fish-shell/doc_src/design.hdr

/** \page design Design document

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<h1 class="interior_title">Design documentation</h1>
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\section design-overview Overview

This is a description of the design principles that have been used to
design fish. The fish design has three high level goals. These are:

-# Everything that can be done in other shell languages should be
possible to do in fish, though fish may rely on external commands in
doing so.
-# Fish should be user friendly, but not at the expense of expressiveness.
Most tradeoffs between power and ease of use can be avoided with careful design.
-# Whenever possible without breaking the above goals, fish should
follow the Posix syntax.

To achieve these high-level goals, the fish design relies on a number
of more specific design principles. These are presented below,
together with a rationale and a few examples for each.

\section ortho The law of orthogonality

The shell language should have a small set of orthogonal features. Any
situation where two features are related but not identical, one of them
should be removed, and the other should be made powerful and general
enough to handle all common use cases of either feature.

Rationale:

Related features make the language larger, which makes it harder to
learn. It also increases the size of the sourcecode, making the
program harder to maintain and update.

Examples:

- Here documents are too similar to using echo inside of a pipeline.
- Subshells, command substitution and process substitution are strongly related. `fish` only supports command substitution, the others can be achieved either using a block or the psub shellscript function.
- Having both aliases and functions is confusing, especially since both of them have limitations and problems. `fish` functions have none of the drawbacks of either syntax.
- The many Posix quoting styles are silly, especially \$''.

\section sep The law of responsiveness

The shell should attempt to remain responsive to the user at all times, even in the face of contended or unresponsive filesystems. It is only acceptable to block in response to a user initiated action, such as running a command.

Rationale:

Bad performance increases user-facing complexity, because it trains users to recognize and route around slow use cases. It is also incredibly frustrating.

Examples:

- Features like syntax highlighting and autosuggestions must perform all of their disk I/O asynchronously.
- Startup should minimize forks and disk I/O, so that fish can be started even if the system is under load.

\section conf Configurability is the root of all evil

Every configuration option in a program is a place where the program
is too stupid to figure out for itself what the user really wants, and
should be considered a failiure of both the program and the programmer
who implemented it.

Rationale:

Different configuration options are a nightmare to maintain, since the
number of potential bugs caused by specific configuration combinations
quickly becomes an issue. Configuration options often imply
assumptions about the code which change when reimplementing the code,
causing issues with backwards compatibility. But mostly, configuration
options should be avoided since they simply should not exist, as the
program should be smart enough to do what is best, or at least a good
enough approximation of it.

Examples:

- Fish allows the user to set various syntax highlighting colors. This is needed because fish does not know what colors the terminal uses by default, which might make some things unreadable. The proper solution would be for text color preferences to be defined centrally by the user for all programs, and for the terminal emulator to send these color properties to fish.
- Fish does not allow you to set the history filename, the number of history entries, different language substyles or any number of other common shell configuration options.

A special note on the evils of configurability is the long list of
very useful features found in some shells, that are not turned on by
default. Both zsh and bash support command specific completions, but
no such completions are shipped with bash by default, and they are
turned off by default in zsh. Other features that zsh support that are
disabled by default include tab-completion of strings containing
wildcards, a sane completion pager and a history file.

\section user The law of user focus

When designing a program, one should first think about how to make a
intuitive and powerful program. Implementation issues should only be
considered once a user interface has been designed.

Rationale:

This design rule is different than the others, since it describes how
one should go about designing new features, not what the features
should be. The problem with focusing on what can be done, and what is
easy to do, is that to much of the implementation is exposed. This
means that the user must know a great deal about the underlying system
to be able to guess how the shell works, it also means that the
language will often be rather low-level.

Examples:

- There should only be one type of input to the shell, lists of commands. Loops, conditionals and variable assignments are all performed through regular commands.
- The differences between builtin commands and shellscript functions should be made as small as possible. Builtins and shellscript functions should have exactly the same types of argument expansion as other commands, should be possible to use in any position in a pipeline, and should support any io redirection.
- Instead of forking when performing command substitution to provide a fake variable scope, all fish commands are performed from the same process, and fish instead supports true scoping.
- All blocks end with the `end` builtin.

\section disc The law of discoverability

A program should be designed to make its features as
easy as possible to discover for the user.

Rationale:

A program whose features are discoverable turns a new user into an
expert in a shorter span of time, since the user will become an expert
on the program simply by using it.

The main benefit of a graphical program over a command line-based
program is discoverability. In a graphical program, one can discover
all the common features by simply looking at the user interface and
guessing what the different buttons, menus and other widgets do. The
traditional way to discover features in commandline programs is
through manual pages. This requires both that the user starts to use a
different program, and then she/he remembers the new information
until the next time she/he uses the same program.

Examples:

- Everything should be tab-completable, and every tab completion should have a description.
- Every syntax error and error in a builtin command should contain an error message describing what went wrong and a relevant help page. Whenever possible, errors should be flagged red by the syntax highlighter.
- The help manual should be easy to read, easily available from the shell, complete and contain many examples
- The language should be uniform, so that once the user understands the command/argument syntax, he will know the whole language, and be able to use tab-completion to discover new featues.

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