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Funx - Functional Programming Patterns for Elixir

⚠️ Beta: Funx is in active development. APIs may change until version 1.0. Feedback and contributions are welcome.

Official website: https://www.funxlib.com Code and API documentation: https://hex.pm/packages/funx

Breaking Changes in 0.6.0

If you're upgrading from 0.6.0 or earlier, be aware of the module reorganization:

Eq changes

# Change protocol implementations
defimpl Funx.Eq, for: MyStruct          # Old
defimpl Funx.Eq.Protocol, for: MyStruct  # New

# Change imports and aliases
alias Funx.Eq.Utils  # Old
use Funx.Eq.Dsl      # Old

use Funx.Eq          # New (imports eq DSL macro)
alias Funx.Eq        # New (for utility functions)

# Example usage
Eq.contramap(&(&1.age))

Ord changes

# Change protocol implementations
defimpl Funx.Ord, for: MyStruct          # Old
defimpl Funx.Ord.Protocol, for: MyStruct  # New

# Change imports and aliases
alias Funx.Ord.Utils  # Old
use Funx.Ord.Dsl      # Old

use Funx.Ord          # New (imports ord DSL macro)
alias Funx.Ord        # New (for utility functions)

# Example usage
Ord.contramap(&(&1.score))

See the CHANGELOG for more details.

Installation

To use Funx, add it to the list of dependencies in mix.exs:

def deps do
  [
    {:funx, "~> 0.8"}
  ]
end

Then, run the following command to fetch the dependencies:

mix deps.get

Usage Rules

Funx includes embedded usage rules in addition to API documentation.
They are written for development workflows assisted by LLMs.

Equality

The Eq protocol defines how two values are compared, making equality explicit and adaptable to your domain.

• Define what “equal” means—compare by ID, name, or any derived attribute.
• Compose multiple comparisons—require all to match or just one.
• Implement for structs, built-in types, or custom comparators.

Ordering

The Ord protocol defines ordering relationships in a structured way, without relying on Elixir's built-in comparison operators.

• Define comparisons based on properties like size, age, or priority.
• Chain orderings to create fallback tiebreakers.
• Implement for any type, including custom structs.

Ord DSL

The Ord module includes a DSL for building custom ordering comparators declaratively:

use Funx.Ord

user_ord = ord do
  desc :priority
  asc :name
  desc :created_at
end

Enum.sort(users, Funx.Ord.comparator(user_ord))

Features:

• Multiple projections with asc and desc directions
• Support for optics (Lens, Prism), functions, and modules
• Ord variables for composing and reversing orderings

Eq DSL

The Eq module includes a DSL for building equality comparators with boolean logic:

use Funx.Eq

contact_eq = eq do
  on :name
  any do
    on :email
    on :username
  end
end

Funx.Eq.eq?(user1, user2, contact_eq)

Features:

• on - Field must be equal
• diff_on - Field must differ (non-equivalence constraint)
• all blocks - All checks must pass (AND logic)
• any blocks - At least one check must pass (OR logic)
• Support for optics, functions, and custom comparators

Monads

Monads encapsulate computations, allowing operations to be chained while handling concerns like optional values, failures, dependencies, or deferred effects.

• Identity: Wraps a value with no additional behavior—useful for organizing transformations.
• Maybe: Represents optional data using Just for presence and Nothing for absence.
• Either: Models computations with two possibilities—Left and Right.
• Effect: Encapsulates deferred execution with error handling, similar to Task.
• Reader: Passes an immutable environment through a computation for dependency injection or configuration.
• Writer: Threads a log alongside a result using any monoid—useful for tracing, reporting, or accumulating metadata during computation.

Either DSL

The Either monad includes a DSL for writing declarative pipelines that handle errors gracefully:

use Funx.Monad.Either

either user_id do
  bind fetch_user()
  bind validate_active()
  map transform_to_dto()
end

Supported operations:

• bind - for operations that return Either or result tuples
• map - for transformations that return plain values
• ap - for applying a function in an Either to a value in an Either
• validate - for accumulating multiple validation errors
• Either functions: filter_or_else, or_else, map_left, flip, tap

Formatter Configuration: Funx exports formatter rules for clean DSL formatting. Add :funx to import_deps in your .formatter.exs:

[
  import_deps: [:funx],
  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
]

See FORMATTER_EXPORT.md for details.

Optics

Optics provide composable, lawful abstractions for focusing on and transforming parts of data structures.

• Lens: Total optic for required fields—raises if focus is missing. Use for fields that should always exist.
• Prism: Partial optic for optional fields—returns Maybe. Use for fields that may be absent or for selecting struct types.
• Traversal: Optic for accessing multiple foci simultaneously. Use for filtering collections, combining multiple optics, or working with list-like structures.
• Iso: Total optic for reversible representation changes. Use when two shapes carry the same information and you need guaranteed round trip conversion (view then review).

Monoids

Monoids combine values using an associative operation and an identity element. They are useful for accumulation, selection, and combining logic.

• Sum: Adds numbers (0 is the identity).
• Product: Multiplies numbers (1 is the identity).
• Eq.All: Values are equal only if all comparators agree.
• Eq.Any: Values are equal if any comparator agrees.
• Predicate.All: All predicates must hold.
• Predicate.Any: At least one predicate must hold.
• Ord: Defines ordering compositionally.
• Max and Min: Select the largest or smallest value by custom ordering.
• ListConcat: Concatenates lists ([] is the identity).
• StringConcat: Concatenates strings ("" is the identity).

Predicates

Predicates are functions that return true or false. Funx provides combinators for composing them cleanly.

• p_and: Returns true if both predicates pass.
• p_or: Returns true if either predicate passes.
• p_not: Negates a predicate.
• p_all: Returns true if all predicates in a list pass.
• p_any: Returns true if any predicate in a list passes.
• p_none: Returns true if none pass.

Pred DSL

The Predicate module includes a DSL for building boolean predicates declaratively:

use Funx.Predicate
alias Funx.Predicate.{GreaterThanOrEqual, IsTrue, In}

check_eligible = pred do
  check :age, {GreaterThanOrEqual, value: 18}
  check :verified, IsTrue
  check :role, {In, values: [:admin, :moderator]}
end

Enum.filter(users, check_eligible)

Features:

• check - Project into a field and test with a predicate
• negate - Invert a predicate (logical NOT)
• all/any blocks - AND/OR logic for complex predicates
• Built-in predicates: Eq, In, GreaterThan, LessThan, MinLength, Pattern, Required, IsTrue, and more
• Support for optics (Lens, Prism), functions, and behaviour modules

Validation

The Validate module provides declarative data validation with applicative error accumulation—all validators run and all errors are collected.

Validate DSL

use Funx.Validate
alias Funx.Monad.Either
alias Funx.Validator.{Required, Email, MinLength, Positive}

user_validation =
  validate do
    at :name, [Required, {MinLength, min: 3}]
    at :email, [Required, Email]
    at :age, Positive
  end

Either.validate(%{name: "Alice", email: "alice@example.com", age: 30}, user_validation)
# => %Right{right: %{name: "Alice", email: "alice@example.com", age: 30}}

Either.validate(%{name: "", email: "bad", age: -5}, user_validation)
# => %Left{left: %ValidationError{errors: ["is required", "must be at least 3 characters", ...]}}

Features:

• at :field, Validator - Field validation using Prism (optional by default)
• at [:a, :b], Validator - Nested path validation
• at Lens.key(:field), Validator - Required field (raises if missing)
• Multiple validators: [Required, Email] or {MinLength, min: 3}
• Root validators for whole-structure validation
• Environment passing for context-dependent validation
• Composable validators that can be nested and reused

Built-in validators: Required, Email, MinLength, MaxLength, Pattern, Positive, Negative, Integer, GreaterThan, LessThan, In, NotIn, Range, Each, Confirmation, Not

Folding

The Foldable protocol defines how to reduce a structure to a single result.

• fold_l: Reduces from the left, applying functions in order.
• fold_r: Reduces from the right, applying functions in reverse.

Useful for accumulating values, transforming collections, or extracting data.

Filtering

The Filterable protocol defines how to conditionally retain values within a context.

• guard: Keeps a value if a condition is met; otherwise returns an empty context.
• filter: Retains values that satisfy a predicate.
• filter_map: Applies a transformation and keeps results only when the transformed value is present.

Sequencing

Sequencing runs a series of mona