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Language/Hoon/Guides/Strings

Strings

This document discusses hoon's two main string types: cords (as well as its subsets knot and term) and tapes. The focus of this document is on their basic properties, syntax and the most common text-related functions you'll regularly encounter. In particular, it discusses conversions and the encoding/decoding of atom auras in strings.

Hoon has a system for writing more elaborate functional parsers, but that is not touched on here. Instead, see the Parsing guide. Hoon also has a type for UTF-32 strings, but those are rarely used and not discussed in this document.

There are a good deal more text manipulation functions than are discussed here. See the Further Reading section for details.

tapes vs. text atoms

As mentioned, urbit mainly deals with two kinds of strings: tapes and cord/knot/terms. The former is a list of individual UTF-8 characters. The latter three encode UTF-8 strings in a single atom.

Cords may contain any UTF-8 characters, while knots and terms only allow a smaller subset. Each of these are discussed below in the Text atoms section.

Text atoms like cords are more efficient to store and move around. They are also more efficient to manipulate with simple bitwise operations. Their downside is that UTF-8 characters vary in their byte-length. ASCII characters are all 8-bit, but others can occupy up to four bytes. Accounting for this variation in character size can complicate otherwise simple functions. Tapes, on the other hand, don't have this problem because each character is a separate item in the list, regardless of its byte-length. This fact makes it much easier to process tapes in non-trivial ways with simple list functions.

In light of this, a general rule of thumb is to use cords for simple things like storing chat messages or exchanging them over the network. If text requires complex processing on the other hand, it is generally easier with tapes. Note there are cord manipulation functions in the standard library, so you needn't always convert cords to tapes for processing, it just depends on the case.

Next we'll look at these different types of strings in more detail.

Text atoms

cord

A cord has an aura of @t. It denotes UTF-8 text encoded in an atom, little-endian. That is, the first character in the text is the least-significant byte. A cord may contain any UTF-8 characters, there are no restrictions.

The hoon syntax for a cord is some text wrapped in single-quotes like:

'This is a cord!'

single-quotes and backslashes must be escaped with a backslash like:

'\'quotes\' \\backslashes\\'

Characters can also be entered as hex, they just have to be escaped by a backslash. For example, '\21\21\21' will render as '!!!'. This is useful for entering special characters such as line breaks like 'foo\0abar'.

Cords divided over multiple lines are allowed. There are two ways to do this. The first is to start and end with three single-quotes like:

'''
foo
bar
baz
'''

The line endings will be encoded Unix-style as line feed characters like:

'foo\0abar\0abaz'

The second is to begin with a single-quote like usual, then break the line by ending it with a backslash and start the next line with a forward-slash like:

'foo\
/bar\
/baz'

This will be parsed to:

'foobarbaz'

knot

A knot has an aura of @ta, and is a subset of a cord. It allows lower-case letters, numbers, and four special characters: Hyphen, tilde, underscore and period. Its restricted set of characters is intended to be URL-safe.

The hoon syntax for a knot is a string containing any of the aforementioned characters prepended with ~. like:

~.abc-123.def_456~ghi

term

A term has an aura of @tas, and is a subset of a knot. It only allows lower-case letters, numbers, and hyphens. Additionally, the first character cannot be a hyphen or number. This is a very restricted text atom, and is intended for naming data structures and the like.

The hoon syntax for a term is a string conforming to the prior description, prepended with a % like:

%foo-123

A note about term type inference

There is actually an even more restricted text atom form with the same %foo syntax as a term, where the type of the text is the text itself. For example, in the dojo:

> `%foo`%foo
%foo

The hoon parser will, by default, infer the type of %foo-style syntax this way. If we try with the dojo type printer:

> ? %foo
  %foo
%foo

This type-as-itself is used for many things, such as unions like:

?(%foo %bar %bas)

In order to give %foo the more generic @tas aura, it must be explicitly upcast like:

> ? `@tas`%foo
  @tas
%foo

This is something to be wary of. For example, if you wanted to form a (set @tas) you might think to do:

(silt (limo ~[%foo %bar %baz]))

However, this will actually form a set of the union ?(%foo %bar %baz) due to the specificity of type inference:

> ? (silt (limo ~[%foo %bar %baz]))
?(%~ [?(n=%bar n=%baz n=%foo) l=nlr(?(%bar %baz %foo)) r=nlr(?(%bar %baz %foo))])
[n=%baz l={%bar} r={%foo}]

One further note about the type-as-itself form: Ocassionally you may wish to form a union of strings which contain characters disallowed in terms. To get around this, you can enclose the text after the % with single-quotes like %'HELLO!'.

Aura type validity

The hoon parser will balk at cords, knots and terms containing invalid characters. However, because they're merely auras, any atom can be cast to them. When cast (or clammed), they will not be validated in terms of whether the characters are allowed in the specified aura.

For example, you can do this:

> `@tas`'!%* $@&'
%!%* $@&

This means you cannot rely on mere aura-casting if you need the text to conform to the specified aura's restrictions. Instead, there are a couple of function in the standard library to check text aura validity: +sane and +sand.

The +sane function takes an argument of either %ta or %tas to validate @ta and @tas respectively (you can technically give it %t for @t too but there's no real point). It will return %.y if the given atom is valid for the given aura, and %.n if it isn't. For example:

> ((sane %tas) 'foo')
%.y
> ((sane %tas) 'foo!')
%.n

The +sand function does the same thing, but rather than returning a ? it returns a unit of the given atom, or ~ if validation failed. For example:

> `(unit @tas)`((sand %tas) 'foo')
[~ %foo]
> `(unit @tas)`((sand %tas) 'foo!')
~

tape

A tape is the other main string type in hoon. Rather than a single atom, it's instead a list of individual @tD characters (the D specifies a bit-length of 8, see the Auras documentation for details). The head of the list is the first character in the string.

The hoon syntax for a tape is some text wrapped in double-quotes like:

"This is a tape!"

Double-quotes, backslashes and left-braces must be escaped by a backslash character:

"\"double-quotes\" \\backslash\\ left-brace:\{"

Like with cords, characters can also be entered as hex escaped by a backslash so "\21\21\21" renders as "!!!".

Tapes divided over multiple lines are allowed. Unlike cords, there is only one way to do this, which is by starting and ending with three double-quotes like:

"""
foo
bar
baz
"""

The line endings will be encoded Unix-style as line feed characters like:

"foo\0abar\0abaz"

As mentioned earlier, tapes are lists of single characters:

> `tape`~['f' 'o' 'o']
"foo"

This means they can be manipulated with ordinary list functions:

> `tape`(turn "foobar" succ)
"gppcbs"

Interpolation

Tapes, unlike cords, allow string interpolation. Arbitrary hoon may be embedded in the tape syntax and its product will be included in the resulting tape. There are two ways to do it:

Manual

In the first case, the code to be evaluated is enclosed in braces. The type of the product of the code must itself be a tape. For example, if the @p of our ship is stored in our, simply doing "{our}" will fail because its type will be @p rather than tape. Instead, we must explicitly use the +scow function to render our as a tape:

> "{(scow %p our)}"
"~zod"

Another example:

> "[{(scow %p our)} {(scow %da now)}]"
"[~zod ~2021.10.3..08.59.10..2335]"

Automatic

Rather than having to manually render data as a tape, angle brackets inside the braces tell the interpreter to automatically pretty-print the product of the expression as a tape. This way we needn't use functions like +scow and can just reference things like our directly:

> "{<our>}"
~zod

Another example:

> "{<(add 1 2)>}"
"3"

And another:

> "{<our now>}"
"[~zod ~2021.10.3..09.01.14..1654]"

Conversions

Tapes can easily be converted to cords and vice versa. There are two stdlib functions for this purpose: +crip and +trip. The former converts a tape to a cord and the latter does the opposite. For example:

> (crip "foobar")
'foobar'
> (trip 'foobar')
"foobar"

Knots and terms can also be converted to tapes with +trip:

> (trip %foobar)
"foobar"
> (trip ~.foobar)
"foobar"

Likewise, the output of +crip can be cast to a knot or term:

> `@tas`(crip "foobar")
%foobar
> `@ta`(crip "foobar")
~.foobar
> `@tas`(need ((sand %tas) (crip "foobar")))
%foobar

Encoding in text

It's common to encode atoms in cords or knots, particularly when constructing a scry path or just a path in general. There are two main functions for this purpose: +scot and +scow. The former produces a knot, and the latter produces a tape. Additionally, there are two more functions for encoding paths in cords and tapes respectively: +spat and +spud.

+scot and +spat

+scot encodes atoms of various auras in a knot (or cord/term with casting). It takes two arguments: the aura in a @tas and the atom to be encoded. For example:

> (scot %p ~zod)
~.~zod


> (scot %da now)
~.~2021.10.4..07.35.54..6d41


> (scot %ux 0xaa.bbbb)
~.0xaa.bbbb

Note the aura of the atom needn't actually match the specified aura:

> (scot %ud ~zod)
~.0

Hoon can of course be evaluated in its arguments as well:

> (scot %ud (add 1 1))
~.2

You'll most commonly see this used in constructing a path like:

> /(scot %p our)/garden/(scot %da now)/foo/(scot %ud 123.456)
[~.~zod %garden ~.~2021.10.4..07.43.14..a556 %foo ~.123.456 ~]


> `path`/(scot %p our)/garden/(scot %da now)/foo/(scot %ud 123.456)
/~zod/garden/~2021.10.4..07.43.23..9a0f/foo/123.456

+spat simply encodes a path in a cord like:

> (spat /foo/bar/baz)
'/foo/bar/baz'

+scow and +spud

+scow is the same as +scot except it produces a tape rather than a knot. For example:

> (scow %p ~zod)
"~zod"


> (scow %da now)
"~2021.10.4..07.45.25..b720"


> (scow %ux 0xaa.bbbb)
"0xaa.bbbb"

+spud simply encodes a path in a tape:

> (spud /foo/bar/baz)
"/foo/bar/baz"

Decoding from text

For decoding atoms of particular auras encoded in cords, there are three functions: +slat, +slav, and +slaw. Additionally, there is +stab for decoding a cord to a path.

+slav parses the given cord with the aura specified as a @tas, crashing if the parsing failed. For example:

> `@da`(slav %da '~2021.10.4..11.26.54')
~2021.10.4..11.26.54


> `@p`(slav %p '~zod')
~zod


> (slav %p 'foo')
dojo: hoon expression failed

+slaw is like +slav except it produces a unit which is null if parsing failed, rather than crashing. For example:

> `(unit @da)`(slaw %da '~2021.10.4..11.26.54')
[~ ~2021.10.4..11.26.54]


> `(unit @p)`(slaw %p '~zod')
[~ ~zod]


> (slaw %p 'foo')
~

+slat is a curried version of +slaw, meaning it's given the aura and produces a new gate which takes the actual cord. For example:

> `(unit @da)`((slat %da) '~2021.10.4..11.26.54')
[~ ~2021.10.4..11.26.54]


> `(unit @p)`((slat %p) '~zod')
[~ ~zod]


> ((slat %p) 'foo')
~

Finally, +stab parses a cord containing a path to a path. For example:

> (stab '/foo/bar/baz')
/foo/bar/baz

Further reading

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Last modified October 8, 2023