Computed values¶

A computed value is derived from answers, data and other computed values. It is declared once and available everywhere, so a template does not repeat the same expression in twelve files.

[computed]
package_name = "{{ project_name | lower | replace(' ', '-') }}"
module_name = "{{ package_name | replace('-', '_') }}"
year_range = "{{ start_year }}–{{ end_year }}"

Computed values sit in the same namespace as answers, so a template reads them the same way:

[package]
name = "{{ package_name }}"

They may depend on:

• answers
• other computed values
• loaded data
• template metadata

Ordering¶

You do not declare an order. It is derived from the expressions, the same way question order is — see Questions § Evaluation order.

So this is fine, even though module_name is declared first:

[computed]
module_name = "{{ package_name | replace('-', '_') }}"
package_name = "{{ project_name | lower }}"

And this is an error, reported before any prompt appears:

[computed]
a = "{{ b }}"
b = "{{ a }}"

Cyclic dependency in template `rust-library`.

  computed.a → computed.b → computed.a

Available while prompting¶

A computed value is resolved as soon as its dependencies are, which means a later question can use it:

[questions.project_name]
type = "string"
prompt = "Project name"

[computed]
package_name = "{{ project_name | lower | replace(' ', '-') }}"

[questions.crate_path]
type = "string"
prompt = "Crate directory"
default = "crates/{{ package_name }}"

The sequence is: ask project_name → compute package_name → ask crate_path with crates/<computed> pre-filled.

Values, not strings¶

An expression that produces a single value keeps that value's type:

[computed]
# a boolean, not the string "true"
needs_tokio = "{{ cli and project_type == 'application' }}"

# a list, not "['a', 'b']"
all_features = "{{ data.features.base + extra_features }}"

This matters when the result is used in {% if %}, iterated, or serialised. An expression that interpolates into surrounding text is a string, as you would expect:

[computed]
title = "{{ project_name }} — a Rust library"

Names must not collide¶

A computed value may not have the same name as a question. They share a namespace, so one would silently shadow the other and which one won would depend on evaluation order.

Name collision in template `rust-library`.

  `package_name` is declared both as a question and as a computed value.

  Answers and computed values share one namespace, so a template cannot
  tell them apart. Rename one.

Computed values are not recorded¶

Only answers are written to .config/git.tpl.toml. Computed values are recomputed on every render, by design: they are a function of the answers and the template, and a template that changes how package_name is derived should change it for existing projects too.

Filtering choices¶

A computed value that resolves to a list can be pointed at by choices_from, which is how a question's choices are filtered:

[questions.kind]
type = "choice"
choices = ["library", "application"]

[computed]
servers = "{{ ['nginx', 'caddy'] if kind == 'application' else [] }}"

[questions.server]
type = "choice"
choices_from = "servers"

The graph guarantees kind is answered before servers is computed, and servers before server is asked. If the list comes out empty the question is skipped entirely. See Choices.