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Specification 6.0 · Not released · Tested product 5.21.3

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    Library · std.json

    JSONValue.kind

    Returns the outermost kind of a JSON value.

    kind() -> string
    Module
    std.json
    Receiver
    JSONValue
    Minimum arity
    0
    Maximum arity
    0

    Declaration and parameters

    Link

    Availability

    Included by default: Pending

    Import std.json in language mode 5.21.

    The current compiler selects std/json.tpz for this language mode. Availability in other released editions and inclusion in each installed distribution are Pending.

    Source evidence

    Receiver

    JSONValue

    Parameter constraints

    Additional contracts are Pending.

    JSONValue.kind() -> string returns exactly one of null, bool, number, string, array, or object. The name describes the JSON value's outermost kind. Both true and false have kind bool. Integer and fractional JSON numbers have kind number.

    Test whether a JSON value is null with value.kind() == "null". This comparison is false for false, zero, the empty string, and empty arrays and objects. Reading the kind does not modify or decode the JSON value.

    A present array element or object member containing JSON null is still present. at and get return Some(value) containing that JSONValue. An absent element or member returns None(). Inspect presence first, then compare the contained value's kind.

    JSONValue does not expose an isNull member. Use the same kind comparison for null as for every other JSON kind.

    Related rule: Inspect JSON kinds with kind

    Result

    Additional contracts are Pending.

    The declared result type is string.

    Effects

    Pending

    Failures

    Pending

    Result and effects

    Link

    The result is null, bool, number, string, array, or object. Compare it with "null" to test for JSON null. This query preserves the value and distinguishes stored null from an absent array element or object member.

    Why

    Link

    One query covers every JSON kind, and ordinary equality expresses each kind test.

    Related entries

    Link

    std.json

    Inspect JSON kinds with kind

    Link
    Ruletopaz.stdlib.contract.json-kind.rule

    Inspect JSON kinds with kind

    JSONValue.kind() -> string returns exactly one of null, bool, number, string, array, or object. The name describes the JSON value's outermost kind. Both true and false have kind bool. Integer and fractional JSON numbers have kind number.

    Test whether a JSON value is null with value.kind() == "null". This comparison is false for false, zero, the empty string, and empty arrays and objects. Reading the kind does not modify or decode the JSON value.

    A present array element or object member containing JSON null is still present. at and get return Some(value) containing that JSONValue. An absent element or member returns None(). Inspect presence first, then compare the contained value's kind.

    JSONValue does not expose an isNull member. Use the same kind comparison for null as for every other JSON kind.

    Why

    One kind query distinguishes every JSON value kind. Comparing that result uses the ordinary equality rule for null, booleans, numbers, strings, arrays, and objects. A separate null predicate would duplicate one case and give it a second spelling. Keeping presence separate also prevents a stored null from being mistaken for a missing entry.

    Examples Distinguish null from every other JSON kindKeep present null values distinct from missing entriesReject the removed JSONValue.isNull memberUse kind to test a JSON value for null

    Diagnostics TPZ5006

    Source evidence

    Distinguish null from every other JSON kind

    Link

    run · running · Verification: not-run

    main.tpzDistinguish null from every other JSON kindOpen in Playground
    1. import std.json
    2. import std.test
    3. import std.io
    5. for source in (["null", "false", "0", "\"\"", "[]", "\{\}"]) {
    6. let value = test.assertOk(json.parse(source))
    7. let kind = value.kind()
    8. let nullKind = kind == "null"
    9. io.writeLine("{kind}:{nullKind}")
    10. }
    Expected outputexit 0
    null:true
bool:false
number:false
string:false
array:false
object:false
    $ topaz run main.tpz · target product 5.21.3
    Expected result

    Expected result

    Execution completes. Exit code 0.

    No stderr.

    Expected stdout
    1. null:true
    2. bool:false
    3. number:false
    4. string:false
    5. array:false
    6. object:false

    This exact example has not been executed.

    Source evidence

    Keep present null values distinct from missing entries

    Link

    run · running · Verification: not-run

    main.tpzKeep present null values distinct from missing entriesOpen in Playground
    1. import std.json
    2. import std.test
    3. import std.io
    5. let array = test.assertOk(json.parse("[null]"))
    6. let object = test.assertOk(json.parse("\{\"present\":null\}"))
    7. let arrayValue = test.assertSome(array.at(0))
    8. let objectValue = test.assertSome(object.get("present"))
    9. test.assertNone(array.at(1))
    10. test.assertNone(object.get("absent"))
    11. io.writeLine("{arrayValue.kind()}:{objectValue.kind()}")
    Expected outputexit 0
    null:null
    $ topaz run main.tpz · target product 5.21.3
    Expected result

    Expected result

    Execution completes. Exit code 0.

    No stderr.

    Expected stdout
    1. null:null

    This exact example has not been executed.

    Source evidence

    Reject the removed JSONValue.isNull member

    Link

    reject · checking · Verification: not-run

    main.tpzReject the removed JSONValue.isNull memberOpen in Playground
    1. import std.json { JSONValue }
    3. function inspect(value: JSONValue) -> () {
    4. let result = value.isNull()
    5. return ()
    6. }
    Expected outputNonzero exit
    No stdout.
    $ topaz check main.tpz --format json · target product 5.21.3
    Expected result

    Expected result

    Rejection during check. Nonzero exit.

    • TPZ5006 stdlib-removed-member at main.tpz:4:22

    No stdout.

    This exact example has not been executed.

    Source evidence

    Use kind to test a JSON value for null

    Link

    check · checking · Verification: not-run

    main.tpzUse kind to test a JSON value for nullOpen in Playground
    1. import std.json { JSONValue }
    3. function inspect(value: JSONValue) -> () {
    4. let result = value.kind() == "null"
    5. return ()
    6. }
    Expected outputexit 0
    No stdout.
    $ topaz check main.tpz --format json · target product 5.21.3
    Expected result

    Expected result

    Checking accepts the program. Exit code 0.

    No stderr.

    No stdout.

    This exact example has not been executed.

    Source evidence

    Diagnostics

    Link

    Unknown member or field

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