +94,7 @@ def test_sibling_can_append_forward(tmpdir):
 
     includes.process(main)
 
-    assert _yaml.node_get(main, list, 'test') == ['a', 'b']
+    assert main.get_sequence('test').as_str_list() == ['a', 'b']
 
 
 def test_lastest_sibling_has_priority(tmpdir):
@@ -111,7 +111,7 @@ def test_lastest_sibling_has_priority(tmpdir):
 
     includes.process(main)
 
-    assert _yaml.node_get(main, list, 'test') == ['b']
+    assert main.get_sequence('test').as_str_list() == ['b']
 
 
 def test_main_keeps_keys(tmpdir):
@@ -127,7 +127,7 @@ def test_main_keeps_keys(tmpdir):
 
     includes.process(main)
 
-    assert _yaml.node_get(main, list, 'test') == ['a']
+    assert main.get_sequence('test').as_str_list() == ['a']
     assert main.get_scalar('something').as_str() == 'else'
 
 
@@ -151,5 +151,5 @@ def test_overwrite_directive_on_later_composite(tmpdir):
 
     includes.process(main)
 
-    assert _yaml.node_get(main, list, 'test') == ['Overwritten']
+    assert main.get_sequence('test').as_str_list() == ['Overwritten']
     assert main.get_scalar('foo').as_str() == 'should be present'
diff --git a/tests/format/options.py b/tests/format/options.py
index 38c0c12c6..965a98307 100644
--- a/tests/format/options.py
+++ b/tests/format/options.py
@@ -223,8 +223,8 @@ def test_deep_nesting_level1(cli, datafiles, option, expected):
         'element.bst'])
     result.assert_success()
     loaded = _yaml.load_data(result.output)
-    shallow_list = _yaml.node_get(loaded, list, 'shallow-nest')
-    first_dict = shallow_list[0]
+    shallow_list = loaded.get_sequence('shallow-nest')
+    first_dict = shallow_list.mapping_at(0)
 
     assert first_dict.get_scalar('animal').as_str() == expected
 
@@ -244,8 +244,8 @@ def test_deep_nesting_level2(cli, datafiles, option, expected):
         'element-deeper.bst'])
     result.assert_success()
     loaded = _yaml.load_data(result.output)
-    shallow_list = _yaml.node_get(loaded, list, 'deep-nest')
-    deeper_list = shallow_list[0]
-    first_dict = deeper_list[0]
+    shallow_list = loaded.get_sequence('deep-nest')
+    deeper_list = shallow_list.sequence_at(0)
+    first_dict = deeper_list.mapping_at(0)
 
     assert first_dict.get_scalar('animal').as_str() == expected
diff --git a/tests/format/projectoverrides.py b/tests/format/projectoverrides.py
index 4b0c3f4d0..730e43b1e 100644
--- a/tests/format/projectoverrides.py
+++ b/tests/format/projectoverrides.py
@@ -24,6 +24,6 @@ def test_prepend_configure_commands(cli, datafiles):
 
     result.assert_success()
     loaded = _yaml.load_data(result.output)
-    config_commands = _yaml.node_get(loaded, list, 'configure-commands')
+    config_commands = loaded.get_sequence('configure-commands').as_str_list()
     assert len(config_commands) == 3
     assert config_commands[0] == 'echo "Hello World!"'