@module builtin pkg/eval
The builtin module contains facilities that are potentially useful to all users.
The builtin module is consulted implicitly when resolving unqualified names, and Elvish's namespacing mechanism makes it impossible for other modules to redefine builtin symbols. It's almost always sufficient (and safe) to use builtin functions and variables with their unqualified names.
Nonetheless, the builtin module is also available as a
pre-defined module. It can be imported with
use builtin
, which makes all the builtin symbols available under the
builtin:
namespace. This can be useful in several cases:
To refer to a builtin function when it is shadowed locally. This is especially useful when the function that shadows the builtin one is a wrapper:
use builtin
fn cd {|@args|
echo running my cd function
builtin:cd $@args
}
Note that the shadowing of cd
is only in effect in the local lexical
scope.
To introspect the builtin module, for example keys $builtin:
.
The usage of a builtin command is described by giving an example usage, using
variables as arguments. For instance, The repeat
command takes two arguments
and are described as:
repeat $n $v
Optional arguments are represented with a trailing ?
, while variadic arguments
with a trailing ...
. For instance, the count
command takes an optional list:
count $inputs?
While the put
command takes an arbitrary number of arguments:
put $values...
Options are given along with their default values. For instance, the echo
command takes an sep
option and arbitrary arguments:
echo &sep=' ' $value...
(When you calling functions, options are always optional.)
Most commands that take value inputs (e.g. count
, each
) can take the inputs
in one of two ways:
From the pipeline:
~> put lorem ipsum | count # count number of inputs
2
~> put 10 100 | each {|x| + 1 $x } # apply function to each input
▶ (num 11)
▶ (num 101)
If the previous command outputs bytes, one line becomes one string input, as
if there is an implicit from-lines
(this behavior is
subject to change):
~> print "a\nb\nc\n" | count # count number of lines
▶ 3
~> use str
~> print "a\nb\nc\n" | each $str:to-upper~ # apply to each line
▶ A
▶ B
▶ C
From an argument -- an iterable value:
~> count [lorem ipsum] # count number of elements in argument
2
~> each {|x| + 1 $x } [10 100] # apply to each element in argument
▶ 11
▶ 101
Strings, and in future, other sequence types are also supported:
~> count lorem
▶ 5
When documenting such commands, the optional argument is always written as
$inputs?
.
Note: You should prefer the first form, unless using it requires explicit
put
commands. Avoid count [(some-command)]
or
each $some-func [(some-command)]
; they are equivalent to
some-command | count
or some-command | each $some-func
.
Rationale: An alternative way to design this is to make (say) count
take
an arbitrary number of arguments, and count its arguments; when there is 0
argument, count inputs. However, this leads to problems in code like count *
;
the intention is clearly to count the number of files in the current directory,
but when the current directory is empty, count
will wait for inputs. Hence it
is required to put the input in a list: count [*]
unambiguously supplies input
in the argument, even if there is no file.
Wherever a command expects a number argument, that argument can be supplied
either with a typed number or a string that can be
converted to a number. This includes numeric comparison commands like ==
.
When a command outputs numbers, it always outputs a typed number.
Examples:
~> + 2 10
▶ (num 12)
~> == 2 (num 2)
▶ $true
Some numeric commands are designated exactness-preserving. When such commands are called with only exact numbers (i.e. integers or rationals), they will always output an exact number. Examples:
~> + 10 1/10
▶ (num 101/10)
~> * 12 5/17
▶ (num 60/17)
If the condition above is not satisfied -- i.e. when a numeric command is not designated exactness-preserving, or when at least one of the arguments is inexact (i.e. a floating-point number), the result is an inexact number, unless otherwise documented. Examples:
~> + 10 0.1
▶ (num 10.1)
~> + 10 1e1
▶ (num 20.0)
~> use math
~> math:sin 1
▶ (num 0.8414709848078965)
There are some cases where the result is exact despite the use of inexact arguments or non-exactness-preserving commands. Such cases are always documented in their respective commands.
The name of some variables and functions have a leading -
. This is a
convention to say that it is subject to change and should not be depended upon.
They are either only useful for debug purposes, or have known issues in the
interface or implementation, and in the worst case will make Elvish crash.
(Before 1.0, all features are subject to change, but those ones are sure to be
changed.)
Those functions and variables are documented near the end of the respective sections. Their known problem is also discussed.