Feature request: literal type

New feature

Hi, I would like to request the addition of a literal type to Nextflow, used to restrict a variable to a small set of specific values. A similar type already exists in Python.

Use case

This is generally useful, but it would especially help with parameter validation, in cases where the parameter must come from a list of options (e.g. the legacy publish_dir_mode, or step in nf-core/sarek).

Suggested implementation

This is largely based on the Python implementation

Definition

The contract allows Literal<v> or Literal<v1, v2, ...>, where v and vn are constants or literals. When multiple vn are passed, they must be of the same type. v cannot be a dataflow (channel or value).

Examples:

• x: Literal<42> = 42 --> correct
• x: Literal<42> = 55 --> error
• y: Literal<"a", "b", "c"> = "b" --> also correct

Allowed constant types

Literals are intended for "atomic" constants, so support for arbitrary values is not required.

The following types are expected to work with literals: Boolean, Duration, Float, Integer, MemoryUnit, Path, String, VersionNumber.

The following types are not expected to work with literals: Iterable and its subtypes (List, Set, etc.), Map, Record.

The following types are categorically forbidden in literals: Channel, Value.

Nested literals

Nested literals are flattened before evaluation. For example:

Literal<Literal<1>, Literal<2>> === Literal<1, 2>

The resulting literal must comply with all assumptions for non-nested literals, in particular type equality.

Behaviour

A variable of type Literal<v> behaves like the type of v. In particular, operations on literals (except setting to a value) return the underlying type. For example:

x: Literal[1] = 1
y: Literal[2] = 2
z = x + y
// the type of z is `Integer`

Equivalence

Literals are equivalent if their underlying values are of the same type and are equal.

Params

Parameters of type Literal[v] default to v when no default value is passed (although this is useless in practice). Parameters of type Literal[v1, v2, ...] default to null.

The issue names no implementation files, tests, or entry points. Start by locating Nextflow's type-checking and parameter-validation code, then compare its existing type behavior with the linked Python Literal specification. Done means Literal values follow the stated allowed types, nesting, equivalence, assignment, operations, and parameter-default rules, with coverage for the examples and restrictions described.