streamly/CONTRIBUTING.md

Contributors' Guide

Bug Reports

Please feel free to open an issue for any questions, suggestions, issues or bugs in the library. In case you are reporting a bug we will appreciate if you provide as much detail as possible to reproduce or understand the problem, but nevertheless you are encouraged to open an issue for any problem that you may encounter.

Pull Requests (PR)

Please feel free to send a pull request (PR) whether it is a single letter typo fix or a complex change. We will accept any PR that makes a net positive change to the package. We encourage you to provide a complete, consistent change with test, documentation and benchmarks. You can contact the maintainers for any help or collaboration needed in that regard. However, if due to lack of time you are not able to complete the PR, you are still welcome to submit it, maintainers will try their best to actively contribute and pick up your change as long as the change is approved.

Pull Request (PR) Checklist

Here is a quick checklist for a PR, for details please see the next section:

• PR contains one logical changeset
• Each commit in the PR consists of a logical change
• Commits are rebased/squashed/fixup/reordered as needed
• Stylistic changes to irrelevant parts of the code are separated in independent commits.
• Code is formatted as per the style of the file or that of other files
• Compiler warnings are fixed
• Reasonable hlint suggestions are accepted
• Tests are added to cover the changed parts
• All tests pass
• Performance benchmarks are added, where applicable
• No significant regressions are reported by performance benchmarks
• Haddock documentation is added to user visible APIs and data types
• Tutorial module, README, and guides are updated if necessary.
• Changelog is updated if needed

Contributing A Change

If the feature makes significant changes to design, we encourage you to open an issue as early as possible so that you do not have to redo much work because of changes in design decisions. However, if you are confident, you can still go ahead and take that risk as the maintainers are supposed to be reasonable people.

Picking Issues to Work on

Beginners are encouraged to pick up issues that are marked help wanted. It is a good idea to update the issue expressing your intent so that others do not duplicate the effort and people with a background on the issue can help.

Changeset in a PR

• Please make sure that a single PR contains a single logical changeset. That helps the reviewers in quickly understanding and reviewing the change.
• You are encouraged to group a logically related set of changes into a single commit. When the overall changeset is largish you can divide it into multiple smaller commits, with each commit having a logically grouped changeset and the title summarizing the logical grouping. Always keep in mind a logical division helps reviewers understand and review your code quickly, easier history tracking and when required clean reversal changes.
• If your commits reflect how you fixed intermediate problems during testing or made random changes at different times you may have to squash your changes (git rebase -i) into a single commit or logically related set of commits.
• Please resist the temptation to make style related changes to surrounding code unless you are changing that code anyway . Make sure that your IDE/Editor is not automatically making sweeping style changes to all the files you are editing. That makes separating the signal from the noise very difficult and makes everything harder. If you would like to make style related changes then please send a separate PR with just those changes and no other functionality changes.

Resolving Conflicts

If during the course of development or before you send the PR you find that your changes are conflicting with the master branch then use git rebase master to rebase your changes on top of master. DO NOT MERGE MASTER INTO YOUR BRANCH.

Testing

It is a good idea to include tests for the changes where applicable. See the existing tests here.

Documentation

For user visible APIs, it is a good idea to provide haddock documentation that is easily understood by the end programmer and does not sound highfalutin, and preferably with examples. If your change affects the tutorial or needs to be mentioned in the tutorial then please update the tutorial. Check if the additional guides are affected or need to updated.

Performance Benchmarks

It is a good idea to run performance benchmarks to see if your change affects any of the existing performance benchmarks. If you introduced something new then you may want to add benchmarks to check if it performs as well as expected by the programmers to deem it usable.

See the README file in the benchmark directory for more details on how to run the benchmarks.

Changelog

Any new changes that affect the user of the library in some way must be documented under Unreleased section at the top of the Changelog. The changes in the changelog must be organized in the following categories, in that order:

• Breaking Changes
• Enhancements
• Bug Fixes
• Deprecations

If there are very few changes then you can just prefix a bullet with these annotations. If there are many changes make sections to group them. A section can be absent if there is nothing to add in it.

If you make changes that are incompatible with the released versions of the library please indicate that in the Changelog as Breaking Changes and also write short notes regarding what the programmers need to do to adapt their existing code to the new change.

Developer documentation

To build haddock documentation for all modules, including the ones not exposed to users of the library, use the following commands:

$ cabal v2-haddock --haddock-option="--show-all"
$ stack haddock --haddock-arguments "--show-all" --no-haddock-deps

For design documentation see the design directory.

Coding

Style

As long as possible please try to match the style of the file or the surrounding code. For haskell coding style guidelines, please see this style guide. Specifically,

• Please use 4 spaces for indentation.
• Do not let the code go beyond 80 columns

Indent the where clause as follows:

f x = ...
    where

    f1 = ...

In general, we avoid using a prime on the variable names, e.g. we use step1 instead of step'. Numbered indexing is better because it is easier on the eyes and we can represent multiple generations of the variables without adding more characters e.g. we can write step2 instead of step''.

Use shorter variable names for shorter scopes, use longer variable names for bigger scopes.

Imports:

• Import symbols explicitly by names as long as possible
• Import qualified is there are too many symbols to be imported

StreamD coding style

Some conventions that we follow in the StreamD code are illustrated by the following example:

mapM f (Stream step1 state1) = Stream step state
    where

    step gst st = do
        r <- step1 (adaptState gst) st
        case r of
            Yield x s -> f x >>= \a -> return $ Yield a s
            Skip s    -> return $ Skip s
            Stop      -> return Stop

• For the input streams use numbering for step and state e.g. step1/state1. For the ouput stream use step and state.
• For state argument of step, use st.
• For result of executing a step use r or res
• For the yielded element use x
• For the yielded state use s

In general, the rule is - the shorter the scope of a variable the shorter its name can be. For example, s has the shortest scope in the above code, st has a bigger scope and state has the biggest scope.

For better fusion we try to keep a single yield point in the state machine.

Organization

Use this simple rule to organize functions in a file: Define before first use. In other words, use the bottom up style to organize functions.

One big benefit of this rule is easier type error debugging. Using a single block comment, at any point in the file up to the end of the file, you can cut the tail and still cleanly compile the remaining head portion of the file.

It is very helpful when we make significant changes to the file. We can start compiling a minimal head portion of the file and keep expanding the head by just moving the start of commented block further down. This keeps the scope of type inference minimal and type errors can be discovered incrementally by increasing the scope a little bit at a time. If you use type signatures on all top level declarations then you do not even need to comment out the code. This organization results in a better order of type errors which makes it easier to fix them and when necessary use elimination method by commenting out some code.

Similarly when you put modules in the cabal file in dependency order you can do the same there, just comment out a tail of the module list and your library will still compile.

Note that this rule does not apply in choosing let vs where clauses. Use of lets leads to bottom up and use of wheres leads to top down style. But we do not encourage let over where, you can freely use where clauses.

Also note that this is not possible in mutually recursive code. The mutually recursive code has to be considered a single block.

Tricky Parts

The state-passing through each API is currently fragile. Every time we run a stream we need to be careful about the state we are passing to it. In case of folds where there is no incoming state, we start with the initial state defState. When we have an incoming state passed to us there are two cases:

1. When we are building a concurrent stream that needs to share the same SVar we pass the incoming state as is.
2. In all other cases we must not share the SVar and every time we pass on the state to run a stream we must use adaptState to reset the SVar in the state.

When in doubt just use adaptState on the state before passing it on, we will at most lose concurrency but the behavior will be correct.

There is no type level enforcement about this as of now, and therefore we need to be careful when coding. There are specific tests to detect and report any problems due to this, all transform operations must be added to those tests.