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October 2015

FileIO2.jl

Description

A very Julian way of describing files leveraging the built-in type system. FileIO2.jl simply provides tools/patterns to register data types with Julia type system.

FileIO2.jl therefore competes with the solution found at: http://github.com/JuliaIO/FileIO.jl.

Advantages (wrt FileIO.jl)

  • FileIO2.jl is a more lightweight module than FileIO.jl (Should probably have been named better).
  • New file/data format types do not have to be registered with the FileIO2.jl module.
  • Only types of imported (used) modules are made available (including those provided with FileIO2.jl).

Disadvantages (wrt FileIO.jl)

  • No facilities to perform dynamic loading of IO module.
  • No ability to auto-detect filetype by extension.
  • No ability to auto-detect filetype by magic bytes.

Points of Consideration

It might be a good idea to consolidate/re-organize the FileIO.jl and FileIO2.jl modules.

  • A base module could provide the base File object hierarchies and manipulation tools.
  • This module should be relatively static (unchanging).
  • A registration module could provide a repository of IO modules to that read/write different files.
  • This module would demonstrate how to register all the PNG readers, the MP3 readers, etc.
  • Users could then create a stripped-down version of the module to meet the needs of their application - thus avoiding complex dependencies.

MIME Support

FileIO2.jl includes simple MIME support for images. As a result, *IJulia notebook users can display IJulia-supported images with very little code:

display(File(:jpeg, "myfile.jpeg"))

Supported image formats include:

  • Pixel formats: :bmp, :png, :gif, :jpeg, :tiff
  • Vector formats: :svg, :cgm, :eps, :emf

More Sample Code

Sample code is provided under the test directory.

Philosophy

open, read, write, load, and save

  • open: Opens a file, and read sufficient header data to return an object of T<:AbstractReader / T<:AbstractWriter. These objects are basically higher-level versions of IO streams.
  • read: Reads data with whatever granularity the module developper desires (ex: individual Int/String/..., a large data block, or even an entire file).
  • read(::Type{TIO<:AbstractReader}, ::File{T<:FileIO2.DataFormat}, ...): Equivalent to load for user-defined reader-only state machines.
  • read(::Type{TIO<:AbstractDataIORW}, ::File{T<:FileIO2.DataFormat}, ...): Equivalent to load for user-defined read/write-capable state machines.
  • read(r::TIO<:{AbstractReader/AbstractDataIORW}, ::Type{T<:DataType}): Reads a value of type T, using r. Example: val = read(myreader, Float64).
  • write: Writes data with whatever granularity the module developper desires (ex: individual Int/String/..., a large data block, or even an entire file).
  • write(::TIO<:AbstractWriter, ::File{T<:FileIO2.DataFormat}, ...): Equivalent to save for user-defined writer-only state machines.
  • write(::TIO<:AbstractDataIORW, ::File{T<:FileIO2.DataFormat}, ...): Equivalent to save for user-defined read/write-capable state machines.
  • load: The simplest interface to reading in data. Performs open/read*/close in one convenient function.
  • DEPRECATED Please overload the read() function as described above.
  • save: The simplest interface to writing data. Performs open/write*/close in one convenient function.
  • DEPRECATED Please overload the write() function as described above.

Principal Types

  • FileIO2.DataFormat: (abstract) Identifies a data format, as opposed to a file.
  • FileIO2.DataEncoding: (abstract) Identifies the type of data encoding used. Can be used to specialize a type T<:DataFormat.
  • Can use UnknownTextEncoding to describe "generic" versions of a type. For example: const TextFmt = TextFormat{UnknownTextEncoding} defines TextFmt to represent any text data format (irrespective of the data encoding used).
  • TODO: Does anybody really need to specialize a DataFormat with a particular encoding?
  • AbstractReader{DataFormat}: (abstract) Used to define an object for reading from a stream formatted with DataFormat.
  • AbstractWriter{DataFormat}: (abstract) Used to define an object for writing to a stream formatted with DataFormat.
  • AbstractDataIORW{DataFormat}: (abstract) Used to define an object for reading/writing to a stream formatted with DataFormat.
  • File{DataFormat}: An object used to dispatch to the appropriate open/load/save/... functions.

Constructing File Objects (Shorthand)

The simplest way to construct a File object is to call the "shorthand" method: File(::Symbol, ::String). For example:

	f1 = File(:text, "myfile.txt")
	f2 = File(:png, "myimage.png")
	f3 = File(:html, "myfile.html")
	
	#When html files need to be recognized as simple text files:
	f3 = File(:text, "myfile.html")

This method allows different File specializations to be constructed without exporting different DataFormat type identifiers (namespace pollution).

User-Defined File Constructors (Shorthand)

To register a user-defined File constructor with this shorthand notation, simply add the following method declaration:

	FileIO2.File(::FileIO2.Shorthand{[NEWFMTSYMBOL]}, path::String) = File{[NEWFMT<:DataFormat]}(path)

For example, the "text" format can be registered using:

	FileIO2.File(::FileIO2.Shorthand{:text}, path::String) = File{FileIO2.TextFmt}(path)

Opening File for read/write

	file = File(:png, "myfile.png")
	reader = open(file, read=true) #Default is read=true

NOTE: A module developper must have first defined a .png reader object:

	type MyPNGReader <: AbstractReader{PNGFmt}; end
	Base.open(r::Type{MyPNGReader}, f::File{PNGFmt}, ...) = ...

A user can therefore call open with this specific reader:

	open(MyPNGReader, file, read=true)

Known Limitations

FileIO2.jl tries to minimize the set of exported types/aliases until a better way is found for the user to import them. At the moment, exporting too much pollutes the symbol namespace - making collisions between modules highly likely.

Compatibility

Extensive compatibility testing of FileIO2.jl has not been performed. The module has been tested using the following environment(s):

  • Linux / Julia-1.1.1 (64-bit).

Disclaimer

The FileIO2.jl module is not yet mature. Expect significant changes.

This software is provided "as is", with no guarantee of correctness. Use at own risk.