169 lines
9.4 KiB
Go
169 lines
9.4 KiB
Go
// Copyright 2020 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package json implements semantic processing of JSON as specified in RFC 8259.
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// JSON is a simple data interchange format that can represent
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// primitive data types such as booleans, strings, and numbers,
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// in addition to structured data types such as objects and arrays.
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//
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// [Marshal] and [Unmarshal] encode and decode Go values
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// to/from JSON text contained within a []byte.
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// [MarshalWrite] and [UnmarshalRead] operate on JSON text
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// by writing to or reading from an [io.Writer] or [io.Reader].
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// [MarshalEncode] and [UnmarshalDecode] operate on JSON text
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// by encoding to or decoding from a [jsontext.Encoder] or [jsontext.Decoder].
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// [Options] may be passed to each of the marshal or unmarshal functions
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// to configure the semantic behavior of marshaling and unmarshaling
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// (i.e., alter how JSON data is understood as Go data and vice versa).
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// [jsontext.Options] may also be passed to the marshal or unmarshal functions
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// to configure the syntactic behavior of encoding or decoding.
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//
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// The data types of JSON are mapped to/from the data types of Go based on
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// the closest logical equivalent between the two type systems. For example,
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// a JSON boolean corresponds with a Go bool,
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// a JSON string corresponds with a Go string,
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// a JSON number corresponds with a Go int, uint or float,
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// a JSON array corresponds with a Go slice or array, and
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// a JSON object corresponds with a Go struct or map.
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// See the documentation on [Marshal] and [Unmarshal] for a comprehensive list
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// of how the JSON and Go type systems correspond.
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//
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// Arbitrary Go types can customize their JSON representation by implementing
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// [Marshaler], [MarshalerTo], [Unmarshaler], or [UnmarshalerFrom].
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// This provides authors of Go types with control over how their types are
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// serialized as JSON. Alternatively, users can implement functions that match
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// [MarshalFunc], [MarshalToFunc], [UnmarshalFunc], or [UnmarshalFromFunc]
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// to specify the JSON representation for arbitrary types.
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// This provides callers of JSON functionality with control over
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// how any arbitrary type is serialized as JSON.
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//
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// # JSON Representation of Go structs
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//
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// A Go struct is naturally represented as a JSON object,
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// where each Go struct field corresponds with a JSON object member.
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// When marshaling, all Go struct fields are recursively encoded in depth-first
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// order as JSON object members except those that are ignored or omitted.
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// When unmarshaling, JSON object members are recursively decoded
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// into the corresponding Go struct fields.
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// Object members that do not match any struct fields,
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// also known as “unknown members”, are ignored by default or rejected
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// if [RejectUnknownMembers] is specified.
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//
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// The representation of each struct field can be customized in the
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// "json" struct field tag, where the tag is a comma separated list of options.
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// As a special case, if the entire tag is `json:"-"`,
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// then the field is ignored with regard to its JSON representation.
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// Some options also have equivalent behavior controlled by a caller-specified [Options].
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// Field-specified options take precedence over caller-specified options.
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//
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// The first option is the JSON object name override for the Go struct field.
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// If the name is not specified, then the Go struct field name
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// is used as the JSON object name. JSON names containing commas or quotes,
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// or names identical to "" or "-", can be specified using
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// a single-quoted string literal, where the syntax is identical to
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// the Go grammar for a double-quoted string literal,
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// but instead uses single quotes as the delimiters.
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// By default, unmarshaling uses case-sensitive matching to identify
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// the Go struct field associated with a JSON object name.
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//
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// After the name, the following tag options are supported:
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//
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// - omitzero: When marshaling, the "omitzero" option specifies that
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// the struct field should be omitted if the field value is zero
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// as determined by the "IsZero() bool" method if present,
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// otherwise based on whether the field is the zero Go value.
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// This option has no effect when unmarshaling.
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//
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// - omitempty: When marshaling, the "omitempty" option specifies that
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// the struct field should be omitted if the field value would have been
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// encoded as a JSON null, empty string, empty object, or empty array.
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// This option has no effect when unmarshaling.
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//
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// - string: The "string" option specifies that [StringifyNumbers]
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// be set when marshaling or unmarshaling a struct field value.
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// This causes numeric types to be encoded as a JSON number
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// within a JSON string, and to be decoded from a JSON string
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// containing the JSON number without any surrounding whitespace.
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// This extra level of encoding is often necessary since
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// many JSON parsers cannot precisely represent 64-bit integers.
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//
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// - case: When unmarshaling, the "case" option specifies how
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// JSON object names are matched with the JSON name for Go struct fields.
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// The option is a key-value pair specified as "case:value" where
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// the value must either be 'ignore' or 'strict'.
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// The 'ignore' value specifies that matching is case-insensitive
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// where dashes and underscores are also ignored. If multiple fields match,
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// the first declared field in breadth-first order takes precedence.
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// The 'strict' value specifies that matching is case-sensitive.
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// This takes precedence over the [MatchCaseInsensitiveNames] option.
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//
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// - inline: The "inline" option specifies that
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// the JSON representable content of this field type is to be promoted
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// as if they were specified in the parent struct.
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// It is the JSON equivalent of Go struct embedding.
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// A Go embedded field is implicitly inlined unless an explicit JSON name
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// is specified. The inlined field must be a Go struct
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// (that does not implement any JSON methods), [jsontext.Value],
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// map[~string]T, or an unnamed pointer to such types. When marshaling,
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// inlined fields from a pointer type are omitted if it is nil.
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// Inlined fields of type [jsontext.Value] and map[~string]T are called
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// “inlined fallbacks” as they can represent all possible
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// JSON object members not directly handled by the parent struct.
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// Only one inlined fallback field may be specified in a struct,
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// while many non-fallback fields may be specified. This option
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// must not be specified with any other option (including the JSON name).
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//
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// - unknown: The "unknown" option is a specialized variant
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// of the inlined fallback to indicate that this Go struct field
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// contains any number of unknown JSON object members. The field type must
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// be a [jsontext.Value], map[~string]T, or an unnamed pointer to such types.
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// If [DiscardUnknownMembers] is specified when marshaling,
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// the contents of this field are ignored.
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// If [RejectUnknownMembers] is specified when unmarshaling,
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// any unknown object members are rejected regardless of whether
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// an inlined fallback with the "unknown" option exists. This option
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// must not be specified with any other option (including the JSON name).
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//
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// - format: The "format" option specifies a format flag
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// used to specialize the formatting of the field value.
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// The option is a key-value pair specified as "format:value" where
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// the value must be either a literal consisting of letters and numbers
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// (e.g., "format:RFC3339") or a single-quoted string literal
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// (e.g., "format:'2006-01-02'"). The interpretation of the format flag
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// is determined by the struct field type.
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//
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// The "omitzero" and "omitempty" options are mostly semantically identical.
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// The former is defined in terms of the Go type system,
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// while the latter in terms of the JSON type system.
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// Consequently they behave differently in some circumstances.
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// For example, only a nil slice or map is omitted under "omitzero", while
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// an empty slice or map is omitted under "omitempty" regardless of nilness.
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// The "omitzero" option is useful for types with a well-defined zero value
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// (e.g., [net/netip.Addr]) or have an IsZero method (e.g., [time.Time.IsZero]).
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//
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// Every Go struct corresponds to a list of JSON representable fields
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// which is constructed by performing a breadth-first search over
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// all struct fields (excluding unexported or ignored fields),
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// where the search recursively descends into inlined structs.
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// The set of non-inlined fields in a struct must have unique JSON names.
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// If multiple fields all have the same JSON name, then the one
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// at shallowest depth takes precedence and the other fields at deeper depths
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// are excluded from the list of JSON representable fields.
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// If multiple fields at the shallowest depth have the same JSON name,
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// but exactly one is explicitly tagged with a JSON name,
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// then that field takes precedence and all others are excluded from the list.
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// This is analogous to Go visibility rules for struct field selection
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// with embedded struct types.
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//
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// Marshaling or unmarshaling a non-empty struct
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// without any JSON representable fields results in a [SemanticError].
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// Unexported fields must not have any `json` tags except for `json:"-"`.
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package json
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// requireKeyedLiterals can be embedded in a struct to require keyed literals.
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type requireKeyedLiterals struct{}
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// nonComparable can be embedded in a struct to prevent comparability.
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type nonComparable [0]func()
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