Code Loading
This chapter covers the technical details of package loading. To install packages, use Pkg
, Julia's built-in package manager, to add packages to your active environment. To use packages already in your active environment, write import X
or using X
, as described in the Modules documentation.
Definitions
Julia has two mechanisms for loading code:
- Code inclusion: e.g.
include("source.jl")
. Inclusion allows you to split a single program across multiple source files. The expressioninclude("source.jl")
causes the contents of the filesource.jl
to be evaluated in the global scope of the module where theinclude
call occurs. Ifinclude("source.jl")
is called multiple times,source.jl
is evaluated multiple times. The included path,source.jl
, is interpreted relative to the file where theinclude
call occurs. This makes it simple to relocate a subtree of source files. In the REPL, included paths are interpreted relative to the current working directory,pwd()
. - Package loading: e.g.
import X
orusing X
. The import mechanism allows you to load a package—i.e. an independent, reusable collection of Julia code, wrapped in a module—and makes the resulting module available by the nameX
inside of the importing module. If the sameX
package is imported multiple times in the same Julia session, it is only loaded the first time—on subsequent imports, the importing module gets a reference to the same module. Note though, thatimport X
can load different packages in different contexts:X
can refer to one package namedX
in the main project but potentially to different packages also namedX
in each dependency. More on this below.
Code inclusion is quite straightforward and simple: it evaluates the given source file in the context of the caller. Package loading is built on top of code inclusion and serves a different purpose. The rest of this chapter focuses on the behavior and mechanics of package loading.
A package is a source tree with a standard layout providing functionality that can be reused by other Julia projects. A package is loaded by import X
or using X
statements. These statements also make the module named X
—which results from loading the package code—available within the module where the import statement occurs. The meaning of X
in import X
is context-dependent: which X
package is loaded depends on what code the statement occurs in. Thus, handling of import X
happens in two stages: first, it determines what package is defined to be X
in this context; second, it determines where that particular X
package is found.
These questions are answered by searching through the project environments listed in LOAD_PATH
for project files (Project.toml
or JuliaProject.toml
), manifest files (Manifest.toml
or JuliaManifest.toml
), or folders of source files.
Federation of packages
Most of the time, a package is uniquely identifiable simply from its name. However, sometimes a project might encounter a situation where it needs to use two different packages that share the same name. While you might be able fix this by renaming one of the packages, being forced to do so can be highly disruptive in a large, shared code base. Instead, Julia's code loading mechanism allows the same package name to refer to different packages in different components of an application.
Julia supports federated package management, which means that multiple independent parties can maintain both public and private packages and registries of packages, and that projects can depend on a mix of public and private packages from different registries. Packages from various registries are installed and managed using a common set of tools and workflows. The Pkg
package manager that ships with Julia lets you install and manage your projects' dependencies. It assists in creating and manipulating project files (which describe what other projects that your project depends on), and manifest files (which snapshot exact versions of your project's complete dependency graph).
One consequence of federation is that there cannot be a central authority for package naming. Different entities may use the same name to refer to unrelated packages. This possibility is unavoidable since these entities do not coordinate and may not even know about each other. Because of the lack of a central naming authority, a single project may end up depending on different packages that have the same name. Julia's package loading mechanism does not require package names to be globally unique, even within the dependency graph of a single project. Instead, packages are identified by universally unique identifiers (UUIDs), which get assigned when each package is created. Usually you won't have to work directly with these somewhat cumbersome 128-bit identifiers since Pkg
will take care of generating and tracking them for you. However, these UUIDs provide the definitive answer to the question of "what package does X
refer to?"
Since the decentralized naming problem is somewhat abstract, it may help to walk through a concrete scenario to understand the issue. Suppose you're developing an application called App
, which uses two packages: Pub
and Priv
. Priv
is a private package that you created, whereas Pub
is a public package that you use but don't control. When you created Priv
, there was no public package by the name Priv
. Subsequently, however, an unrelated package also named Priv
has been published and become popular. In fact, the Pub
package has started to use it. Therefore, when you next upgrade Pub
to get the latest bug fixes and features, App
will end up depending on two different packages named Priv
—through no action of yours other than upgrading. App
has a direct dependency on your private Priv
package, and an indirect dependency, through Pub
, on the new public Priv
package. Since these two Priv
packages are different but are both required for App
to continue working correctly, the expression import Priv
must refer to different Priv
packages depending on whether it occurs in App
's code or in Pub
's code. To handle this, Julia's package loading mechanism distinguishes the two Priv
packages by their UUID and picks the correct one based on its context (the module that called import
). How this distinction works is determined by environments, as explained in the following sections.
Environments
An environment determines what import X
and using X
mean in various code contexts and what files these statements cause to be loaded. Julia understands two kinds of environments:
- A project environment is a directory with a project file and an optional manifest file, and forms an explicit environment. The project file determines what the names and identities of the direct dependencies of a project are. The manifest file, if present, gives a complete dependency graph, including all direct and indirect dependencies, exact versions of each dependency, and sufficient information to locate and load the correct version.
- A package directory is a directory containing the source trees of a set of packages as subdirectories, and forms an implicit environment. If
X
is a subdirectory of a package directory andX/src/X.jl
exists, then the packageX
is available in the package directory environment andX/src/X.jl
is the source file by which it is loaded.
These can be intermixed to create a stacked environment: an ordered set of project environments and package directories, overlaid to make a single composite environment. The precedence and visibility rules then combine to determine which packages are available and where they get loaded from. Julia's load path forms a stacked environment, for example.
These environment each serve a different purpose:
- Project environments provide reproducibility. By checking a project environment into version control—e.g. a git repository—along with the rest of the project's source code, you can reproduce the exact state of the project and all of its dependencies. The manifest file, in particular, captures the exact version of every dependency, identified by a cryptographic hash of its source tree, which makes it possible for
Pkg
to retrieve the correct versions and be sure that you are running the exact code that was recorded for all dependencies. - Package directories provide convenience when a full carefully-tracked project environment is unnecessary. They are useful when you want to put a set of packages somewhere and be able to directly use them, without needing to create a project environment for them.
- Stacked environments allow for adding tools to the primary environment. You can push an environment of development tools onto the end of the stack to make them available from the REPL and scripts, but not from inside packages.
At a high-level, each environment conceptually defines three maps: roots, graph and paths. When resolving the meaning of import X
, the roots and graph maps are used to determine the identity of X
, while the paths map is used to locate the source code of X
. The specific roles of the three maps are:
roots:
name::Symbol
⟶uuid::UUID
An environment's roots map assigns package names to UUIDs for all the top-level dependencies that the environment makes available to the main project (i.e. the ones that can be loaded in
Main
). When Julia encountersimport X
in the main project, it looks up the identity ofX
asroots[:X]
.graph:
context::UUID
⟶name::Symbol
⟶uuid::UUID
An environment's graph is a multilevel map which assigns, for each
context
UUID, a map from names to UUIDs, similar to the roots map but specific to thatcontext
. When Julia seesimport X
in the code of the package whose UUID iscontext
, it looks up the identity ofX
asgraph[context][:X]
. In particular, this means thatimport X
can refer to different packages depending oncontext
.paths:
uuid::UUID
×name::Symbol
⟶path::String
The paths map assigns to each package UUID-name pair, the location of that package's entry-point source file. After the identity of
X
inimport X
has been resolved to a UUID via roots or graph (depending on whether it is loaded from the main project or a dependency), Julia determines what file to load to acquireX
by looking uppaths[uuid,:X]
in the environment. Including this file should define a module namedX
. Once this package is loaded, any subsequent import resolving to the sameuuid
will create a new binding to the already-loaded package module.
Each kind of environment defines these three maps differently, as detailed in the following sections.
For ease of understanding, the examples throughout this chapter show full data structures for roots, graph and paths. However, Julia's package loading code does not explicitly create these. Instead, it lazily computes only as much of each structure as it needs to load a given package.
Project environments
A project environment is determined by a directory containing a project file called Project.toml
, and optionally a manifest file called Manifest.toml
. These files may also be called JuliaProject.toml
and JuliaManifest.toml
, in which case Project.toml
and Manifest.toml
are ignored. This allows for coexistence with other tools that might consider files called Project.toml
and Manifest.toml
significant. For pure Julia projects, however, the names Project.toml
and Manifest.toml
are preferred.
The roots, graph and paths maps of a project environment are defined as follows:
The roots map of the environment is determined by the contents of the project file, specifically, its top-level name
and uuid
entries and its [deps]
section (all optional). Consider the following example project file for the hypothetical application, App
, as described earlier:
name = "App"
uuid = "8f986787-14fe-4607-ba5d-fbff2944afa9"
[deps]
Priv = "ba13f791-ae1d-465a-978b-69c3ad90f72b"
Pub = "c07ecb7d-0dc9-4db7-8803-fadaaeaf08e1"
This project file implies the following roots map, if it was represented by a Julia dictionary:
roots = Dict(
:App => UUID("8f986787-14fe-4607-ba5d-fbff2944afa9"),
:Priv => UUID("ba13f791-ae1d-465a-978b-69c3ad90f72b"),
:Pub => UUID("c07ecb7d-0dc9-4db7-8803-fadaaeaf08e1"),
)
Given this roots map, in App
's code the statement import Priv
will cause Julia to look up roots[:Priv]
, which yields ba13f791-ae1d-465a-978b-69c3ad90f72b
, the UUID of the Priv
package that is to be loaded in that context. This UUID identifies which Priv
package to load and use when the main application evaluates import Priv
.
The dependency graph of a project environment is determined by the contents of the manifest file, if present. If there is no manifest file, graph is empty. A manifest file contains a stanza for each of a project's direct or indirect dependencies. For each dependency, the file lists the package's UUID and a source tree hash or an explicit path to the source code. Consider the following example manifest file for App
:
[[Priv]] # the private one
deps = ["Pub", "Zebra"]
uuid = "ba13f791-ae1d-465a-978b-69c3ad90f72b"
path = "deps/Priv"
[[Priv]] # the public one
uuid = "2d15fe94-a1f7-436c-a4d8-07a9a496e01c"
git-tree-sha1 = "1bf63d3be994fe83456a03b874b409cfd59a6373"
version = "0.1.5"
[[Pub]]
uuid = "c07ecb7d-0dc9-4db7-8803-fadaaeaf08e1"
git-tree-sha1 = "9ebd50e2b0dd1e110e842df3b433cb5869b0dd38"
version = "2.1.4"
[Pub.deps]
Priv = "2d15fe94-a1f7-436c-a4d8-07a9a496e01c"
Zebra = "f7a24cb4-21fc-4002-ac70-f0e3a0dd3f62"
[[Zebra]]
uuid = "f7a24cb4-21fc-4002-ac70-f0e3a0dd3f62"
git-tree-sha1 = "e808e36a5d7173974b90a15a353b564f3494092f"
version = "3.4.2"
This manifest file describes a possible complete dependency graph for the App
project:
- There are two different packages named
Priv
that the application uses. It uses a private package, which is a root dependency, and a public one, which is an indirect dependency throughPub
. These are differentiated by their distinct UUIDs, and they have different deps:- The private
Priv
depends on thePub
andZebra
packages. - The public
Priv
has no dependencies.
- The private
- The application also depends on the
Pub
package, which in turn depends on the publicPriv
and the sameZebra
package that the privatePriv
package depends on.
This dependency graph represented as a dictionary, looks like this:
graph = Dict(
# Priv – the private one:
UUID("ba13f791-ae1d-465a-978b-69c3ad90f72b") => Dict(
:Pub => UUID("c07ecb7d-0dc9-4db7-8803-fadaaeaf08e1"),
:Zebra => UUID("f7a24cb4-21fc-4002-ac70-f0e3a0dd3f62"),
),
# Priv – the public one:
UUID("2d15fe94-a1f7-436c-a4d8-07a9a496e01c") => Dict(),
# Pub:
UUID("c07ecb7d-0dc9-4db7-8803-fadaaeaf08e1") => Dict(
:Priv => UUID("2d15fe94-a1f7-436c-a4d8-07a9a496e01c"),
:Zebra => UUID("f7a24cb4-21fc-4002-ac70-f0e3a0dd3f62"),
),
# Zebra:
UUID("f7a24cb4-21fc-4002-ac70-f0e3a0dd3f62") => Dict(),
)
Given this dependency graph
, when Julia sees import Priv
in the Pub
package—which has UUID c07ecb7d-0dc9-4db7-8803-fadaaeaf08e1
—it looks up:
graph[UUID("c07ecb7d-0dc9-4db7-8803-fadaaeaf08e1")][:Priv]
and gets 2d15fe94-a1f7-436c-a4d8-07a9a496e01c
, which indicates that in the context of the Pub
package, import Priv
refers to the public Priv
package, rather than the private one which the app depends on directly. This is how the name Priv
can refer to different packages in the main project than it does in one of its package's dependencies, which allows for duplicate names in the package ecosystem.
What happens if import Zebra
is evaluated in the main App
code base? Since Zebra
does not appear in the project file, the import will fail even though Zebra
does appear in the manifest file. Moreover, if import Zebra
occurs in the public Priv
package—the one with UUID 2d15fe94-a1f7-436c-a4d8-07a9a496e01c
—then that would also fail since that Priv
package has no declared dependencies in the manifest file and therefore cannot load any packages. The Zebra
package can only be loaded by packages for which it appear as an explicit dependency in the manifest file: the Pub
package and one of the Priv
packages.
The paths map of a project environment is extracted from the manifest file. The path of a package uuid
named X
is determined by these rules (in order):
- If the project file in the directory matches
uuid
and nameX
, then either:
- It has a toplevel
path
entry, thenuuid
will be mapped to that path, interpreted relative to the directory containing the project file. - Otherwise,
uuid
is mapped tosrc/X.jl
relative to the directory containing the project file.
- If the above is not the case and the project file has a corresponding manifest file and the manifest contains a stanza matching
uuid
then:
- If it has a
path
entry, use that path (relative to the directory containing the manifest file). - If it has a
git-tree-sha1
entry, compute a deterministic hash function ofuuid
andgit-tree-sha1
—call itslug
—and look for a directory namedpackages/X/$slug
in each directory in the JuliaDEPOT_PATH
global array. Use the first such directory that exists.
If any of these result in success, the path to the source code entry point will be either that result, the relative path from that result plus src/X.jl
; otherwise, there is no path mapping for uuid
. When loading X
, if no source code path is found, the lookup will fail, and the user may be prompted to install the appropriate package version or to take other corrective action (e.g. declaring X
as a dependency).
In the example manifest file above, to find the path of the first Priv
package—the one with UUID ba13f791-ae1d-465a-978b-69c3ad90f72b
—Julia looks for its stanza in the manifest file, sees that it has a path
entry, looks at deps/Priv
relative to the App
project directory—let's suppose the App
code lives in /home/me/projects/App
—sees that /home/me/projects/App/deps/Priv
exists and therefore loads Priv
from there.
If, on the other hand, Julia was loading the other Priv
package—the one with UUID 2d15fe94-a1f7-436c-a4d8-07a9a496e01c
—it finds its stanza in the manifest, see that it does not have a path
entry, but that it does have a git-tree-sha1
entry. It then computes the slug
for this UUID/SHA-1 pair, which is HDkrT
(the exact details of this computation aren't important, but it is consistent and deterministic). This means that the path to this Priv
package will be packages/Priv/HDkrT/src/Priv.jl
in one of the package depots. Suppose the contents of DEPOT_PATH
is ["/home/me/.julia", "/usr/local/julia"]
, then Julia will look at the following paths to see if they exist:
/home/me/.julia/packages/Priv/HDkrT
/usr/local/julia/packages/Priv/HDkrT
Julia uses the first of these that exists to try to load the public Priv
package from the file packages/Priv/HDKrT/src/Priv.jl
in the depot where it was found.
Here is a representation of a possible paths map for our example App
project environment, as provided in the Manifest given above for the dependency graph, after searching the local file system:
paths = Dict(
# Priv – the private one:
(UUID("ba13f791-ae1d-465a-978b-69c3ad90f72b"), :Priv) =>
# relative entry-point inside `App` repo:
"/home/me/projects/App/deps/Priv/src/Priv.jl",
# Priv – the public one:
(UUID("2d15fe94-a1f7-436c-a4d8-07a9a496e01c"), :Priv) =>
# package installed in the system depot:
"/usr/local/julia/packages/Priv/HDkr/src/Priv.jl",
# Pub:
(UUID("c07ecb7d-0dc9-4db7-8803-fadaaeaf08e1"), :Pub) =>
# package installed in the user depot:
"/home/me/.julia/packages/Pub/oKpw/src/Pub.jl",
# Zebra:
(UUID("f7a24cb4-21fc-4002-ac70-f0e3a0dd3f62"), :Zebra) =>
# package installed in the system depot:
"/usr/local/julia/packages/Zebra/me9k/src/Zebra.jl",
)
This example map includes three different kinds of package locations (the first and third are part of the default load path):
- The private
Priv
package is "vendored" inside theApp
repository. - The public
Priv
andZebra
packages are in the system depot, where packages installed and managed by the system administrator live. These are available to all users on the system. - The
Pub
package is in the user depot, where packages installed by the user live. These are only available to the user who installed them.
Package directories
Package directories provide a simpler kind of environment without the ability to handle name collisions. In a package directory, the set of top-level packages is the set of subdirectories that "look like" packages. A package X
is exists in a package directory if the directory contains one of the following "entry point" files:
X.jl
X/src/X.jl
X.jl/src/X.jl
Which dependencies a package in a package directory can import depends on whether the package contains a project file:
- If it has a project file, it can only import those packages which are identified in the
[deps]
section of the project file. - If it does not have a project file, it can import any top-level package—i.e. the same packages that can be loaded in
Main
or the REPL.
The roots map is determined by examining the contents of the package directory to generate a list of all packages that exist. Additionally, a UUID will be assigned to each entry as follows: For a given package found inside the folder X
...
- If
X/Project.toml
exists and has auuid
entry, thenuuid
is that value. - If
X/Project.toml
exists and but does not have a top-level UUID entry,uuid
is a dummy UUID generated by hashing the canonical (real) path toX/Project.toml
. - Otherwise (if
Project.toml
does not exist), thenuuid
is the all-zero nil UUID.
The dependency graph of a project directory is determined by the presence and contents of project files in the subdirectory of each package. The rules are:
- If a package subdirectory has no project file, then it is omitted from graph and import statements in its code are treated as top-level, the same as the main project and REPL.
- If a package subdirectory has a project file, then the graph entry for its UUID is the
[deps]
map of the project file, which is considered to be empty if the section is absent.
As an example, suppose a package directory has the following structure and content:
Aardvark/
src/Aardvark.jl:
import Bobcat
import Cobra
Bobcat/
Project.toml:
[deps]
Cobra = "4725e24d-f727-424b-bca0-c4307a3456fa"
Dingo = "7a7925be-828c-4418-bbeb-bac8dfc843bc"
src/Bobcat.jl:
import Cobra
import Dingo
Cobra/
Project.toml:
uuid = "4725e24d-f727-424b-bca0-c4307a3456fa"
[deps]
Dingo = "7a7925be-828c-4418-bbeb-bac8dfc843bc"
src/Cobra.jl:
import Dingo
Dingo/
Project.toml:
uuid = "7a7925be-828c-4418-bbeb-bac8dfc843bc"
src/Dingo.jl:
# no imports
Here is a corresponding roots structure, represented as a dictionary:
roots = Dict(
:Aardvark => UUID("00000000-0000-0000-0000-000000000000"), # no project file, nil UUID
:Bobcat => UUID("85ad11c7-31f6-5d08-84db-0a4914d4cadf"), # dummy UUID based on path
:Cobra => UUID("4725e24d-f727-424b-bca0-c4307a3456fa"), # UUID from project file
:Dingo => UUID("7a7925be-828c-4418-bbeb-bac8dfc843bc"), # UUID from project file
)
Here is the corresponding graph structure, represented as a dictionary:
graph = Dict(
# Bobcat:
UUID("85ad11c7-31f6-5d08-84db-0a4914d4cadf") => Dict(
:Cobra => UUID("4725e24d-f727-424b-bca0-c4307a3456fa"),
:Dingo => UUID("7a7925be-828c-4418-bbeb-bac8dfc843bc"),
),
# Cobra:
UUID("4725e24d-f727-424b-bca0-c4307a3456fa") => Dict(
:Dingo => UUID("7a7925be-828c-4418-bbeb-bac8dfc843bc"),
),
# Dingo:
UUID("7a7925be-828c-4418-bbeb-bac8dfc843bc") => Dict(),
)
A few general rules to note:
- A package without a project file can depend on any top-level dependency, and since every package in a package directory is available at the top-level, it can import all packages in the environment.
- A package with a project file cannot depend on one without a project file since packages with project files can only load packages in
graph
and packages without project files do not appear ingraph
. - A package with a project file but no explicit UUID can only be depended on by packages without project files since dummy UUIDs assigned to these packages are strictly internal.
Observe the following specific instances of these rules in our example:
Aardvark
can import on any ofBobcat
,Cobra
orDingo
; it does importBobcat
andCobra
.Bobcat
can and does import bothCobra
andDingo
, which both have project files with UUIDs and are declared as dependencies inBobcat
's[deps]
section.Bobcat
cannot depend onAardvark
sinceAardvark
does not have a project file.Cobra
can and does importDingo
, which has a project file and UUID, and is declared as a dependency inCobra
's[deps]
section.Cobra
cannot depend onAardvark
orBobcat
since neither have real UUIDs.Dingo
cannot import anything because it has a project file without a[deps]
section.
The paths map in a package directory is simple: it maps subdirectory names to their corresponding entry-point paths. In other words, if the path to our example project directory is /home/me/animals
then the paths
map could be represented by this dictionary:
paths = Dict(
(UUID("00000000-0000-0000-0000-000000000000"), :Aardvark) =>
"/home/me/AnimalPackages/Aardvark/src/Aardvark.jl",
(UUID("85ad11c7-31f6-5d08-84db-0a4914d4cadf"), :Bobcat) =>
"/home/me/AnimalPackages/Bobcat/src/Bobcat.jl",
(UUID("4725e24d-f727-424b-bca0-c4307a3456fa"), :Cobra) =>
"/home/me/AnimalPackages/Cobra/src/Cobra.jl",
(UUID("7a7925be-828c-4418-bbeb-bac8dfc843bc"), :Dingo) =>
"/home/me/AnimalPackages/Dingo/src/Dingo.jl",
)
Since all packages in a package directory environment are, by definition, subdirectories with the expected entry-point files, their paths
map entries always have this form.
Environment stacks
The third and final kind of environment is one that combines other environments by overlaying several of them, making the packages in each available in a single composite environment. These composite environments are called environment stacks. The Julia LOAD_PATH
global defines an environment stack—the environment in which the Julia process operates. If you want your Julia process to have access only to the packages in one project or package directory, make it the only entry in LOAD_PATH
. It is often quite useful, however, to have access to some of your favorite tools—standard libraries, profilers, debuggers, personal utilities, etc.—even if they are not dependencies of the project you're working on. By adding an environment containing these tools to the load path, you immediately have access to them in top-level code without needing to add them to your project.
The mechanism for combining the roots, graph and paths data structures of the components of an environment stack is simple: they are merged as dictionaries, favoring earlier entries over later ones in the case of key collisions. In other words, if we have stack = [env₁, env₂, …]
then we have:
roots = reduce(merge, reverse([roots₁, roots₂, …]))
graph = reduce(merge, reverse([graph₁, graph₂, …]))
paths = reduce(merge, reverse([paths₁, paths₂, …]))
The subscripted rootsᵢ
, graphᵢ
and pathsᵢ
variables correspond to the subscripted environments, envᵢ
, contained in stack
. The reverse
is present because merge
favors the last argument rather than first when there are collisions between keys in its argument dictionaries. There are a couple of noteworthy features of this design:
- The primary environment—i.e. the first environment in a stack—is faithfully embedded in a stacked environment. The full dependency graph of the first environment in a stack is guaranteed to be included intact in the stacked environment including the same versions of all dependencies.
- Packages in non-primary environments can end up using incompatible versions of their dependencies even if their own environments are entirely compatible. This can happen when one of their dependencies is shadowed by a version in an earlier environment in the stack (either by graph or path, or both).
Since the primary environment is typically the environment of a project you're working on, while environments later in the stack contain additional tools, this is the right trade-off: it's better to break your development tools but keep the project working. When such incompatibilities occur, you'll typically want to upgrade your dev tools to versions that are compatible with the main project.
Conclusion
Federated package management and precise software reproducibility are difficult but worthy goals in a package system. In combination, these goals lead to a more complex package loading mechanism than most dynamic languages have, but it also yields scalability and reproducibility that is more commonly associated with static languages. Typically, Julia users should be able to use the built-in package manager to manage their projects without needing a precise understanding of these interactions. A call to Pkg.add("X")
will add to the appropriate project and manifest files, selected via Pkg.activate("Y")
, so that a future call to import X
will load X
without further thought.