Template typing in task interfaces

[tclose]

What would you like changed/added and why?

The ability to use template types (i.e. typing.TypeVar) when defining task interfaces, e.g.

@mask.task
def mytask(x: typing.T) -> typing.T:
    ...

What would be the benefit? Does the change make something easier to use?

The most important benefit would be to enable you to specify that a task operates on a generic object/files but returns the same type as it was passed

What would be required

• When passed a type-var, TypeParser checks the type-var's "bound-type"
• When defining an output-spec, check that any type-vars referenced in it are referenced in the input-spec
• When initialising inputs, check whether all type-var types can be resolved to a single type (respecting the bound-types of the type-vars)
• When accessing lazy-outputs of a task resolve any type-var types from their resolved values in the input-spec
  • Check that all type-var inputs have been already specified
  • Guard against input template type changes if the lzout attribute has already been generated
• When initialising outputs type-check against their resolved types

[tclose]

@effigies I think this is all that would be required to add support for template typing, probably something that could be looked at in a future hack-week