Mutator: Mutator

In muttest: Mutation Testing

Mutator

Description

Mutator

Mutator

Details

Represents a single code mutation — a pattern to find and a replacement to apply. Every mutator function (operator(), boolean_literal(), etc.) returns an instance of this class.

Public fields

from

The token or operator to replace.

to

The replacement token or operator.

query

Tree-sitter query used to locate candidate nodes.

match_fn

Optional ⁠function(node_text)⁠ returning logical; overrides the default node_text == from equality check.

replacement_fn

Optional ⁠function(node_text)⁠ returning a string; overrides the static to value as the replacement text.

mutate_fn

Optional ⁠function(code)⁠ that fully replaces the default mutation logic when set.

Methods

Public methods

• Mutator$new()

• Mutator$mutate()

• Mutator$print()

• Mutator$clone()

---

Method new()

Create a new Mutator.

Usage

Mutator$new(
  from,
  to,
  query,
  match_fn = NULL,
  replacement_fn = NULL,
  mutate_fn = NULL
)

Arguments

from

Token to replace.

to

Replacement token.

query

Tree-sitter query string.

match_fn

Optional custom match function.

replacement_fn

Optional custom replacement function.

mutate_fn

Optional ⁠function(code)⁠ that fully overrides the default mutation logic.

---

Method mutate()

Apply this mutator to a character vector of source lines.

Usage

Mutator$mutate(code)

Arguments

code

Character vector of source lines.

Returns

A list of mutated code variants (one per match), or NULL if the pattern was not found.

---

Method print()

Print a short summary of the mutator.

Usage

Mutator$print()

---

Method clone()

The objects of this class are cloneable with this method.

Usage

Mutator$clone(deep = FALSE)

Arguments

deep

Whether to make a deep clone.

muttest documentation built on May 14, 2026, 5:10 p.m.