Destructuring objects within other objects

[arlyon]

There seems to be a 'feature hole' for destructuring objects within other objects. In python:

> x = {"a": 1, "b": 2}
> {"c": 3, **x}
{'c': 3, 'a': 1, 'b': 2}

This is useful if using top-level functions for your definitions. Say I'd like to create a TLA that dynamically composes other objects:

local clients = import 'compose/clients.libsonnet';
local servers = import 'compose/servers.libsonnet';

// generate a config for a given client / server config
function(client, server) {
    local clientFn = clients[client.name],
    local serverFn = servers[server.name],

    local clientCompose = clientFn(client.version, client.config),
    local serverCompose = serverFn(server.version, server.config),
    local dependency = {services+: {client+: {depends_on+: ["frontend"]}}},

    **(clientCompose + serverCompose + dependency)
}

The alternative (some dictionary expression) doesn't work.

function(client, server) {
    ...
    local compose = clientCompose + serverCompose + dependency,
    [key]: compose[key] for key in std.objectFields(compose)
}

compose.jsonnet:14:52-59 Unknown variable: compose

    [key]: compose[key] for key in std.objectFields(compose)

Originally posted by @arlyon in #307 (comment)

[sbarzowski]

The "for" part in object comprehension doesn't have access to object locals (they are scope "inside" the for and have access to the variable you're iterating over). This is why you get the error.

The problem with a flattening operator like you describe, is that objects in Jsonnet are more than dicts in Python. In Python dicts are just mapping from key to value and here we have objects with OOP sense with self and super.

I think you can achieve your goal with just plain inheritance (+ operator):

local clients = import 'compose/clients.libsonnet';
local servers = import 'compose/servers.libsonnet';

// generate a config for a given client / server config
function(client, server)
    local clientFn = clients[client.name];
    local serverFn = servers[server.name];
    local clientCompose = clientFn(client.version, client.config);
    local serverCompose = serverFn(server.version, server.config);
    local dependency = {services+: {client+: {depends_on+: ["frontend"]}}};
    clientCompose + serverCompose + dependency + { whatever_additional_field_I_want: 42 }

Please let me know if that works for you.

[arlyon]

Hi thanks for the info, maybe I should have added it to the initial description but this example reproduces the syntax error in the trivial case:

function(a, b) {
    local thisA = {a: a},
    local thisB = {b: b},

    thisA + thisB
}

❯ jsonnet tla.jsonnet --tla-str a="hello" --tla-str b="world"
STATIC ERROR: tla.jsonnet:5:5: expected one of :, ::, :::, +:, +::, +:::, got: +

[sbarzowski]

Yes. Please take a look at where the curly braces are placed.

This works:

local fun(a, b) =
    local thisA = {a: a};
    local thisB = {b: b};
    thisA + thisB;
fun(1, 2)

The curly braces are not a part of the function syntax – they are a part of the object literal syntax. You have no enclosing object here, so no curly braces.

[arlyon]

Thanks for the quick reply. It seems it is not possible to apply this syntax to a top level function. Omitting the local keyword produces a new syntax error:

function(a, b) =
  local thisA = { a: a };
  local thisB = { b: b };
  thisA + thisB;

❯ jsonnet tla.jsonnet --tla-str a="hello" --tla-str b="world"
STATIC ERROR: tla.jsonnet:1:16: not a unary operator: =

edit: solved it, thanks for your help

function(a, b)
  local thisA = { a: a };
  local thisB = { b: b };
  thisA + thisB