# Query package

## Overview

Package query performs JSONPath-like queries on a TOML document.

The query path implementation is based loosely on the JSONPath specification:
http://goessner.net/articles/JsonPath/.

The idea behind a query path is to allow quick access to any element, or set
of elements within TOML document, with a single expression.

```go
result, err := query.CompileAndExecute("$.foo.bar.baz", tree)
```

This is roughly equivalent to:

```go
next := tree.Get("foo")
if next != nil {
    next = next.Get("bar")
    if next != nil {
        next = next.Get("baz")
    }
}
result := next
```

err is nil if any parsing exception occurs.

If no node in the tree matches the query, result will simply contain an empty list of
items.

As illustrated above, the query path is much more efficient, especially since
the structure of the TOML file can vary. Rather than making assumptions about
a document's structure, a query allows the programmer to make structured
requests into the document, and get zero or more values as a result.

## Query syntax

The syntax of a query begins with a root token, followed by any number
sub-expressions:

```
$
                    Root of the TOML tree. This must always come first.
.name
                    Selects child of this node, where 'name' is a TOML key
                    name.
['name']
                    Selects child of this node, where 'name' is a string
                    containing a TOML key name.
[index]
                    Selcts child array element at 'index'.
..expr
                    Recursively selects all children, filtered by an a union,
                    index, or slice expression.
..*
                    Recursive selection of all nodes at this point in the
                    tree.
.*
                    Selects all children of the current node.
[expr,expr]
                    Union operator - a logical 'or' grouping of two or more
                    sub-expressions: index, key name, or filter.
[start:end:step]
                    Slice operator - selects array elements from start to
                    end-1, at the given step. All three arguments are
                    optional.
[?(filter)]
                    Named filter expression - the function 'filter' is
                    used to filter children at this node.
```

## Query Indexes And Slices

Index expressions perform no bounds checking, and will contribute no
values to the result set if the provided index or index range is invalid.
Negative indexes represent values from the end of the array, counting backwards.

```go
// select the last index of the array named 'foo'
query.CompileAndExecute("$.foo[-1]", tree)
```

Slice expressions are supported, by using ':' to separate a start/end index pair.

```go
// select up to the first five elements in the array
query.CompileAndExecute("$.foo[0:5]", tree)
```

Slice expressions also allow negative indexes for the start and stop
arguments.

```go
// select all array elements except the last one.
query.CompileAndExecute("$.foo[0:-1]", tree)
```

Slice expressions may have an optional stride/step parameter:

```go
// select every other element
query.CompileAndExecute("$.foo[0::2]", tree)
```

Slice start and end parameters are also optional:

```go
// these are all equivalent and select all the values in the array
query.CompileAndExecute("$.foo[:]", tree)
query.CompileAndExecute("$.foo[::]", tree)
query.CompileAndExecute("$.foo[::1]", tree)
query.CompileAndExecute("$.foo[0:]", tree)
query.CompileAndExecute("$.foo[0::]", tree)
query.CompileAndExecute("$.foo[0::1]", tree)
```

## Query Filters

Query filters are used within a Union [,] or single Filter [] expression.
A filter only allows nodes that qualify through to the next expression,
and/or into the result set.

```go
// returns children of foo that are permitted by the 'bar' filter.
query.CompileAndExecute("$.foo[?(bar)]", tree)
```

There are several filters provided with the library:

```
tree
        Allows nodes of type Tree.
int
        Allows nodes of type int64.
float
        Allows nodes of type float64.
string
        Allows nodes of type string.
time
        Allows nodes of type time.Time.
bool
        Allows nodes of type bool.
```

## Query Results

An executed query returns a Result object. This contains the nodes
in the TOML tree that qualify the query expression. Position information
is also available for each value in the set.

```go
// display the results of a query
results := query.CompileAndExecute("$.foo.bar.baz", tree)
for idx, value := results.Values() {
    fmt.Println("%v: %v", results.Positions()[idx], value)
}
```

## Compiled Queries

Queries may be executed directly on a Tree object, or compiled ahead
of time and executed discretely. The former is more convenient, but has the
penalty of having to recompile the query expression each time.

```go
// basic query
results := query.CompileAndExecute("$.foo.bar.baz", tree)

// compiled query
query, err := toml.Compile("$.foo.bar.baz")
results := query.Execute(tree)

// run the compiled query again on a different tree
moreResults := query.Execute(anotherTree)
```

## User Defined Query Filters

Filter expressions may also be user defined by using the SetFilter()
function on the Query object. The function must return true/false, which
signifies if the passed node is kept or discarded, respectively.

```go
// create a query that references a user-defined filter
query, _ := query.Compile("$[?(bazOnly)]")

// define the filter, and assign it to the query
query.SetFilter("bazOnly", func(node interface{}) bool{
    if tree, ok := node.(*Tree); ok {
        return tree.Has("baz")
    }
    return false // reject all other node types
})

// run the query
query.Execute(tree)
```
