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Many Lints

prefer_from_nullable

v1.0.0 Warning Fix fpdart

This rule flags a conditional that tests a value against null and builds Option.of(value) in one branch and a None in the other.

Why use this rule

Section titled “Why use this rule”

Option.fromNullable makes exactly this decision, so the conditional spells out a step the constructor already performs.

The manual form is not just longer — it names the value twice, once in the condition and once inside the Some. That is where the copy-paste bug lives: name != null ? Option.of(other) : const None() compiles cleanly and quietly wraps the wrong variable, or wraps one that is still nullable.

See also: fpdart: Option.fromNullable

Don’t

Section titled “Don’t”
Option<String> displayName(String? name) =>
    name != null ? Option.of(name) : Option<String>.none();

The inverted spelling is the same thing:

Option<String> displayName(String? name) =>
    name == null ? Option<String>.none() : Option.of(name);

Do

Section titled “Do”
Option<String> displayName(String? name) => Option.fromNullable(name);

optionOf(name) is the shorthand for the same constructor.

Quick fix

Section titled “Quick fix”

A quick fix replaces the whole conditional with Option.fromNullable(value), re-deriving the tested value from the condition. It can be applied across a whole file at once.

Known limitations

Section titled “Known limitations”

The Some branch must wrap the same expression the condition tested — compared by source text. When it wraps something else the rule stays silent, because rewriting it would change behaviour:

// Not reported — tests `name`, wraps `nickname`. This is the copy-paste bug
// the constructor form makes impossible, but the rule cannot tell it from a
// deliberate choice.
Option<String> displayName(String? name, String? nickname) =>
    name != null ? Option.of(nickname) : Option<String>.none();

Only Option is covered. Either.fromNullable takes an onNull callback, so the equivalent conditional carries a value the rewrite would have to invent.

Configuration

Section titled “Configuration”

This rule is in the opinionated preset. With a lower preset, enable it by name with prefer_from_nullable: true.

To turn it off:

many_lints.yaml
rules:
  prefer_from_nullable: false

To keep the rule on but skip certain paths, use per-rule exclude.

Section titled “Related rules”