#toml #operator


Implement Toml pointer following the json path syntax, with type Option<&toml::Value>. Overload / as path operator to point into a node in toml tree, as well as some other meaningfull operator overload

1 unstable release

0.1.0 Mar 13, 2023

#694 in Parser implementations


767 lines

Toml Path and Pointer


Implement Toml pointer following the json path syntax, with type Option<&toml::Value>, which dependent on crate toml. Overload / as path operator to point into a node in toml tree, as well as some other meaningfull operator overload. Such as pipe operator | to get primitive value from scalar leaf node, push operator << to overwrite scalar node or push new item to array or table, and push assign operator <<= to re-assign to toml node unconditionally. While / or operator << may invalidate the pointer, we can use ! operator or is_none() method to test such failed case.


use toml_ops::PathOperator;
let tv = r#"
proto=["tcp", "udp"]
let mut v: toml::Value = tv.parse().unwrap(); 

let port = v.path() / "host" / "port" | 0;
assert_eq!(port, 8080);

let node = v.path_mut() / "host" / "port" << 8989;
let port = node | 0;
assert_eq!(port, 8989);

let proto = v.path() / "host" / "proto" / 0 | "";
assert_eq!(proto, "tcp");

let host = v.path_mut() / "host";
let host = host << ("newkey", "newval") << ("morekey", 1234);
assert_eq!(host.is_none(), false);
assert_eq!(!host, false);

let mut proto = v.path_mut() / "host" / "proto";
proto = proto << ("json", ) << ["protobuf"];
assert_eq!(proto.as_ref().unwrap().as_array().unwrap().len(), 4);

proto <<= "default";
assert_eq!(proto.as_ref().unwrap().is_str(), true);
let proto = v.path() / "host" / "proto" | "";
assert_eq!(proto, "default");

let invalid = v.path() / "host" / "no-key";
assert_eq!(!invalid, true);
assert_eq!(invalid.is_none(), true);

Path Syntax

Some like file path in unix file system, separate each component by slash /, espacially for array index also use / not []. So you should not use numeric key in table to avoid confuse.

Please refer detail or standard json path syntax, as toml is roughly equivalent to json in their data model.

Toml Operater Overload Guide

Operator Trigger

We cannot overload operator form toml::Value directly outside the toml crate. So I define a trait named PathOperator in this toml_ops crate, and implent it for toml::Value. Then you can call the following methods from a toml::Value to create a toml pointer:

  • path(): point to self toml value.
  • pathto(subpath: &str): point to some sub node of self toml value.
  • path_mut(): mutable version of path().
  • pathto_mut(subpath: &str): mutable version of pathto().

The pointer is just a struct wrapper of Option<&toml::Value> or Option<&mut toml::Value> for mutable version. But usually no need to care about this, only use overloaded operators after it.

The mutable version pointer can not implement the Copy trait, pay attention that many operator afterward would consume (move) it.

Path Operator /

It is the core operator for toml path pointer, and it is straightforward to overload / (Div) as path operator purpose.

For example, the following statements is roughtly equivalent:

let node = toml_value.path() / "path" / "to" / "node";
let node = toml_value.pathto("path/to/node");
let node = toml_value.path() / "path/to/node";
let node = toml_value.path() / "path.to.node";

Note that the later two forms with one string for long path is slightly inefficient in performance as it involes to split and parse the path syntax, and it may confuse when there is numeric key in table node(there is subtle bug only in mutable version now).

It's better to point to array item like this:

let node = toml_value.path() / "sub-array" / 0 / "sub-key";
// not so good to use:
// let node = toml_value.path() / "sub-array/0/sub-key";

While it is impossible to override operator ., the dot . can also used as path seperator in the long single path form. But not mix use slash and dot to confuse youself. For example, path/../to/node is not point to parrent as in file system, it just same as path/to/node or path.to.node, because path operator ignore continuous separators.

Pipe Operator |

User pipe operator | to get the primitive scalar value from toml node. You can view | as the vertical form of /, and read it as "or default", means that if the node is invalid or mistype, return the default value in the right hand of |.

let str_val: &str = toml_value().path() / "path" / "to" / "node" | "";
let int_val: i64 = toml_value().path() / "path" / "to" / "node" | 0;
let float_val: f64 = toml_value().path() / "path" / "to" / "node" | 0.0;
let bool_val: bool = toml_value().path() / "path" / "to" / "node" | false;

The type notation such as : i64 is not necessary as it can derived from the right hand of |.

And it is obvious that the | would finalize the / operator chain as it return a value of primitve type.

Put and Push Operator <<

It is used in the mutable pointer version, modify the node it refers to.

Put operator << apply to leaf node. Because a toml leaf node can only hold a scalar value, put operator will overwrite the value it hold, provided that the value type is match with the origin one.

let node = toml_vaule.path_mut() / "path" / "to" / "int-node" << 314;
let node = toml_vaule.path_mut() / "path" / "to" / "float-node" << 3.14;
let _ = toml_vaule.path_mut() / "path" / "to" / "string-node" << "PI";
let _ = toml_vaule.path_mut() / "path" / "to" / "bool-node" << true;

Put operator consume the left pointer and return a new pointer, and may result in None poiner, if the node is non-existed or mistype. Use let _ to depreess warning if not want to use the returned pointer.

Push operator which also overload <<, would push new item to array node or table node.

let node = toml_vaule.path_mut() / "table" << ("int", 314) << ("float", 3.14);
let _ = toml_vaule.path_mut() / "array" << (314,) << [3.14];

As << return also a poiner, the push operator can be chained to append more items to array or table, provided the origin refered array or table is valid. You can push any value type the can convert to toml::Value.

Push Assign Operator <<=

Because in rust cannot oversion assgin operator =, so choose <<= instead, which would has some consistent meanning as previous put or push operator <<. It would re-assign to node unconditionally to any value that can convert to toml::Value.

let node = toml_vaule.path_mut() / "path" / "to" / "bool-node" << true;
node <<= "true";

Note that <<= receive reference to self and won't return new pointer, and should not to chain it as <<. Beside that, the following tow statements have the same effect for leaf node, if << successes:

let node = node << val; // may fail when mistype
node <<= val; // always sucesse expect node already invalid pointer.

Valid Operator !

Overlaod not operator ! which can be used to test if the pointer is invalid. In the fowllong cases would return invalid pointer:

  • path operator / when point to non-existed node.
  • put operator << when try to put value of mistype.

Deref operator * is also overloaded, support to use pointer implicitly as Option<&toml::Value>. And so is_none() method will behave the same as operator !.

let node = toml_value.path() / "path" / "to" / "nil";
if (!node) { return; }
if (node.is_none()) { return; }

However, in mutable verion pointer, the operator ! would consume the pointer and so better use is_none() method.

Save Intermediate Pointer

You can save intermediate pointer as you want, for some reasons:

let root = toml_value.path();
let table = root / "path" / "to" / "table";
let val1 = table / "key1" | 0;
let val2 = table / "key2" | "";

While the mutable pointer may have some limitation, follow the compiler prompt to fix any question.

Path Operator vs Nest Struct

When deal with simple toml, it is fairly convenient to deserialize to some nested struct, and then can use chained dot . operator to access the depper fields.

rust let node = toml_value.path.to.node;

Where in above, suppose path and to and node are all filed of some struct in deffirent level, that can map to the toml keys.

However, this easy way can also be broken easily in some practice case:

  • when some keys(fields) are not determined in compile time but changed in rutime, or just change it's type in runtime.
  • when you want to handle optional field in strict way, and then wrap many fields in Option<>.

Then in these case, it is impossible to keep the simplest path.to.node style any more, and become more tedious than "path"/"to"/"node" way.

While in any cases, it can be used path operator in the consistent way, it may be more straightforward to write business code according to the sample data structure, and the json path syntax is some what language neutral.

The pay off for this flexibility is typo in coding and may lost IDE completion tips.


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