muss/mps-interpreter/README.md

# mps-interpreter

All necessary components to interpret and run a MPS script.

MpsInterpretor uses a non-standard Iterator implementation,
so it is recommended to use MpsRunner to execute a script.
Since MPS is centered around iterators, script execution is also done by iterating.

MpsInterpretor is misspelt to emphasise that it behaves strangely:
after every MPS statement, a None item is returned even when the script is not complete.
MpsRunner wraps MpsInterpretor so that this behaviour is hidden when iterating.

```rust
use std::io::Cursor;
use mps_interpreter::*;

let cursor = Cursor::new(
"files(folder=`~/Music/`, recursive=true)" // retrieve all files from Music folder
);

let interpreter = MpsRunner::with_stream(cursor);

// warning: my library has ~3800 songs, so this outputs too much information to be useful.
for result in interpreter {
    match result {
        Ok(item) => println!("Got song `{}` (file: `{}`)", item.title, item.filename),
        Err(e) => panic!("Got error while executing: {}", e),
    }
}
```

## Standard Vocabulary
By default, the standard vocabulary is used to parse the stream when iterating the interpreter.
The standard vocabulary defines the valid statement syntax for MPS and parses syntax into special Rust Iterators which can be used to execute the statement.
To declare your own vocabulary, use MpsRunner::with_settings to provide a MpsInterpretor with a custom vocabulary (I'm not sure why you would, but I'm not going to stop you).

### Oddities
The MPS standard syntax does a few things that most other languages don't, because I wanted it to.

\` can be used in place of " -- To make it easier to write SQL, string literals may be surrounded by backticks instead of quotation marks.

; -- The REPL will automatically place semicolons when Enter is pressed and it's not inside of brackets or a literal. MPS requires semicolons at the end of every statement, though, so MPS files must use semicolons.

### Filters
Operations to reduce the items in an iterable: `iterable.(filter);`.
Filters are statements of the format `something.(predicate)`, where "something" is a variable name or another valid statement, and "predicate" is a valid filter predicate (see below).
E.g. `files(folder="~/Music/", recursive=true).(title == "Romantic Traffic");` is valid filter syntax to filter all songs in the Music folder for songs named "Romantic Traffic" (probably just one song).

#### field == something

#### field != something

#### field >= something

#### field > something

#### field <= something

#### field < something -- e.g. `iterable.(title == "Romantic Traffic");`

Compare all items, keeping only those that match the condition. Valid field names are those of the MpsMusicItem (title, artist, album, genre, track, etc.), though this will change when proper object support is added. Optionally, a ? or ! can be added to the end of the field name to skip items whose field is missing/incomparable, or keep all items whose field is missing/incomparable (respectively).

#### start..end -- e.g. `iterable.(0..42);`

Keep only the items that are at the start index up to the end index. Start and/or end may be omitted to start/stop at the iterable's existing start/end (respectively). This stops once the end condition is met, leaving the rest of the iterator unconsumed.

#### start..=end -- e.g. `iterable.(0..=42);`

Keep only the items that are at the start index up to and including the end index. Start may be omitted to start at the iterable's existing start. This stops once the end condition is met, leaving the rest of the iterator unconsumed.

#### index -- e.g. `iterable.(4);`

Keep only the item at the given index. This stops once the index is reached, leaving the rest of the iterator unconsumed.

#### predicate1 || predicate2 -- e.g. `iterable.(4 || 5);`

Keep only the items that meet the criteria of predicate1 or predicate2. This will always consume the full iterator.

#### [empty] -- e.g. `iterable.();`

Matches all items

#### if filter: operation1 else operation2 -- e.g. `iterable.(if title == "Romantic Traffic": repeat(item, 2) else item.());`

Replace items matching the filter with operation1 and replace items not matching the filter with operation2. The `else operation2` part may be omitted to preserve items not matching the filter. To perform operations with the current item, use the special variable `item`. The replacement filter may not contain || -- instead, use multiple filters chained together.

### Functions
Similar to most other languages: `function_name(param1, param2, etc.);`.
These always return an iterable which can be manipulated.
Functions are statements of the format `function_name(params)`, where "function_name" is one of the function names (below) and params is a valid parameter input for the function.
Each function is responsible for parsing it's own parameters when the statement is parsed, so this is very flexible.
E.g. `files(folder="~/Music/", recursive=true);` is valid function syntax to execute the files function with parameters `folder="~/Music/", recursive=true`.


#### sql_init(generate = true|false, folder = "path/to/music");

Initialize the SQLite database connection using the provided parameters. This must be performed before any other database operation (otherwise the database will already be connected with default settings).

#### sql("SQL query here");

Perform a raw SQLite query on the database which MPS auto-generates. An iterator of the results is returned.

#### song("something");

Retrieve all songs in the database with a title like something.

#### album("something");

Retrieve all songs in the database with an album title like something.

#### artist("something");

Retrieve all songs in the database with an artist name like something.

#### genre("something");

Retrieve all songs in the database with a genre title like something.

#### repeat(iterable, count);

Repeat the iterable count times, or infinite times if count is omitted.

#### files(folder = "path/to/music", recursive = true|false, regex = "pattern");

Retrieve all files from a folder, matching a regex pattern.

### Sorters
Operations to sort the items in an iterable: iterable~(sorter) OR iterable.sort(sorter)

#### field -- e.g. iterable~(filename)

Sort by a MpsItem field. Valid field names change depending on what information is available when the MpsItem is populated, but usually title, artist, album, genre, track, filename are valid fields. Items with a missing/incomparable fields will be sorted to the end.

#### distance bliss -- e.g. iterable~(distance bliss)

Sort by the distance (similarity) between songs. Songs which are more similar (lower distance) to the first song in the iterator will be placed closer to the first song, while less similar songs will be sorted to the end. This uses the [bliss music analyser](https://github.com/polochon-street/bliss-rs), which is a very slow operation and can cause music playback interruptions for large iterators.


License: LGPL-2.1-only OR GPL-2.0-or-later