How to modify a cty.ObjectVal?

[steeve85]

I am new to Go and to go-cty, and I am wondering how to add a new element into a["type"] based on the following sample:

a := map[string]cty.Value{
	"type": cty.ObjectVal(map[string]cty.Value{
		"name": cty.ObjectVal(map[string]cty.Value{
			"id": cty.StringVal("value"),
		}),
	}),
}

I tried something like a["type"]["test"] = cty.StringVal("value") and a["type"].AsValueMap()["test"] = cty.StringVal("test2") but it didn't work.

How can I add a new element into a["type"]? Thanks

[apparentlymart]

Hi @steeve85,

cty values are, by design, always immutable. Instead of modifying an existing object, you must construct a new object.

If the change you want to make is shallow (that is, it only involves changes to the top-level map or object) then AsValueMap can be part of the solution, but you then need to pass the modified map back to cty.MapVal or cty.ObjectVal (depending on whether you intend to create a map-typed or object-typed value) to construct the new value:

foo1 := cty.ObjectVal(map[string]cty.Value{
  "example1": cty.StringVal("a"),
})
fooMap := foo1.AsValueMap()
fooMap["example2"] = cty.StringVal("b")
foo2 := cty.ObjectVal(fooMap)

If you want to make a change deep inside the structure then things can get more involved: you'll need to reconstruct each level you want to modify one at a time.

cty is designed to be used as part of implementing languages that use a functional programming style with no mutation, so there are no functions for modifying values in-place. In practice applications that use values resulting from such a language, such as Terraform providers consuming the result of evaluating a Terraform configuration, do it by converting from cty.Value to some suitable application-specific data type -- which, in the case of Terraform providers, is mutable -- and then convert that data type back into an entirely new cty.Value to return, so the cty representation is used only for transport to and from the language runtime and not for internal processing.

[steeve85]

@apparentlymart Great, totally make sense. Thank you for the help!