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176 lines
6.1 KiB
Go
176 lines
6.1 KiB
Go
// Package query performs JSONPath-like queries on a TOML document.
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//
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// The query path implementation is based loosely on the JSONPath specification:
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// http://goessner.net/articles/JsonPath/.
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//
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// The idea behind a query path is to allow quick access to any element, or set
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// of elements within TOML document, with a single expression.
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//
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// result, err := query.CompileAndExecute("$.foo.bar.baz", tree)
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//
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// This is roughly equivalent to:
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//
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// next := tree.Get("foo")
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// if next != nil {
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// next = next.Get("bar")
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// if next != nil {
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// next = next.Get("baz")
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// }
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// }
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// result := next
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//
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// err is nil if any parsing exception occurs.
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//
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// If no node in the tree matches the query, result will simply contain an empty list of
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// items.
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//
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// As illustrated above, the query path is much more efficient, especially since
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// the structure of the TOML file can vary. Rather than making assumptions about
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// a document's structure, a query allows the programmer to make structured
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// requests into the document, and get zero or more values as a result.
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//
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// Query syntax
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//
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// The syntax of a query begins with a root token, followed by any number
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// sub-expressions:
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//
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// $
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// Root of the TOML tree. This must always come first.
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// .name
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// Selects child of this node, where 'name' is a TOML key
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// name.
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// ['name']
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// Selects child of this node, where 'name' is a string
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// containing a TOML key name.
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// [index]
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// Selcts child array element at 'index'.
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// ..expr
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// Recursively selects all children, filtered by an a union,
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// index, or slice expression.
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// ..*
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// Recursive selection of all nodes at this point in the
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// tree.
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// .*
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// Selects all children of the current node.
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// [expr,expr]
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// Union operator - a logical 'or' grouping of two or more
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// sub-expressions: index, key name, or filter.
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// [start:end:step]
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// Slice operator - selects array elements from start to
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// end-1, at the given step. All three arguments are
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// optional.
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// [?(filter)]
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// Named filter expression - the function 'filter' is
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// used to filter children at this node.
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//
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// Query Indexes And Slices
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//
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// Index expressions perform no bounds checking, and will contribute no
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// values to the result set if the provided index or index range is invalid.
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// Negative indexes represent values from the end of the array, counting backwards.
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//
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// // select the last index of the array named 'foo'
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// query.CompileAndExecute("$.foo[-1]", tree)
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//
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// Slice expressions are supported, by using ':' to separate a start/end index pair.
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//
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// // select up to the first five elements in the array
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// query.CompileAndExecute("$.foo[0:5]", tree)
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//
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// Slice expressions also allow negative indexes for the start and stop
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// arguments.
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//
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// // select all array elements.
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// query.CompileAndExecute("$.foo[0:-1]", tree)
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//
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// Slice expressions may have an optional stride/step parameter:
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//
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// // select every other element
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// query.CompileAndExecute("$.foo[0:-1:2]", tree)
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//
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// Slice start and end parameters are also optional:
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//
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// // these are all equivalent and select all the values in the array
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// query.CompileAndExecute("$.foo[:]", tree)
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// query.CompileAndExecute("$.foo[0:]", tree)
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// query.CompileAndExecute("$.foo[:-1]", tree)
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// query.CompileAndExecute("$.foo[0:-1:]", tree)
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// query.CompileAndExecute("$.foo[::1]", tree)
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// query.CompileAndExecute("$.foo[0::1]", tree)
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// query.CompileAndExecute("$.foo[:-1:1]", tree)
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// query.CompileAndExecute("$.foo[0:-1:1]", tree)
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//
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// Query Filters
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//
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// Query filters are used within a Union [,] or single Filter [] expression.
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// A filter only allows nodes that qualify through to the next expression,
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// and/or into the result set.
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//
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// // returns children of foo that are permitted by the 'bar' filter.
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// query.CompileAndExecute("$.foo[?(bar)]", tree)
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//
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// There are several filters provided with the library:
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//
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// tree
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// Allows nodes of type Tree.
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// int
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// Allows nodes of type int64.
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// float
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// Allows nodes of type float64.
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// string
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// Allows nodes of type string.
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// time
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// Allows nodes of type time.Time.
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// bool
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// Allows nodes of type bool.
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//
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// Query Results
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//
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// An executed query returns a Result object. This contains the nodes
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// in the TOML tree that qualify the query expression. Position information
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// is also available for each value in the set.
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//
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// // display the results of a query
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// results := query.CompileAndExecute("$.foo.bar.baz", tree)
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// for idx, value := results.Values() {
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// fmt.Println("%v: %v", results.Positions()[idx], value)
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// }
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//
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// Compiled Queries
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//
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// Queries may be executed directly on a Tree object, or compiled ahead
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// of time and executed discretely. The former is more convenient, but has the
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// penalty of having to recompile the query expression each time.
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//
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// // basic query
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// results := query.CompileAndExecute("$.foo.bar.baz", tree)
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//
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// // compiled query
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// query, err := toml.Compile("$.foo.bar.baz")
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// results := query.Execute(tree)
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//
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// // run the compiled query again on a different tree
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// moreResults := query.Execute(anotherTree)
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//
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// User Defined Query Filters
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//
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// Filter expressions may also be user defined by using the SetFilter()
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// function on the Query object. The function must return true/false, which
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// signifies if the passed node is kept or discarded, respectively.
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//
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// // create a query that references a user-defined filter
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// query, _ := query.Compile("$[?(bazOnly)]")
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//
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// // define the filter, and assign it to the query
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// query.SetFilter("bazOnly", func(node interface{}) bool{
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// if tree, ok := node.(*Tree); ok {
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// return tree.Has("baz")
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// }
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// return false // reject all other node types
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// })
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//
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// // run the query
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// query.Execute(tree)
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//
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package query
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