Zip

• Why this library is written
  • zip-archive
  • LibZip
  • zip-conduit
• Features
  • Compression methods
  • Encryption
  • Sources of file data
  • ZIP64
  • Filenames
  • Meta-information about files
• Quick start
• Contribution
• License

This is a feature-rich, memory-efficient, and type-safe library to manipulate Zip archives in Haskell. The library is the most complete and efficient implementation of .ZIP specification in pure Haskell (at least from open-sourced ones). In particular, it’s created with large multimedia data in mind and provides all features users might expect, comparable in terms of feature-set with libraries like libzip in C.

Why this library is written

There are a few libraries to work with Zip archives, yet every one of them provides only a subset of all the functionality a user may need (obviously the libraries provide functionality that their authors needed) and otherwise is flawed in some way so it cannot be easily used in some situations. Let’s examine all the libraries available on Hackage to understand motivation for this package.

zip-archive

zip-archive is a widely used library. It’s quite old, well-known and simple to use. However it creates Zip archives purely, as ByteStringss in memory that you can then write to the file system. This is not acceptable if you work with more-or-less big data. For example, if you have collection of files with total size of 500 MB and you want to pack them into an archive, you can easily consume up to 1 GB of memory (the files plus resulting archive). Not always you can afford to do this or do this at scale. Even if you want just to look at list of archive entries it will read it into memory in all its entirety. For my use-case it’s not acceptable.

LibZip

This is a binding to C library libzip. There is always a certain kind of trouble when you are using bindings. For example, you need to take care that target library is installed and its version is compatible with the version of your binding. Yes, this means additional headaches. It should be just “plug and play”, but now you need to watch out for compatibility.

It’s not that bad with libraries that do not break their API for years, but it’s not the case with libzip. As the maintainer of LibZip puts it:

libzip 0.10, 0.11, and 1.0 are not binary compatible. If your C library is 0.11.x, then you should use LibZip 0.11. If your C library is 1.0, then you should use LibZip master branch (not yet released to Hackage).

Now, on my machine I have version 1.0. To put the package on Stackage we had to use version 0.10, because Stackage uses Ubuntu to build packages and libraries on Ubuntu are always ancient. This means that I cannot use the version of the library from Stackage, and I don’t yet know what will be on the server.

After much frustration with all these things I decided to avoid using of LibZip, because after all, this is not that sort of a project that shouldn’t be done in pure Haskell. By rewriting this in Haskell, I also can make it safer to use.

zip-conduit

This one uses the right approach: leverage a good streaming library (conduit) for memory-efficient processing. This is however is not feature-rich and has certain problems (including programming style, it uses error if an entry is missing in archive, among other things), some of them are reported on its issue tracker. It also does not appear to be maintained (the last sign of activity was on December 23, 2014).

Features

The library supports all features specified in the modern .ZIP specification except for encryption and multi-disk archives. See more about this below.

For reference, here is a copy of the specification.

Compression methods

zip supports the following compression methods:

• Store (no compression, just store files “as is”)
• DEFLATE
• Bzip2

The best way to add a new compression method to the library is to write a conduit that will do the compression and publish it as a library. zip can then depend on it and add it to the list of supported compression methods. The current list of compression methods reflects what is available on Hackage at the moment.

Encryption

Encryption is currently not supported. Encryption system described in the .ZIP specification is known to be seriously flawed, so it’s probably not the best way to protect your data anyway. The encryption method seems to be a proprietary technology of PKWARE (at least that’s what stated about it in the .ZIP specification), so to hell with it.

Sources of file data

The library gives you many options how to get file contents to compress and how to get extracted data. The following methods are supported:

• File name. This is an efficient method to perform compression or decompression. You just specify where to get data or where to save it and the rest is handled by the library.
• Conduit source or sink.
• ByteString. Use it only with small data.
• Copy file from another archive. An efficient operation, file is copied “as is”—no re-compression is performed.

ZIP64

When necessary, the ZIP64 extension is automatically used. It’s necessary when anything from this list takes place:

• Total size of archive is greater than 4 GB.
• Size of compressed/uncompressed file in archive is greater than 4 GB.
• There are more than 65535 entries in archive.

The library is particularly suited for processing of large files. For example, I’ve been able to easily create 6.5 GB archive with reasonable speed and without significant memory consumption.

Filenames

The library has API that makes it impossible to create archive with non-portable or invalid file names in it.

As of .ZIP specification 6.3.2, files with Unicode symbols in their names can be put into Zip archives. The library supports mechanisms for this and uses them automatically when needed. Besides UTF-8, CP437 is also supported as it’s required in the specification.

Meta-information about files

The library allows to attach comments to entire archive or individual files, and also gives its user full control over extra fields that are written to file headers, so the user can store arbitrary information about file in the archive.

Quick start

The module Codec.Archive.Zip provides everything you may need to manipulate Zip archives. There are three things that should be clarified right away, to avoid confusion in the future.

First, we use the EntrySelector type that can be obtained from relative FilePaths (paths to directories are not allowed). This method may seem awkward at first, but it will protect you from the problems with portability when your archive is unpacked on a different platform.

The second thing, that is rather a consequence of the first, is that there is no way to add directories, or to be precise, empty directories to your archive. This approach is used in Git, and I find it quite sane.

Finally, the third feature of the library is that it does not modify archive instantly, because doing so on every manipulation would often be inefficient. Instead we maintain a collection of pending actions that can be turned into an optimized procedure that efficiently modifies archive in one pass. Normally this should be of no concern to you, because all actions are performed automatically when you leave the realm of ZipArchive monad. If, however, you ever need to force an update, the commit function is your friend. There are even “undo” functions, by the way.

Let’s take a look at some examples that show how to accomplish most typical tasks with help of the library.

To get full information about archive entries, use getEntries:

λ> withArchive archivePath (M.keys <$> getEntries)

This will return a list of all entries in the archive at archivePath. It’s possible to extract contents of an entry as a strict ByteString:

λ> withArchive archivePath (getEntry entrySelector)

…to stream them to a given sink:

λ> withArchive archivePath (sourceEntry entrySelector mySink)

…to save a specific entry to a file:

λ> withArchive archivePath (saveEntry entrySelector pathToFile)

…and finally just unpack the entire archive into some directory:

λ> withArchive archivePath (unpackInto destDir)

See also getArchiveComment and getArchiveDescription.

Modifying is also easy, efficient, and powerful. When you want to create a new archive use createArchive, otherwise withArchive will do. To add an entry from ByteString:

λ> createArchive archivePath (addEntry Store "Hello, World!" entrySelector)

You can stream from Source as well:

λ> createArchive archivePath (sinkEntry Deflate source entrySelector)

To add contents from a file, use loadEntry:

λ> let toSelector = const (mkEntrySelector "my-entry.txt")
λ> createArchive archivePath (loadEntry BZip2 toSelector myFilePath)

Finally, you can copy an entry from another archive without re-compression (unless you use recompress, see below):

λ> createArchive archivePath (copyEntry srcArchivePath selector selector)

It’s often desirable to just pack a directory:

λ> createArchive archivePath (packDirRecur Deflate mkEntrySelector dir)

It’s also possible to:

• rename an entry with renameEntry
• delete an entry with deleteEntry
• change compression method with recompress
• change comment associated with an entry with setEntryComment
• delete comment with deleteEntryComment
• set modification time with setModTime
• manipulate extra fields with addExtraField and deleteExtraField
• check if entry is intact with checkEntry
• undo changes with undoEntryCanges, undoArchiveChanges, and undoAll
• force changes to be written to file system with commit

This should cover all your needs. Feel free to open an issue if you’re missing something.

Contribution

You can contact the maintainer via the issue tracker.

Pull requests are also welcome and will be reviewed quickly.

License

Copyright © 2016–2018 Mark Karpov

Distributed under BSD 3 clause license.