---
title: Typed parameters
description: Declare statically typed pipeline parameters in Nextflow using the params block.
---

# Typed parameters

<AddedInVersion version="25.10" />

Typed parameters let you declare a pipeline's parameters, their types, and their default values in a single `params` block. Nextflow uses the annotations to validate every parameter reference against its declared type.

```nextflow
params {
    // Path to input data.
    input: Path

    // Whether to save intermediate files.
    save_intermeds: Boolean

    // Maximum number of iterations.
    max_iterations: Integer = 100
}

workflow {
    analyze(params.input, params.save_intermeds, params.max_iterations)
}
```

The `params` block does not require the `nextflow.enable.types` feature flag. For an overview of static typing and how to enable it, see [Static typing][static-typing-page]. For the complete syntax reference, see [Params block][syntax-params-block].

## Supported types

Parameters can use any [standard type][stdlib-types]. This includes primitive types such as `Path`, `String`, `Integer`, and `Boolean`, as well as collections, records, and the dataflow types (`Channel` and `Value`).

<AddedInVersion version="26.10" />

A parameter can be declared with a dataflow type:

- A `Channel<E>` parameter accepts a samplesheet path, which Nextflow loads as a channel of records. The samplesheet file can be CSV, JSON, or YAML. The element type must be `Map`, `Record`, or a record type. Each row is validated against and converted to the declared type.

- A `Value<V>` parameter accepts a value of type `V`, which Nextflow wraps in a value channel.

For example:

```nextflow
params {
    samples: Channel<Sample>
    index: Value<Path>
}

workflow {
    RNASEQ(params.samples, params.index)
}

record Sample {
    id: String
    fastq_1: Path
    fastq_2: Path
}
```

The pipeline can be run with a samplesheet and an index file:

```console
$ nextflow run main.nf --samples samples.csv --index genome.fa
```

Dataflow types are useful when a pipeline is [included by another pipeline][pipeline-composition], because they allow the calling pipeline to provide a parameter from dataflow logic.

## Default and required parameters

A parameter can optionally specify a default value:

- A parameter that specifies a default value is *optional*. If no value is supplied at runtime, the default is used.
- A parameter that does not specify a default value is *required*. If a required parameter has no value at runtime, the run fails.

<AddedInVersion version="26.04"/>

Boolean parameters that don't specify a default value default to `false` rather than being treated as required.

Input files declared as `Path` must already exist when the pipeline is launched. Use `String` for file paths that are created by the pipeline.

## Overriding parameters

A parameter's default value can be overridden on the command line, in a params file, or in a config file. When a parameter is set from more than one source, Nextflow resolves the final value in the order described in [Pipeline parameters][cli-params].

Nextflow converts command-line parameters to the type set by their annotation. For example, a parameter declared as `Integer` receives an integer value, not the string entered on the command line.

## Referencing parameters

Typed parameters should only be referenced in the entry workflow or the `output` block. To use a parameter elsewhere, pass it to a workflow or process as an explicit input.

## Validation

The language server validates each parameter reference against its declared type. Referencing a parameter in a context that expects a different type is reported as an error before the pipeline runs.

## See also

- [Static typing][static-typing-page]: Overview of static typing and how to enable it.
- [Standard types][stdlib-types]: The standard types available for annotations.
- [Pipeline parameters][cli-params]: How parameter values are resolved across sources.

[cli-params]: ./cli#pipeline-parameters
[pipeline-composition]: ./workflow-typed#pipeline-composition
[static-typing-page]: ./static-typing
[stdlib-types]: ./reference/stdlib-types
[syntax-params-block]: ./reference/syntax#params-block