Specification: BQN system-provided values

This portion of the spec is still potentially subject to major changes.

The • symbol is used to access values other than primitives provided by BQN.

All system values described in the BQN specification are optional: an implementation does not have to include any of them. However, if a system value with one of the names given below is included, then it must have the specified behavior. For namespaces this rule applies to individual fields as well: a namespace may be provided with only some of the fields, but a field with one of the given names must behave as specified.

Execution and scope manipulation

Name	Summary
`•BQN`	Evaluate the argument string in an isolated scope
`•Eval`	Evaluate the argument string in the current scope
`•ScopedEval`	Evaluate the argument string in a child scope
`•Using`	Import all values from the argument namespace

The effect of •Eval should be the same as if its argument were written as source code in the scope where •Eval appears. It can define variables, and modify those in the current scope or a parent.

•ScopedEval creates as new scope for evaluation as it is loaded. Other than its syntactic role, it is effectively equivalent to {•Eval}. Parent scopes are visible from the created scope; to make a scope without this property use •BQN"•Eval" or •BQN"•ScopedEval".

Scripts

Name	Summary
`•Import`	Load a script file
`•args`	Arguments passed to current file
`•path`	Current file's path
`•name`	Current filename

•Import loads another BQN script. The script is evaluated in its own isolated scope, and its result is either the result of the last line, or a module if it exports with ⇐ at the top level. If it is a module, then it must be destructured immediately unless first-class namespaces are possible.

The right argument is a filename, which may be relative or absolute. Relative paths are taken relative to the source file where this instance of •Import was written. The left argument, if given, is the list of arguments that should be passed through to the file as •args. If no left argument is given then ⟨⟩ is used for •args. However, the behavior is different in this case. The same file will only be loaded once in a given BQN program by •Import calls with no left argument: the first such call saves the returned value, even if it is mutable, and subsequent calls return this saved value. To avoid this and reload the file, pass a left argument of ⟨⟩.

•args is the arguments passed as the file was invoked, either from the command line or •Import. For command line calls it is a list of strings.

•path simply gives the path of the file in which it appears. It includes a trailing slash but not the name of the file itself.

•name gives the name, including the extension, of the file in which it appears. It doesn't include the path.

Files

The system namespace value •file deals with file operations. For the purposes of •file, paths in the filesystem are always strings. As with •Import, file paths may be relative or absolute, and relative paths are relative to •path, except in •file.At which allows 𝕨 to specify an alternate base directory. The value •path used for a particular instance of •file is determined by the file that contains that instance.

When a •file function returns a file path or portion of a path, the path is always absolute and canonical, with . and .. components removed.

Possible fields of •file are given in the subsections below.

File paths

The following functions manipulate paths and don't access files. Each takes a relative or absolute path 𝕩, and At may also take a base directory 𝕨.

Name	Summary
`path`	Path of this source file, that is, `•path`
`At`	Absolute path of file, with optional base `𝕨`
`Name`	File name including extension
`Parent`	Path of the containing directory, with trailing backslash
`BaseName`	File name, with dot and extension removed
`Extension`	File extension, including leading dot
`Parts`	List of parent, base name, and extension

File metadata

Metadata functions may query information about a file or directory but do not read to or write from it. Each takes a path 𝕩, and Permissions also allows new data in 𝕨. The returned data in any case is the specified property.

Name	Summary
`Exists`	`1` if the file exists and `0` otherwise
`Type`	`"none"`, `"file"`, or `"dir"`
`Created`	Time created
`Accessed`	Time of last access
`Modified`	Time of last modification
`Size`	Total size in bytes
`Permissions`	Query or set file permissions

Times are Unix timestamps, that is, seconds since the Unix epoch, as used by time system values.

File access

File access functions read or write files, either by manipulating files as a whole or interacting with the contents. Whole-file functions cannot overwrite target files: that is, Rename and Copy must give an error if a file exists at 𝕨, while Chars, Lines, and Bytes can overwrite the contents of an existing file 𝕨. However, these three functions must give an error if 𝕨 exists and is a directory.

Name	Summary
`Open`	Return an open file object based on `𝕩`
`Rename`	Rename file `𝕩` with path `𝕨`
`Copy`	Copy file `𝕩` to path `𝕨`
`Remove`	Delete file `𝕩`
`RemoveDir`	Recursively delete directory `𝕩` and all contents
`Chars`	Read from or write to entire file, as characters
`Lines`	Read from or write to entire file, as lines
`Bytes`	Read from or write to entire file, as bytes

Rename and Copy return the path of the new file. Remove and RemoveDir return 1 to indicate successful removal (and error otherwise).

Functions Chars, Lines, and Bytes are all ambivalent. If only 𝕩 is given, then it is a filename, and the result is the contents of the file in the appropriate format. If there are two arguments, then 𝕨 is the filename and 𝕩 is the desired contents. These are written to the file, overwriting its contents, and the absolute filename 𝕨 is returned. The three formats are:

Chars: BQN characters, or UTF-32. The file is assumed to be UTF-8 encoded.
Lines: BQN strings. The file is decoded as with chars, then split into lines by CR, LR, or CRLF line endings.
Bytes: Single-byte values, stored as BQN characters from @ to @+255.

The following short names can also be provided for file access. They can be provided, and use the definitions from above even if •file is not provided.

Name	Equivalent
`•FChars`	`•file.Chars`
`•FLines`	`•file.Lines`
`•FBytes`	`•file.Bytes`

Open file object

Input and output

Name	Summary
`•Out`	Print argument string
`•Fmt`	Format value for printing

Operation properties

Name	Summary
`•Type`	Return a number indicating type
`•Glyph`	Return the glyph for a primitive
`•Source`	Return the source of a block, as a string
`•Decompose`	Show the parts of a compound function

Each function in this section is monadic.

•Type gives its argument's type, as a number from the table below:

Number	Type
0	Array
1	Number
2	Character
3	Function
4	1-modifier
5	2-modifier
6	Namespace

•Glyph gives the glyph corresponding to a primitive as a single character, for example returning '+' given an argument matching +. It causes an error if the argument is not a primitive.

•Source gives a string containing a block's source, including the enclosing braces {}. It causes an error if the argument is not a block. In contrast to •Glyph, this function does not give full information about 𝕩 because the result cannot convey environment or mutable identity.

•Decompose breaks down one level of a compound function or modifier, returning a list with a code giving what kind of structure it has (as listed in the table below) followed by each of its components. Non-operations do not cause an error, but return code -1, then the argument as a single component. The result is thus a list of length 2 to 4, and •Decompose cannot cause an error.

Kind	Code	Components
Non-operation	-1	`𝕩`
Primitive	0	`𝕩`
Block	1	`𝕩`
2-train	2	`g,h`
3-train	3	`f,g,h`
1-mod	4	`𝕗,𝕣`
2-mod	5	`𝕗,𝕣,𝕘`
Left partial	6	`𝕗,𝕣`
Right partial	7	`𝕣,𝕘`

Time

Name	Summary
`•UnixTime`	Time between Unix epoch and function call
`•MonoTime`	Monotonically-increasing time counter for relative measurement
`•Delay`	Wait at least `𝕩` seconds, and return the actual wait time
`•_timed`	Call `𝔽` on `𝕩` `𝕨⊣1` times, and return the average duration
`•_maxTime_`	Call `𝔽` on the arguments, but fail if it takes over `𝕨𝔾𝕩` seconds

All times are measured in seconds.

The Unix epoch is 1970-01-01 00:00:00 UTC. •UnixTime is intended for absolute time measurement and should be implemented with the method that gives the most accurate result at any given time. •MonoTime is intended for relative measurement and should use the method that gives the most precise time differences over the course of the program. Its return value must never decrease between calls.

•_timed returns the total time taken divided by the number of function calls (𝕨 if provided and 1 otherwise), including the overhead required for the outer loop that counts iterations (which will typically be negligible in comparison to the BQN code).

More accurately the modifier •_maxTime_ may fail if execution of 𝔽 takes over 𝕨𝔾𝕩 seconds, and should fail as quickly as it is practically able to. The most likely way to implement this modifier is to interrupt execution at the given time. If 𝔽 completes before the interrupt there is no need to measure the amount of time it actually took.