attr: Consider Adding ValueType Method to Type Interface

[bflad]

Module version

v0.13.0

Use-cases

When the internal reflection package is attempting to build values or convert them into given types, it needs to raise errors when the given target type/value is incompatible. Today, these errors can tell provider developers what went wrong, but not necessarily offer help for how to resolve it.

An example diagnostic that we'd like to raise:

Received unknown value, however the target type cannot handle unknown values. Use the corresponding `types` package type or a custom type that handles unknown values.

Path: example
Target Type: *bool
Suggested Type: types.Bool <-- this (but also working for any attr.Value)

Attempted Solutions

It's technically possible to try to get the associated attr.Value of an attr.Type by doing something like:

attrValue, err := attrType.ValueFromTerraform(ctx, tftypes.NewValue(attrType.TerraformType(ctx), nil))

However, supporting null values in an attr.Type is not a requirement, as edge case-y as that may be, since in all implementations may be Required in the schema.

Proposal

Consider introducing a new method in the attr.Type interface:

type Type interface {
  // ... existing methods ...

  // ValueType should return the attr.Value type returned by ValueFromTerraform.
  // The returned attr.Value can be any null, unknown, or known value for the type,
  // as this is only intended for type detection.
  ValueType(context.Context) attr.Value
}

Or, make it optional, where the reflection package can pick it up if it is detected:

type TypeWithValueType interface {
  // ValueType should return the attr.Value type returned by ValueFromTerraform.
  // The returned attr.Value can be any null, unknown, or known value for the type,
  // as this is only intended for type detection.
  ValueType(context.Context) attr.Value
}

References

• Better error messaging for "unhandled {null,unknown} value" #191
• Improve diagnostics when types that cannot handle unknown or null are used #495

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