Stanford CS240h Lab 3

Please use this skeleton code as your starting point: lab3.tar.gz.

You only need to edit the src/MergeIni.hs file!


In this lab, you will implement a git merge driver for INI files. The usage will be:

    inimerge [-w] headFile baseFile mergeFile

headFile corresponds to the latest version of the file. mergeFile consists of a file that you want to merge in. baseFile is the common ancestor of headFile and mergeFile. By default your merge driver will print the merged file to standard output (since this is easiest for debugging). However, the -w flag will cause it to overwrite headFile, which is the behavior expected by git. The intended use will be to put the following in your .git/config file, then use gitattributes to assign this merge driver to INI files.

    [merge "inimerge"]
        name = INI file merge driver
        driver = inimerge -w %A %O %B
        recursive = text


The overall strategy will be as follows. For any (section, key, values) tuple that is the same in both the headFile and mergeFile, use that version since the two files agree. Otherwise, if headFile agrees with baseFile, then assume that mergeFile has more recently updated the file and use the values stored in mergeFile. Conversely, if mergeFile and baseFile are identical, then use the values from headFile. Otherwise, you have a conflict.

If there is an update conflict, you will preserve all values. Assume that when a key has multiple values, the more recent values are added earlier in the file. Thus, you will date the list of values in headFile and the list of values in mergeFile and find the longest common suffix. The merge file will contain first the unique prefix of mergeFile, then the unique prefix of headFile, then the common suffix of the two lists of values.

The assignment

The assignment will consist of implementing a function in the file MergeIni.hs. The other files should be usable as-is, though you may learn from reading them.

So as not to penalize people who did not do well in lab 2, we have supplied you with an INI file parser and pretty-printer. This parser has a couple of differences from the lab 2 assignment. Most importantly, the INIVal type is just a strict bytestring, as we don't care about parsing booleans and integers differently. Second, the parser we provided guarantees that when there is a duplicate key in a section, the list of values preserves the order of the file (so the head of the list is the value corresponding to the first occurrence of the key in the file).


Please ask (early!) any qualifying questions about the specification on Piazza.

Due Date

Lab 3 should be submitted by the start of class on Wednesday, February 10th.

You have 48 hours of late days for the three labs. They are consumed in 24 hour blocks and are used automatically. After they are used, you'll have the maximum grade you can receive for a late lab reduced by 25% each day.

Stack -- Build & Test Tool

We are using the stack build tool for this course. Once getting the skeleton code, you should be able to run:

    stack setup
    stack build
    stack test
    stack exec inimerge

We have provided an overview of Stack here.

Provided Files

You only need to edit the src/MergeIni.hs file!

The files provided to get started are:


Testing Lab 3

Some skeleton code for a test framework is provided in test/Spec.hs. You'll need to edit it to add your own tests. The test framework uses a Haskell package called test-framework. Please refer to it for documentation on how to use it.


While we strongly encourage you to take testing seriously and write a comprehensive test suite, we are only going to grade you on your inimerge matching implementation.

Grading will be done only on functionality. We will provide an example solution later to learn from. Come to office hours for more feedback.


First, simply type:

    stack sdist

This will generate a tar file of your code. Please don't add any extra source files without changing the inimerge.cabal file correspondingly. Otherwise, your submission will be broken and missing files.

Then go to and submit your work through the online form. You can resubmit as many times as you want up until the deadline.

If you have any trouble submitting on-line, then please post on Piazza, or email the staff mailing .

Suggested Music

We suggest listening to Newmangenerator. if needing music while programming.