Merge pull request #4302 from garbas/cli-guideline

Adds Nix CLI Guideline to docs

4 files changed, 593 insertions, 1 deletions

diff --git a/doc/manual/src/SUMMARY.md.in b/doc/manual/src/SUMMARY.md.in
index b5ae34cfa..448fee803 100644
--- a/doc/manual/src/SUMMARY.md.in
+++ b/doc/manual/src/SUMMARY.md.in
@@ -66,7 +66,9 @@
   - [Files](command-ref/files.md)
     - [nix.conf](command-ref/conf-file.md)
 - [Glossary](glossary.md)
-- [Hacking](hacking.md)
+- [Contributing](contributing/contributing.md)
+  - [Hacking](contributing/hacking.md)
+  - [CLI guideline](contributing/cli-guideline.md)
 - [Release Notes](release-notes/release-notes.md)
   - [Release 2.3 (2019-09-04)](release-notes/rl-2.3.md)
   - [Release 2.2 (2019-01-11)](release-notes/rl-2.2.md)
diff --git a/doc/manual/src/contributing/cli-guideline.md b/doc/manual/src/contributing/cli-guideline.md
new file mode 100644
index 000000000..0132867c8
--- /dev/null
+++ b/doc/manual/src/contributing/cli-guideline.md
@@ -0,0 +1,589 @@
+# CLI guideline
+
+## Goals
+
+Purpose of this document is to provide a clear direction to **help design
+delightful command line** experience. This document contain guidelines to
+follow to ensure a consistent and approachable user experience.
+
+## Overview
+
+`nix` command provides a single entry to a number of sub-commands that help
+**developers and system administrators** in the life-cycle of a software
+project. We particularly need to pay special attention to help and assist new
+users of Nix.
+
+# Naming the `COMMANDS`
+
+Words matter. Naming is an important part of the usability. Users will be
+interacting with Nix on a regular basis so we should **name things for ease of
+understanding**.
+
+We recommend following the [Principle of Least
+Astonishment](https://en.wikipedia.org/wiki/Principle_of_least_astonishment).
+This means that you should **never use acronyms or abbreviations** unless they
+are commonly used in other tools (e.g. `nix init`). And if the command name is
+too long (> 10-12 characters) then shortening it makes sense (e.g.
+“prioritization” → “priority”).
+
+Commands should **follow a noun-verb dialogue**. Although noun-verb formatting
+seems backwards from a speaking perspective (i.e. `nix store copy` vs. `nix
+copy store`) it allows us to organize commands the same way users think about
+completing an action (the group first, then the command).
+
+## Naming rules
+
+Rules are there to guide you by limiting your options. But not everything can
+fit the rules all the time. In those cases document the exceptions in [Appendix
+1: Commands naming exceptions](#appendix-1-commands-naming-exceptions) and
+provide reason. The rules want to force a Nix developer to look, not just at
+the command at hand, but also the command in a full context alongside other
+`nix` commands.
+
+```shell
+$ nix [<GROUP>] <COMMAND> [<ARGUMENTS>] [<OPTIONS>]
+```
+
+- `GROUP`, `COMMAND`, `ARGUMENTS` and `OPTIONS` should be lowercase and in a
+  singular form.
+- `GROUP` should be a  **NOUN**.
+- `COMMAND` should be a **VERB**.
+- `ARGUMENTS` and `OPTIONS` are discussed in [*Input* section](#input).
+
+## Classification
+
+Some commands are more important, some less. While we want all of our commands
+to be perfect we can only spend limited amount of time testing and improving
+them.
+
+This classification tries to separate commands in 3 categories in terms of
+their importance in regards to the new users. Users who are likely to be
+impacted the most by bad user experience.
+
+- **Main commands**
+
+  Commands used for our main use cases and most likely used by new users. We
+  expect attention to details, such as:
+
+    - Proper use of [colors](#colors), [emojis](#special-unicode-characters)
+      and [aligning of text](#text-alignment).
+    - [Autocomplete](#shell-completion) of options.
+    - Show [next possible steps](#next-steps).
+    - Showing some [“tips”](#educate-the-user) when running logs running tasks
+      (eg. building / downloading) in order to teach users interesting bits of
+      Nix ecosystem.
+    - [Help pages](#help-is-essential) to be as good as we can write them
+      pointing to external documentation and tutorials for more.
+
+  Examples of such commands: `nix init`, `nix develop`, `nix build`, `nix run`,
+  ...
+
+- **Infrequently used commands**
+
+  From infrequently used commands we expect less attention to details, but
+  still some:
+
+    - Proper use of [colors](#colors), [emojis](#special-unicode-characters)
+      and [aligning of text](#text-alignment).
+    - [Autocomplete](#shell-completion) of options.
+
+  Examples of such commands: `nix doctor`, `nix edit`, `nix eval`, ...
+
+- **Utility and scripting commands**
+
+  Commands that expose certain internal functionality of `nix`, mostly used by
+  other scripts.
+
+    - [Autocomplete](#shell-completion) of options.
+
+  Examples of such commands: `nix store copy`, `nix hash base16`, `nix store
+  ping`, ...
+
+
+# Help is essential
+
+Help should be built into your command line so that new users can gradually
+discover new features when they need them. 
+
+## Looking for help
+
+Since there is no standard way how user will look for help we rely on ways help
+is provided by commonly used tools. As a guide for this we took `git` and
+whenever in doubt look at it as a preferred direction.
+
+The rules are:
+
+- Help is shown by using `--help` or `help` command (eg `nix` `--``help` or
+  `nix help`).
+- For non-COMMANDs (eg. `nix` `--``help`  and `nix store` `--``help`) we **show
+  a summary** of most common use cases. Summary is presented on the STDOUT
+  without any use of PAGER.
+- For COMMANDs (eg. `nix init` `--``help` or `nix help init`) we display the
+  man page of that command. By default the PAGER is used (as in `git`).
+- At the end of either summary or man page there should be an URL pointing to
+  an online version of more detailed documentation.
+- The structure of summaries and man pages should be the same as in `git`.
+
+## Anticipate where help is needed
+
+Even better then requiring the user to search for help is to anticipate and
+predict when user might need it. Either because the lack of discoverability,
+typo in the input or simply taking the opportunity to teach the user of
+interesting - but less visible - details.
+
+### Shell completion
+
+This type of help is most common and almost expected by users. We need to
+**provide the best shell completion** for `bash`, `zsh` and `fish`.
+
+Completion needs to be **context aware**, this mean when a user types:
+
+```shell
+$ nix build n<TAB>
+```
+
+we need to display a list of flakes starting with `n`.
+
+### Wrong input
+
+As we all know we humans make mistakes, all the time. When a typo - intentional
+or unintentional - is made, we should prompt for closest possible options or
+point to the documentation which would educate user to not make the same
+errors. Here are few examples:
+
+In first example we prompt the user for typing wrong command name:
+
+
+```shell
+$ nix int
+------------------------------------------------------------------------
+  Error! Command `int` not found.
+------------------------------------------------------------------------
+  Did you mean:
+    |> nix init
+    |> nix input
+```
+
+Sometimes users will make mistake either because of a typo or simply because of
+lack of discoverability. Our handling of this cases needs to be context
+sensitive.
+
+
+```shell
+$ nix init --template=template#pyton
+------------------------------------------------------------------------
+  Error! Template `template#pyton` not found.
+------------------------------------------------------------------------
+Initializing Nix project at `/path/to/here`.
+      Select a template for you new project:
+          |> template#pyton
+             template#python-pip
+             template#python-poetry
+```
+
+### Next steps
+
+It can be invaluable to newcomers to show what a possible next steps and what
+is the usual development workflow with Nix. For example:
+
+
+```shell
+$ nix init --template=template#python
+Initializing project `template#python`
+          in `/home/USER/dev/new-project`
+
+  Next steps
+    |> nix develop   -- to enter development environment
+    |> nix build     -- to build your project
+```
+
+### Educate the user
+
+We should take any opportunity to **educate users**, but at the same time we
+must **be very very careful to not annoy users**. There is a thin line between
+being helpful and being annoying.
+
+An example of educating users might be to provide *Tips* in places where they
+are waiting.
+
+```shell
+$ nix build
+    Started building my-project 1.2.3
+ Downloaded python3.8-poetry 1.2.3 in 5.3 seconds
+ Downloaded python3.8-requests 1.2.3 in 5.3 seconds
+------------------------------------------------------------------------
+      Press `v` to increase logs verbosity
+         |> `?` to see other options
+------------------------------------------------------------------------
+      Learn something new with every build...
+         |> See last logs of a build with `nix log --last` command.
+------------------------------------------------------------------------
+  Evaluated my-project 1.2.3 in 14.43 seconds
+Downloading [12 / 200]
+         |> firefox 1.2.3 [#########>       ] 10Mb/s | 2min left
+   Building [2 / 20]
+         |> glibc 1.2.3 -> buildPhase: <last log line>
+------------------------------------------------------------------------
+```
+
+Now **Learn** part of the output is where you educate users. You should only
+show it when you know that a build will take some time and not annoy users of
+the builds that take only few seconds.
+
+Every feature like this should go though a intensive review and testing to
+collect as much a feedback as possible and to fine tune every little detail. If
+done right this can be an awesome features beginners and advance users will
+love, but if not done perfectly it will annoy users and leave bad impression.
+
+# Input
+
+Input to a command is provided via `ARGUMENTS` and `OPTIONS`. 
+
+`ARGUMENTS` represent a required input for a function. When choosing to use
+`ARGUMENT` over function please be aware of the downsides that come with it:
+
+- User will need to remember the order of `ARGUMENTS`. This is not a problem if
+  there is only one `ARGUMENT`.
+- With `OPTIONS` it is possible to provide much better auto completion.
+- With `OPTIONS` it is possible to provide much better error message.
+- Using `OPTIONS` it will mean there is a little bit more typing.
+
+We don’t discourage the use of `ARGUMENTS`, but simply want to make every
+developer consider the downsides and choose wisely.
+
+## Naming the `OPTIONS`
+
+Then only naming convention - apart from the ones mentioned in Naming the
+`COMMANDS` section is how flags are named.
+
+Flags are a type of `OPTION` that represent an option that can be turned ON of
+OFF. We can say **flags are boolean type of** `**OPTION**`.
+
+Here are few examples of flag `OPTIONS`:
+
+- `--colors` vs. `--no-colors` (showing colors in the output)
+- `--emojis` vs. `--no-emojis` (showing emojis in the output)
+
+## Prompt when input not provided
+
+For *main commands* (as [per classification](#classification)) we want command
+to improve the discoverability of possible input. A new user will most likely
+not know which `ARGUMENTS` and `OPTIONS` are required or which values are
+possible for those options.
+
+In cases, the user might not provide the input or they provide wrong input,
+rather then show the error, prompt a user with an option to find and select
+correct input (see examples).
+
+Prompting is of course not required when TTY is not attached to STDIN. This
+would mean that scripts wont need to handle prompt, but rather handle errors.
+
+A place to use prompt and provide user with interactive select
+
+
+```shell
+$ nix init
+Initializing Nix project at `/path/to/here`.
+      Select a template for you new project:
+          |> py
+             template#python-pip
+             template#python-poetry
+             [ Showing 2 templates from 1345 templates ]
+```
+
+Another great place to add prompts are **confirmation dialogues for dangerous
+actions**. For example when adding new substitutor via `OPTIONS` or via
+`flake.nix` we should prompt - for the first time - and let user review what is
+going to happen.
+
+
+```shell
+$ nix build --option substitutors https://cache.example.org
+------------------------------------------------------------------------
+  Warning! A security related question need to be answered.
+------------------------------------------------------------------------
+  The following substitutors will be used to in `my-project`: 
+    - https://cache.example.org
+
+  Do you allow `my-project` to use above mentioned substitutors?
+    [y/N] |> y
+```
+
+# Output
+
+Terminal output can be quite limiting in many ways. Which should forces us to
+think about the experience even more. As with every design the output is a
+compromise between being terse and being verbose, between showing help to
+beginners and annoying advance users. For this it is important that we know
+what are the priorities.
+
+Nix command line should be first and foremost written with beginners in mind.
+But users wont stay beginners for long and what was once useful might quickly
+become annoying. There is no golden rule that we can give in this guideline
+that would make it easier how to draw a line and find best compromise.
+
+What we would encourage is to **build prototypes**, do some **user testing**
+and collect **feedback**. Then repeat the cycle few times.
+
+First design the *happy path* and only after your iron it out, continue to work
+on **edge cases** (handling and displaying errors, changes of the output by
+certain `OPTIONS`, etc…)
+
+## Follow best practices
+
+Needless to say we Nix must be a good citizen and follow best practices in
+command line.
+
+In short: **STDOUT is for output, STDERR is for (human) messaging.**
+
+STDOUT and STDERR provide a way for you to output messages to the user while
+also allowing them to redirect content to a file. For example:
+
+```shell
+$ nix build > build.txt
+------------------------------------------------------------------------
+  Error! Atrribute `bin` missing at (1:94) from string.
+------------------------------------------------------------------------
+
+  1| with import <nixpkgs> { }; (pkgs.runCommandCC or pkgs.runCommand) "shell" { buildInputs = [ (surge.bin) ]; } ""
+```
+
+Because this warning is on STDERR, it doesn’t end up in the file.
+
+But not everything on STDERR is an error though. For example, you can run `nix
+build` and collect logs in a file while still seeing the progress.
+
+```
+$ nix build > build.txt
+  Evaluated 1234 files in 1.2 seconds
+ Downloaded python3.8-poetry 1.2.3 in 5.3 seconds
+ Downloaded python3.8-requests 1.2.3 in 5.3 seconds
+------------------------------------------------------------------------
+      Press `v` to increase logs verbosity
+         |> `?` to see other options
+------------------------------------------------------------------------
+      Learn something new with every build...
+         |> See last logs of a build with `nix log --last` command.
+------------------------------------------------------------------------
+  Evaluated my-project 1.2.3 in 14.43 seconds
+Downloading [12 / 200]
+         |> firefox 1.2.3 [#########>       ] 10Mb/s | 2min left
+   Building [2 / 20]
+         |> glibc 1.2.3 -> buildPhase: <last log line>
+------------------------------------------------------------------------
+```
+
+## Errors (WIP)
+
+**TODO**: Once we have implementation for the *happy path* then we will think
+how to present errors.
+
+## Not only for humans
+
+Terse, machine-readable output formats can also be useful but shouldn’t get in
+the way of making beautiful CLI output. When needed, commands should offer a
+`--json` flag to allow users to easily parse and script the CLI.
+
+When TTY is not detected on STDOUT we should remove all design elements (no
+colors, no emojis and using ASCII instead of Unicode symbols). The same should
+happen when TTY is not detected on STDERR. We should not display progress /
+status section, but only print warnings and errors.
+
+## Dialog with the user
+
+CLIs don't always make it clear when an action has taken place. For every
+action a user performs, your CLI should provide an equal and appropriate
+reaction, clearly highlighting the what just happened. For example:
+
+```shell
+$ nix build
+ Downloaded python3.8-poetry 1.2.3 in 5.3 seconds
+ Downloaded python3.8-requests 1.2.3 in 5.3 seconds
+...
+   Success! You have successfully built my-project.
+$
+```
+
+Above command clearly states that command successfully completed. And in case
+of `nix build`, which is a command that might take some time to complete, it is
+equally important to also show that a command started.
+
+## Text alignment 
+
+Text alignment is the number one design element that will present all of the
+Nix commands as a family and not as separate tools glued together.
+
+The format we should follow is:
+
+```shell
+$ nix COMMAND
+   VERB_1 NOUN and other words
+  VERB__1 NOUN and other words
+       |> Some details 
+```
+
+Few rules that we can extract from above example:
+
+- Each line should start at least with one space.
+- First word should be a VERB and must be aligned to the right.
+- Second word should be a NOUN and must be aligned to the left.
+- If you can not find a good VERB / NOUN pair, don’t worry make it as
+  understandable to the user as possible.
+- More details of each line can be provided by `|>` character which is serving
+  as the first word when aligning the text
+
+Don’t forget you should also test your terminal output with colors and emojis
+off (`--no-colors --no-emojis`).
+
+## Dim / Bright
+
+After comparing few terminals with different color schemes we would **recommend
+to avoid using dimmed text**. The difference from the rest of the text is very
+little in many terminal and color scheme combinations. Sometimes the difference
+is not even notable, therefore relying on it wouldn’t make much sense.
+
+**The bright text is much better supported** across terminals and color
+schemes. Most of the time the difference is perceived as if the bright text
+would be bold. 
+
+## Colors
+
+Humans are already conditioned by society to attach certain meaning to certain
+colors. While the meaning is not universal, a simple collection of colors is
+used to represent basic emotions. 
+
+Colors that can be used in output
+
+- Red = error, danger, stop
+- Green = success, good
+- Yellow/Orange = proceed with caution, warning, in progress
+- Blue/Magenta = stability, calm
+
+While colors are nice, when command line is used by machines (in automation
+scripts) you want to remove the colors. There should be a global `--no-colors`
+option that would remove the colors.
+
+## Special (Unicode) characters
+
+Most of the terminal have good support for Unicode characters and you should
+use them in your output by default. But always have a backup solution that is
+implemented only with ASCII characters and will be used when `--ascii` option
+is going to be passed in. Please make sure that you test your output also
+without Unicode characters
+
+More they showing all the different Unicode characters it is important to
+**establish common set of characters** that we use for certain situations.
+
+## Emojis
+
+Emojis help channel emotions even better than text, colors and special
+characters.
+
+We recommend **keeping the set of emojis to a minimum**. This will enable each
+emoji to stand out more.
+
+As not everybody is happy about emojis we should provide an `--no-emojis`
+option to disable them. Please make sure that you test your output also without
+emojis.
+
+## Tables
+
+All commands that are listing certain data can be implemented in some sort of a
+table. It’s important that each row of your output is a single ‘entry’ of data.
+Never output table borders. It’s noisy and a huge pain for parsing using other
+tools such as `grep`.
+
+Be mindful of the screen width. Only show a few columns by default with the
+table header, for more the table can be manipulated by the following options:
+
+- `--no-headers`: Show column headers by default but allow to hide them.
+- `--columns`: Comma-separated list of column names to add.
+- `--sort`: Allow sorting by column. Allow inverse and multi-column sort as well.
+
+## Interactive output
+
+Interactive output was selected to be able to strike the balance between
+beginners and advance users. While the default output will target beginners it
+can, with a few key strokes, be changed into and advance introspection tool.
+
+### Progress
+
+For longer running commands we should provide and overview of the progress.
+This is shown best in `nix build` example:
+
+```shell
+$ nix build
+    Started building my-project 1.2.3
+ Downloaded python3.8-poetry 1.2.3 in 5.3 seconds
+ Downloaded python3.8-requests 1.2.3 in 5.3 seconds
+------------------------------------------------------------------------
+      Press `v` to increase logs verbosity
+         |> `?` to see other options
+------------------------------------------------------------------------
+      Learn something new with every build...
+         |> See last logs of a build with `nix log --last` command.
+------------------------------------------------------------------------
+  Evaluated my-project 1.2.3 in 14.43 seconds
+Downloading [12 / 200]
+         |> firefox 1.2.3 [#########>       ] 10Mb/s | 2min left
+   Building [2 / 20]
+         |> glibc 1.2.3 -> buildPhase: <last log line>
+------------------------------------------------------------------------
+```
+
+### Search
+
+Use a `fzf` like fuzzy search when there are multiple options to choose from.
+
+```shell
+$ nix init
+Initializing Nix project at `/path/to/here`.
+      Select a template for you new project:
+          |> py
+             template#python-pip
+             template#python-poetry
+             [ Showing 2 templates from 1345 templates ]
+```
+
+### Prompt
+
+In some situations we need to prompt the user and inform the user about what is
+going to happen.
+
+```shell
+$ nix build --option substitutors https://cache.example.org
+------------------------------------------------------------------------
+  Warning! A security related question need to be answered.
+------------------------------------------------------------------------
+  The following substitutors will be used to in `my-project`: 
+    - https://cache.example.org
+
+  Do you allow `my-project` to use above mentioned substitutors?
+    [y/N] |> y
+```
+
+## Verbosity
+
+There are many ways that you can control verbosity.
+
+Verbosity levels are: 
+
+- `ERROR` (level 0)
+- `WARN` (level 1)
+- `NOTICE` (level 2)
+- `INFO` (level 3)
+- `TALKATIVE` (level 4)
+- `CHATTY` (level 5)
+- `DEBUG` (level 6)
+- `VOMIT` (level 7)
+
+The default level that the command starts is `ERROR`. The simplest way to
+increase the verbosity by stacking `-v` option (eg: `-vvv == level 3 == INFO`).
+There are also two shortcuts, `--debug` to run in `DEBUG` verbosity level and
+`--quiet` to run in `ERROR` verbosity level.
+
+----------
+
+# Appendix 1: Commands naming exceptions
+
+`nix init` and `nix repl` are well established 
diff --git a/doc/manual/src/contributing/contributing.md b/doc/manual/src/contributing/contributing.md
new file mode 100644
index 000000000..854139a31
--- /dev/null
+++ b/doc/manual/src/contributing/contributing.md
@@ -0,0 +1 @@
+# Contributing
diff --git a/doc/manual/src/hacking.md b/doc/manual/src/contributing/hacking.md
index 2a1e55e5b..2a1e55e5b 100644
--- a/doc/manual/src/hacking.md
+++ b/doc/manual/src/contributing/hacking.md