exists

Used in a template function to create a condition that is true if a matching fact exists.

static exists<T>(
    template: Partial<T>
): Condition<T>;

Parameters

template - A JSON object with the desired type and predecessors

Returns

A condition that can be used in suchThat or not

Examples

Return only facts that have a specified successor.

function publishedPostsInFolder(f) {
    return j.match({
        type: 'Blog.Post',
        folder: f
    }).suchThat(postIsPublished);
}

function postIsPublished(p) {
    return j.exists({
        type: 'Blog.Post.Published',
        post: p
    });
}

Return only facts that do not have a specified successor.

function messagesInChannel(c) {
    return j.match({
        type: 'Chat.Message',
        channel: c
    }).suchThat(j.not(messageIsRedacted));
}

function messageIsRedacted(m) {
    return j.exists({
        type: 'Chat.Message.Redacted',
        message: m
    });
}

The above can be expressed in terms of notExists. The result is the same.

function messagesInChannel(c) {
    return j.match({
        type: 'Chat.Message',
        channel: c
    }).suchThat(messageIsNotRedacted);
}

function messageIsNotRedacted(m) {
    return j.notExists({
        type: 'Chat.Message.Redacted',
        message: m
    });
}

Continue With

notExists