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December 2012


This package is for exploring options for help in Julia.


The main idea

The main idea is that everything is decentralized. Nothing needs to be generated when you add a package. Help commands and browser-based help both go looking for help. From the REPL, you could ask for help in several ways:

@help mydataframe["col1"]   # figures out the function name and package based on the function call
@help DataFrames.ref        # lookup by module and keyword name (must be loaded)
@help DataFrames ref        # lookup by package and keyword name
help(DataFrames.ref)        # same but from a function definition 
help("DataFrames", "ref")   # string-based lookups

All of the above end up calling the same function: help("DataFrames", "ref"). The main mapping is by package and then keyword. Keywords would normally be defined for every exported function and type. They could also be used for constants, data sets, vignettes, and documentation sections.

Single-argument help calls might do the following:

@help DataFrames   # does what help() does now then looks for packages and/or modules named "DataFrames"
apropos("DataFrames", "within")   # looks up the keyword within in the DataFrames package 
apropos("within")      # looks up the keyword in base and then all packages 

Help for a package might look for a specific file (like "ModuleName.txt" or "README.md"). @help package could show the given help file, and it could also show help keywords available within the package.

Browser-based help

We could feed web pages to the browser from the Julia webserver. Using live feeds from our own local webserver could allow us to do the following:

  • Common header -- We could splice in a header with links to the Julia home page, github, and more.

  • Cross referencing between packages -- Cross references of the form /foo/bar can be interpretted as package foo and keyword bar.

  • Search bar -- This could be similar to the arguments for @help.

  • On-the fly conversion of Markdown or reStructuredText -- This could be done on the Julia side or in Javascript. (I'm not sure this is wise, but it's possible.)

Package metadata

A key question is when one does @help packagename keyword or clicks a link for /packagename/keyword in the browser, where are the files stored, and what format are they in?

Here's one option that's pretty easy. The doc/_JL_INDEX_ is an indexing file with one row per keyword with the following three pieces of information.

keyword filename one-line description of the module/function/doc/whatever

Here is the start of the _JL_INDEX_ file for RDatasets.jl:

RDatasets  README  A package with datasets originally distributed with R or R add-on packages
BOD  datasets/BOD  Biochemical Oxygen Demand
Formaldehyde  datasets/Formaldehyde  Determination of Formaldehyde
HairEyeColor  datasets/HairEyeColor  Hair and Eye Color of Statistics Students
InsectSprays  datasets/InsectSprays  Effectiveness of Insect Sprays

filename could be looked up by the REPL or the webserver, and multiple formats could be supported. A simple package with just a few functions could have a _JL_INDEX_ file where all keywords point to one README.md file. The one-liner is especially useful for searching with apropos.

filename could be treated several ways. The extension could be left off, optional, or required. There could also be a bookmark or line number indicator to reference one part of a file.


There are many help files stashed in doc/. These were generated from help for the Modelica Standard Library. I chose this example because this library is huge, and because I'm working on a lite version of this for Julia: Sims.jl.

Files were converted from html by pandoc. I also used pandoc to create reStructuredText (in doc/), plain text (in doc/txt), html (in doc/html), and Markdown (in doc/md). The idea is that we can use this as a testbed on a ridiculously large package with help in different formats. Most of the files in this library document multiple objects. Keywords are repeated.

Packages that support this format


julia> load("HelpTestbed"); using Help

julia> @help sin
Loading help data...

   Compute sine of "x"

julia> @help sin(1)
sin(Real,) at math.jl:86


   Compute sine of "x"

julia> m = Modelica.MyType(1)

julia> @help sin(m)
sin(MyType,) at /home/tshort/.julia/HelpTestbed/src/HelpTestbed.jl:166

Package `HelpTestbed`,  sin

Miscellaneous trig functions

# Basic trig operations


That's it...

Package `HelpTestbed`,  sin

Sine of complex number



Library of complex mathematical functions (e.g., sin, cos) and of
functions operating on copmlex vectors and matrices

-- [long output truncated by hand] --

julia> @help m[1]
ref(MyType,Int64) at /home/tshort/.julia/HelpTestbed/src/HelpTestbed.jl:171

Package `HelpTestbed`,  ref

Referencing for MyType's

# Test help file for referencing MyType's

ref(x::MyType, a) 
ref(x::MyType, a::Int)
ref(x::MyType, a, b) 

That's it...

julia> apropos("regression")
Base.linreg(x, y)
Base.linreg(x, y, w)

 -- Package documentation available -- 

DataFrames		model_matrix		A ModelMatrix type used for regression modeling
RDatasets		anscombe		Anscombe's Quartet of 'Identical' Simple Linear Regressions
RDatasets		longley		Longley's Economic Regression Data
RDatasets		exAM		Example Data of Antille and May - for Simple Regression
RDatasets		Quartet		Four Regression Datasets
RDatasets		bivariate		Sample data for bivariate probit regression
RDatasets		grunfeld		Simulation Data for model Seemingly Unrelated Regression (sur) that corresponds to method SUR of systemfit

julia> apropos("HelpTestbed", "leakage")
HelpTestbed		LeakageWithCoefficient		Leakage reluctance with respect to the reluctance of a useful flux path (not for dynamic simulation of actuators)
HelpTestbed		PartialLeakage		Base class for leakage flux tubes with position-independent permeance and hence no force generation; mu\_r=1
HelpTestbed		LeakageAroundPoles		Leakage flux tube around cylindrical or prismatic poles
HelpTestbed		QuarterCylinder		Leakage flux from one edge to the opposite plane through a quarter cylinder
HelpTestbed		QuarterHollowCylinder		Leakage flux in circumferential direction through a quarter hollow cylinder
HelpTestbed		HalfCylinder		Leakage flux through the edges of a half cylinder
HelpTestbed		HalfHollowCylinder		Leakage flux in circumferential direction through a half hollow cylinder
HelpTestbed		QuarterSphere		Leakage flux through the corners of a quarter sphere
HelpTestbed		QuarterHollowSphere		Leakage flux through the edges of a qarter hollow sphere
HelpTestbed		EighthOfSphere		Leakage flux through one edge and the opposite plane of an eighth of a sphere
HelpTestbed		EighthOfHollowSphere		Leakage flux through one edge and the opposite plane of an eighth of a hollow sphere
HelpTestbed		CoaxCylindersEndFaces		Leakage flux between the end planes of a inner solid cylinder and a coaxial outer hollow cylinder
HelpTestbed		Leakage		Leakage flux tubes with position-independent permeance and hence no force generation; mu\_r=1

julia> help("HelpTestbed", "Analog")
Package `HelpTestbed`,  Analog

Library for analog electrical models

Modelica.Electrical.Analog: Modelica.Electrical.Analog

Library for analog electrical models



This package contains packages for analog electrical components:

-   Basic: basic components (resistor, capacitor, conductor, inductor,
    transformer, gyrator)
-   Semiconductors: semiconductor devices (diode, bipolar and MOS
-   Lines: transmission lines (lossy and lossless)
-   Ideal: ideal elements (switches, diode, transformer, idle, short,
-   Sources: time-dependend and controlled voltage and current sources
-   Sensors: sensors to measure potential, voltage, and current

-- [long output truncated by hand] --

Handling multiple dispatch

With the @help macro, we can find the signature of a function call. We can use that to find the package. That narrows down the mapping from a function signature to a documentation keyword.

That doesn't cover the case where functions refer to different things within one package. For example, Tim Holy's HDF5 has read for plain HDF5 files, Matlab HDF5 files, and JLD (Julia data) files. If these are documented all in one place, the existing approach works fine.

We could go a step further and allow packagers to enter the function signature as part of the _JL_INDEX_ file. This would allow separate documentation of the different versions of read within a package. Entering many function signatures might be too tedious, especially since Julia code can tend to have lots of versions of a function defined.

If it's better to document these in separate places, the packager could offer different keywords: "read_jld", "read_plain", "read_matio", and "read" which is a short description pointing to the other keywords.

We could also allow a keyword to point to the file. Every function signature has a pointer back to the file it was written in. We could allow function keywords of the form "read#matio" and "read#jld" where matio points to "src/matio.jl". The following would try to open help for "read#matio"

file = jldopen("mydata.jld", "r")
@help read(file, "A")


Aliases might help for cases where multiple keywords point to the same thing. We could have a _JL_ALIASES_ file in addition to _JL_INDEX_. This could map from keyword to main keyword. Here is an example:

ref_dataframe   dataframe  Indexing DataFrames
ref_datavec     datavec    Indexing DataVecs

ref           ref_dataframe
ref#dataframe ref_dataframe
ref#datavec   ref_datavec  

What we can learn from R's help

R generally has comprehensive documentation. Here are it's main strengths and how that might apply to Julia:

  • Functions well documented -- Almost every function points to a documentation page. Each of these has a consistent structure. Two things may help Julia achieve this: (1) making it as easy as possible to document things, and (2) have some sort of Pkg.check(packagename) that runs to make sure every exported function has a documentation entry.

  • Examples -- Most function documentation has an "Examples" section. You can also run examples with example(mean). For Julia, we can encourage having an "Examples" documentation section. To get the equivalent of example(mean), we may need to specify exactly how to bookmark example sections.

  • Multiple backends -- Users can access help in multiple ways: in the REPL, HTML help, PDF files of help, and Emacs/ESS help. This should be straightforward to implement in Julia.

  • Nice HTML interface -- R includes a built-in webserver that organizes and feeds results to the browser. It allows on-the-fly lookup of help keywords. It provides a really nice interface. We could use Julia's webserver to do something similar.

Here are some weaknesses with R's help system and how we might improve upon them in Julia:

  • Packages have awful default documentation -- The default package documentation is just a straight listing of the help for individual functions and data in the package with no organization. Julia and most Julia packages already do a better job of providing the "big picture".

  • Little connection between user's manuals, vignettes, and function documentation -- In Julia, it would be nice to have ways to cross-reference between these.

  • Poor documentation for S4 classes -- S4 is R's advanced type system that supports multiple dispatch. I can never figure out help for packages that use S4. For Julia to handle documentation for multiple dispatch, tying function calls to documentation keywords is key. The @help macro can help with that.

  • No graphics support -- Help files and manuals don't include graphics. R's documentation format is rather "old-fashioned". HTML, Markdown, or reStructuredText shouldn't have these problems. One question is whether we should try to support vector graphic formats.

R's system is a decentralized system. Each package has a "help/AnIndex" or "help/aliases.rds" (binary) file that gives a keyword lookups. There's also a main "Index" file with keyword entries and one-line descriptions. The help system does lookups of these on the fly. It is relatively speedy even with many packages. R's indexing files are generated when a package is installed. I'm proposing to leave that up to the package manager. Tools like Sphinx could be set up to generate that file automatically. We could provide a function that can check the _JL_INDEX_ file to make sure that all keywords point to documentation in one of the required formats.


This is mainly just a playground so far.

Browser-based help hasn't been started.

Code is MIT licensed. The help files are were released by the Modelica organization under the Modelica license v2.