it may be a `string` or a `bool` depending on how/where it's used.
Overriding a variable with a complex type can only be done when the type is provided.
This is still done via environment variables, but the values can be provided via CSV or JSON.

#### CSV overrides

This is considered the canonical method and is well suited to interactive usage.
It is assumed that `list` and `set` will be the most common complex type,
as well as the most common complex type designed to be overridden.
Thus, there is full CSV support for `list` and `set`
(and `tuple`; despite being considered a structural type, it is more like a collection type in this regard).


There is limited support for `map` and `object` and no support for composite types;
for these advanced cases, an alternative mechanism [using JSON](#json-overrides) is available.

#### JSON overrides

Overrides can also be provided via JSON.
This is the only method available for providing some complex types and may be convenient if overrides are already JSON
(for example, if they come from a JSON API).
It can also be used when dealing with values are difficult or impossible to specify using CSV (e.g., values containing quotes or commas).
To use JSON, simply append `_JSON` to the variable name.
In this contrived example, CSV cannot handle the second value; despite being a supported CSV type, JSON must be used:
```hcl
variable "VALS" {
  type = list(string)
  default = ["some", "list"]
}
```
```console
$ cat data.json
["hello","with,comma","with\"quote"]
$ VALS_JSON=$(< data.json) docker buildx bake

# CSV equivalent, though the second value cannot be expressed at all 
$ VALS='hello,"with""quote"' docker buildx bake
```

This example illustrates some precedence and usage rules:
```hcl
variable "FOO" {
  type = string
  default = "foo"
}

variable "FOO_JSON" {
  type = string
  default = "foo"
}
```

The variable `FOO` can *only* be overridden using CSV because `FOO_JSON`, which would typically used for a JSON override,
is already a defined variable.
Since `FOO_JSON` is an actual variable, setting that environment variable would be expected to a CSV value.
A JSON override *is* possible for this variable, using environment variable `FOO_JSON_JSON`.

```Console
# These three are all equivalent, setting variable FOO=bar
$ FOO=bar docker buildx bake <...>
$ FOO='bar' docker buildx bake <...>
$ FOO="bar" docker buildx bake <...>

# Sets *only* variable FOO_JSON; FOO is untouched
$ FOO_JSON=bar docker buildx bake <...>

# This also sets FOO_JSON, but will fail due to not being valid JSON
$ FOO_JSON_JSON=bar docker buildx bake <...>

# These are all equivalent
$ cat data.json
"bar"
$ FOO_JSON_JSON=$(< data.json) docker buildx bake <...>
$ FOO_JSON_JSON='"bar"' docker buildx bake <...>
$ FOO_JSON=bar docker buildx bake <...>

# This results in setting two different variables, both specified as CSV (FOO=bar and FOO_JSON="baz")
$ FOO=bar FOO_JSON='"baz"' docker buildx bake <...>