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#166 in Encoding

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Implicit Data Markup

IDM is a non-self-describing data serialization format intended to be both written and read by humans.

It uses an indentation-based outline syntax with semantically significant indentation. It requires an external type schema to direct parsing the input, the same input can be parsed in different ways if a different type is expected. In the Rust version, the type schema is provided by the Serde data model. Because the format is not self-describing, IDM files can get by with less syntax than almost any other human-writable data serialization language.

The motivation is to provide a notation that is close to freeform handwritten plaintext notes. IDM can be thought of as a long-form user interface to a program rather than just a data exchange protocol for computers.

It's basically a fixie bike serialization format. Simple, arguably fun and having it as your daily driver might cause a horrific crash sooner or later. It is expected that the user controls both the data and the types when using it, and can work around corner cases which it can't handle.

If you need robust serialization of any Rust data structure, hassle-free collaboration between multiple people who aren't always communicating closely with each other, or just general high reliability, you probably want a more verbose serialization language like JSON, YAML or RON.


IDM implements Serde serialization.

Use idm::from_str to deserialize Serde-deserializable data. Use idm::to_string to serialize Serde-serializable data.

IDM is a non-self-describing data format

Depending on the expected type, the same input can be parsed in several ways. Take something like the following:

    1 2 3
    4 5 6
    7 8 9

When expecting a single String, the whole thing gets read verbatim as a three-line paragraph. When expecting a list Vec<String> sequence, it's three lines, ["1 2 3", "4 5 6", "7 8 9"]. When expecting a matrix Vec<Vec<i32>>, it's three lists of three numbers, [[1, 2, 3], [4, 5, 6], [7, 8, 9]].

A sequence can be read either vertically as an outline of items or horizontally as a sequence of whitespace-separated words. For the horizontal sequence, whitespace is the only recognized separator. If the representations of the elements of a sequence contain spaces, a vertical sequence can always be used.

Basic syntax

An IDM outline document consists of one or more lines of text terminated by newline. It must contain at least one newline, or it will be considered an inline fragment document instead. All indentation in a single IDM document must use only ASCII spaces or only ASCII tabs, these can not be mixed in any way.

IDM treats only ASCII tabs and spaces (U+0009 and U+0020) as whitespace. In the subsequent text, 'whitespace' will refer only to these characters and to newlines. For example NBSP (U+00A0) characters are treated as non-whitespace.

Outlines are defined recursively to consist of items that consist of a line indented to the outline's indent depth, with an optional body outline with a deeper indentation under the line. If the item consists of only the line, it is called a line, if it has a nonempty body outline, it is called a section, it's top line is called a headline and the child outline is called a body.

The indentation of body items in one document must use either only ASCII spaces or only ASCII tabs, these can not be mixed in any way. All items of a section body must be indented to the same depth. The following outline has invalid syntax:

    Item 1 at indent depth 4
  Item 2 at indent depth 2, inconsistent dedentation!

Blank lines are interpreted to have the indent depth of the first non-blank line after them, or 0 if no non-blank line exists after them. This means that a blank line can never be a section headline, since the headline must have a shallower indent depth than the line immediately following it.

The presence of a trailing newline is syntactically significant for single-line documents. A single line with a trailing newline is read as a single-item outline, while a single line without a trailing newline is read as a fragment that may be interpreted as a horizontal sequence.

// No trailing newline, parses as horizontal sequence.
    idm::from_str::<Vec<String>>("a b").unwrap(),
    vec!["a".to_string(), "b".to_string()]);

// Trailing newline present, parses as single-item vertical sequence.
    idm::from_str::<Vec<String>>("a b\n").unwrap(),
    vec!["a b".to_string()]);

Some parts of an IDM document may correspond to multi-line string values from serialized user data, but they must still follow IDM's indentation conventions along with the rest of the document. String values with leading whitespace, inconsistent dedentation or indentation that mixes tabs and spaces cannot be serialized. If the serialization is using a different indentation style than the multi-line string (tabs instead of spaces or vice versa), the string is rewritten to use the different indentation style. Precise indentation depths from the original indentation may be lost when this happens, though the logical structure of the indentation should always be preserved.

Special syntax

Aside from significant whitespace, IDM has only two built-in syntax elements, comments and colon blocks.

Comments are always the only element on a line, and they must either be only "--" or start with "-- ", followed by arbitrary text after the space. Besides the usual role of leaving notes in documents, comments also serve as syntax separators. A sequence of vertical sequences must be separated with dedented comment lines between the sequences:

  1 2
  3 4
  4 5
  6 7

Colon lines start with a colon immediately followed by a non-whitespace character. A colon block is a contiguous run of colon lines, with only comments and blank lines allowed in between:

:a 1
:b 2
Not part of the attribute block

A colon block is syntactic sugar for an extra level of indentation and a comment line that separates the headline-less outline. The fragment above is equivalent to

  a 1
  b 2
Not part of the attribute block

Special forms

Because you hopefully aren't using them for anything else, IDM repurposes singleton tuples ((A,)) as a marker for special forms in IDM documents. Special forms are needed so that IDM can read entire files, comments, blanks and all, into standard outline structures that preserve all the file contents.

A String singleton in the head position of a pair (((String,), _)) matches a line in raw mode. The headline of the current item, even if it's a comment or a blank line, is read into the pair's head string. The body of the section is parsed normally into the tail of the pair.

-- This gets read into the String at pair head (even with comment syntax)
  These lines get
  Read into the body
  Of the pair type

A pair with a map-like type (struct or map) in the head singleton will read an outline (not an item as the string-headed pair), where it will expect to find the initial map-like value in an indented block, and will then read all the remaining outline items into the tail of the pair as a single block. So a type like ((BTreeMap<String, String>,), Vec<String>) would expect something like

  key1 value1
  key2 value2
First element in pair tail
Second element in pair tail...

However, this is exactly the thing colon blocks are made for. Instead of writing the indented block explicitly, the idiomatic way to write the value is

:key1 value1
:key2 value2
First element in pair tail
Second element in pair tail...

All map-like values at pair head position must be in vertical form. If the type is a struct, it cannot be written in the horizontal inline struct form here.

Tuples and sequences

The two types of simple collections supported by IDM are sequences of homogeneous item type and unknown length and tuples of known length and heterogeneous item type. Sequences match standard forms of a vertical sequence of lines or blocks and a horizontal sequence of words. Tuples match the same patterns, but also some other ones. Since singleton tuples are reserved for IDM special forms, actual tuples used must have length of at least 2.

Since the length of a tuple is known, special rules can be applied to its final item. While all elements of a single-line horizontal sequence must be single words, the final element of a tuple is the entire rest of the line, and can contain whitespace. The line key A multi-line value can be matched into (String, String) with values ("key", "A multi-line value"). The last value of a tuple can also be an item body, so the pair tuple could also match

  Multiple lines
  of body

Structs and maps

Structs and maps (map-like types) are treated very similarly. Their actual syntax is an outline of unadorned keys followed by attributes, but they are often placed in colon blocks which gives the illusion of a syntax where the colon prefix is how you write map keys. To keep up the charade, any standalone map-like value can be written as a colon block, and the parser will detect the additional block nesting and pop out the inner value.

Parsing items of a map or a vertical struct is equivalent to parsing a sequence of tuples of the key and value types of a map or of strings for the struct fields and corresponding field value types for a struct.

Unlike most other IDM types, maps only have a vertical form. This is important, since it makes it possible to parse an absence of a map in the special form where the map is parsed at the head of a pair.

Structs do have a horizontal form. In this form, the struct value consists of only the values of struct fields, the field names are not included. The values are listed in the exact order they show up in struct declaration. This form is usually used when writing tabular data. The user needs to be cautious with this form since it does not include field names, so any change in the number or order of the fields in a struct type will make inline values written against a previous version of the type invalid.

IDM has very limited capabilities for representing missing values, so the convention for Option values is to omit the entire item (both key and value) from a map or a struct if the value is None. Structs written in horizontal form cannot have missing values. Completely empty structs or maps can be matched in the special pair head singleton position but not elsewhere.

Complex structure example

Complex structures can be written using maps from object names to objects, which transform into outlines of sections, and using pairs of structs with such maps, which become colon blocks with followed by child elements. The following example shows how a database of stars and their orbiting planets is represented. The planet data is represented compactly using inline structs.

The IDM data:

  :age 4.6e9
  :mass 1.0
  --    :orbit :mass
  Earth  1.0   1.0
  Mars   1.52  0.1
Alpha Centauri
  :age 5.3e9
  :mass 1.1
  --    :orbit :mass
  Chiron 1.32  1.33

The type signature and parse test:

use serde::Deserialize;
use std::collections::BTreeMap;

type StarSystem = ((Star,), BTreeMap<String, Planet>);
type Starmap = BTreeMap<String, StarSystem>;

#[derive(PartialEq, Debug, Deserialize)]
struct Star {
    age: f32,
    mass: f32,

#[derive(PartialEq, Debug, Deserialize)]
struct Planet {
    orbit: f32,
    mass: f32,

  :age 4.6e9
  :mass 1.0
  --    :orbit :mass
  Earth  1.0   1.0
  Mars   1.52  0.1
Alpha Centauri
  :age 5.3e9
  :mass 1.1
  --    :orbit :mass
  Chiron 1.32  1.33

        ((Star { age: 4.6e9, mass: 1.0 },),
           ("Earth".into(), Planet { orbit: 1.0, mass: 1.0 }),
           ("Mars".into(),  Planet { orbit: 1.52, mass: 0.1 })]))),
    ("Alpha Centauri".into(),
        ((Star { age: 5.3e9, mass: 1.1 },),
           ("Chiron".into(),  Planet { orbit: 1.32, mass: 1.33 })])))]));

For an example on how to add syntax to IDM using user-defined types, see the inline maps example.

Outline forms

The uses for IDM seen so far expect you to have an explicit, application-specific type to serialize to. However, IDM can also serialize generic outline types, which can parse most plaintext files.

A simple outline has a type signature like

struct Outline(Vec<((String,), Outline)>);

This matches the form of a generic IDM outline, with the outline items being the ((String,), Outline) pairs. The pair tuple with the string head makes IDM parse each item in raw mode, so that comment and blank lines are included in the data structure and will get echoed back when the structure gets reserialized.

A richer structure is a data outline, which supports an arbitrary data map for each item:

use indexmap::IndexMap;

struct DataOutline((IndexMap<String, String>,), Vec<((String,), DataOutline)>);

You can now read the named attributes from any item in the outline. The values will all be strings, but an application which knows the expected type for a specific attribute can deserialize the attribute value again using IDM into the more appropriate type.

use indexmap::IndexMap;
use serde::Deserialize;

#[derive(Debug, Deserialize)]
struct DataOutline(
    (IndexMap<String, String>,),
    Vec<((String,), DataOutline)>,

let outline = idm::from_str::<DataOutline>(
Example outline
    :tags foo bar
    This part has stuff

// Raw access patterns are pretty rough.
// A proper app would need some sort of selection API here,
// the explicit indexing gets very rough very fast.
assert_eq!(outline.1[0].1 .1[0].0 .0, "Stuff"); // On the right track...
let tags = &outline.1[0].1 .1[0].1 .0 .0["tags"]; // Grab tags field.
assert_eq!(tags, "foo bar");

// Cast to a more appropriate format.
let tags: Vec<String> = idm::from_str(tags).unwrap();
assert_eq!(tags, vec!["foo", "bar"]);

A caveat for the reserialization of data outlines is that the attribute block is not read in raw mode. Comment lines among the attributes will therefore not be preserved when the outline is deserialized and serialized from memory, and therefore should be avoided when writing outlines that are meant to be rewritten through IDM.

The outline types are intentionally not included in the IDM crate. There's literally nothing going on with them other than the type signature, and you're expected to copy that in your own application. You will also probably want to implement your own application-specific methods for the type, which will be easier if you own the type.

For another example of an outline structure with data mixed in, see the minimal blog engine and corresponding content file in examples.

Accepted shapes

Type Vertical value Horizontal value
atom block, section line, word
special string head section, line-as-section -
special map head block -
map block -
tuple / map element block, section line
struct block line
seq block line

Some types support both vertical (each item on their own line) and horizontal (every item on one line) values, the rest support only vertical values.

Pairs with a singleton string tuple in the first position trigger raw mode. Raw mode has the unique line-as-section parsing mode where it interprets a line as a section with an empty body. Normally a line is interpreted as the horizontal variant of a structured type.


  • Serde's #[serde(flatten)] attribute does not work well with IDM if used with structs. It switches from struct-like parsing to map-like parsing, and stops providing types for values. The result is that all values to the flattened struct are provided as strings and won't deserialize correctly. It can still be useful for maps that collect string values or structs that have string values (or deserialize via strings) for all their fields.

  • A special pair where the first half is a colon-indented struct or map can't have the second half be another special pair with a map head. Due to how the second half is fused in the first, the second map cannot be syntactically distinguished from the first.

  • Primitive types, chars, numeric types and booleans, are trimmed of Unicode whitespace like NBSP before being parsed. The main IDM algorithm treats NBSP as content instead of indentation. This allows you to do left-padded table rows without breaking IDM parsing by padding with NBSP, and still having primitive elements parse correctly from the leftmost table column. User types with custom parsing from a string value may need to trim the input string on their own.


IDM is dual-licensed under Apache-2.0 and MIT.


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