Convert Clojure to Elm

Convert Clojure code to idiomatic Elm. This skill extends meta-convert-dev with Clojure-to-Elm specific type mappings, idiom translations, and architectural patterns.

This Skill Extends

• meta-convert-dev - Foundational conversion patterns (APTV workflow, testing strategies)

For general concepts like the Analyze → Plan → Transform → Validate workflow, testing strategies, and common pitfalls, see the meta-skill first.

This Skill Adds

• Type mappings: Dynamic Clojure types → Static Elm types
• Architecture translation: REPL-driven development → The Elm Architecture (TEA)
• Effect handling: Side effects anywhere → Cmd/Sub managed effects
• Null handling: nil → Maybe/Nothing pattern
• Error handling: Exceptions → Result types
• Data structures: Persistent collections → Immutable records
• Macro translation: Compile-time macros → Type-driven design

This Skill Does NOT Cover

• General conversion methodology - see meta-convert-dev
• Clojure language fundamentals - see lang-clojure-dev
• Elm language fundamentals - see lang-elm-dev
• Reverse conversion (Elm → Clojure) - see convert-elm-clojure
• ClojureScript → Elm (similar but with JS runtime considerations)

---

Quick Reference

Clojure	Elm	Notes
nil	Nothing	Explicit Maybe type
{:key "value"}	{ key = "value" }	Records are explicit types
(defn f [x] ...)	f : a -> b f x = ...	Type signatures required
(map f coll)	List.map f list	Module-qualified
(try ... (catch ...))	Result type	No exceptions
(atom 0)	Model in TEA	State in architecture
(assoc m :k v)	{ model | k = v }	Record update syntax
(get m :k)	model.k	Field access
(or x y)	Maybe.withDefault y x	Explicit Maybe handling
(when pred ...)	if pred then ... else ...	Always need else

---

8 Pillars Validation

Before converting, verify coverage of the 8 pillars essential for code conversion:

Pillar	lang-clojure-dev	lang-elm-dev	Coverage
Module System	✓ ns, :require	✓ module, import	Green
Error Handling	✓ try/catch, ex-info	✓ Result, Maybe	Green
Concurrency Model	✓ atoms, refs, agents	✓ Cmd/Sub, Task	Green
Metaprogramming	✓ Macros, syntax quote	✓ No macros (intentional)	Green
Zero/Default Values	~ nil everywhere	✓ Explicit Maybe, no null	Green
Serialization	✓ EDN, JSON, Transit, spec	✓ JSON decoders/encoders	Green
Build/Deps	✓ Leiningen, deps.edn	✓ elm.json, elm install	Green
Testing	✓ clojure.test, test.check	✓ elm-test, fuzz testing	Green

Status: Green (8/8 pillars covered)

Recommendation: Proceed with conversion - both skills have comprehensive coverage.

---

When Converting Code

1. Define types first - Clojure is dynamic, Elm requires explicit types for everything
2. Map nil → Maybe - Identify all nullable values and make them explicit
3. Extract pure functions - Separate logic from side effects
4. Design TEA architecture - Model, View, Update before writing code
5. Handle all cases - Elm's exhaustive pattern matching replaces runtime checks
6. No runtime errors - Elm guarantees no null pointers, no type mismatches

---

Type System Mapping

Primitive Types

Clojure	Elm	Notes
nil	Nothing	Part of Maybe type
true/false	True/False	Capitalized in Elm
"string"	"string"	Strings are identical
42 (integer)	42 : Int	Explicit type
3.14 (double)	3.14 : Float	Explicit type
:keyword	No direct equivalent	Use String or custom type
'symbol	No equivalent	Use String or custom type

Collection Types

Clojure	Elm	Notes
[1 2 3] (vector)	[ 1, 2, 3 ] : List Int	Lists are linked, not vectors
'(1 2 3) (list)	[ 1, 2, 3 ] : List Int	Same as vector in Elm
{:a 1 :b 2} (map)	{ a = 1, b = 2 }	Record with type alias
#{1 2 3} (set)	Set.fromList [ 1, 2, 3 ]	Import Set module
[x y] (tuple)	( x, y )	Parentheses, max 3 elements
(seq coll)	list	All lists are lazy-ish in Elm

Composite Types

Clojure	Elm	Notes
{:name "Alice" :age 30}	type alias User = { name : String, age : Int }	Explicit type required
(defrecord User [name age])	type alias User = { name : String, age : Int }	Type alias for records
(deftype Point [x y])	type Point = Point Float Float	Custom type
Tagged literal	type Msg = Clicked | Typed String	Union types
nil-able value	Maybe User	Explicit optional

Function Types

Clojure	Elm	Notes
(defn f [x] ...)	f : a -> b	Type signature required
(fn [x] ...)	\x -> ...	Anonymous function
#(* % 2)	\x -> x * 2	Lambda syntax
Multi-arity (defn f ([x] ...) ([x y] ...))	Separate functions	No multi-arity
Variadic (defn f [& args])	List a parameter	No varargs

---

Idiom Translation

Pattern: Dynamic Map → Typed Record

Clojure:

;; Maps are dynamic - any keys, any values
(def user {:name "Alice" :email "alice@example.com"})

(defn greet [user]
  (str "Hello, " (:name user)))

(defn update-email [user email]
  (assoc user :email email))

Elm:

-- Records have explicit types - all fields known at compile time
type alias User =
    { name : String
    , email : String
    }

greet : User -> String
greet user =
    "Hello, " ++ user.name

updateEmail : User -> String -> User
updateEmail user email =
    { user | email = email }

Why this translation:

• Elm requires all record fields to be declared in a type alias
• Field access is compile-time checked
• No runtime key errors possible
• Type inference helps but explicit types are idiomatic

---

Pattern: nil Handling → Maybe

Clojure:

;; nil can appear anywhere
(defn find-user [id users]
  (first (filter #(= (:id %) id) users)))
  ;; Returns user or nil

(defn display-name [user]
  (or (:name user) "Anonymous"))

Elm:

-- Explicit Maybe for optional values
findUser : Int -> List User -> Maybe User
findUser id users =
    users
        |> List.filter (\u -> u.id == id)
        |> List.head  -- Returns Maybe User

displayName : Maybe User -> String
displayName maybeUser =
    maybeUser
        |> Maybe.map .name
        |> Maybe.withDefault "Anonymous"

Why this translation:

• Elm has no null/nil - compiler forces you to handle absence
• Maybe makes optionality explicit in type signatures
• Pattern matching ensures all cases are handled
• No NullPointerException possible

---

Pattern: Sequence Operations

Clojure:

;; Lazy sequences with threading
(->> data
     (filter active?)
     (map :value)
     (reduce +))

;; List comprehension
(for [x (range 10)
      :when (even? x)]
  (* x 2))

Elm:

-- List pipeline (not lazy by default)
data
    |> List.filter active
    |> List.map .value
    |> List.foldl (+) 0

-- No list comprehension - use map/filter
List.range 0 9
    |> List.filter (\x -> modBy 2 x == 0)
    |> List.map (\x -> x * 2)

Why this translation:

• Elm's pipeline operator |> is similar to Clojure's ->>
• Elm lists are not lazy (except for user-defined streams)
• Use explicit List module functions
• No for comprehension - compose map/filter instead

---

Pattern: Atoms/State → TEA Model

Clojure:

;; Mutable state with atom
(def counter (atom 0))

(defn increment! []
  (swap! counter inc))

(defn get-count []
  @counter)

Elm:

-- Immutable model in TEA
type alias Model =
    { counter : Int
    }

type Msg
    = Increment

update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        Increment ->
            ( { model | counter = model.counter + 1 }, Cmd.none )

-- No direct "get" - view is pure function of model
view : Model -> Html Msg
view model =
    div [] [ text (String.fromInt model.counter) ]

Why this translation:

• Elm has no mutable state - all state in Model
• Updates happen through messages (Msg type)
• State changes are pure functions: Msg -> Model -> (Model, Cmd Msg)
• View is always pure function: Model -> Html Msg

---

Pattern: Macros → Type-Driven Design

Clojure:

;; Macro for compile-time abstraction
(defmacro unless [condition & body]
  `(if (not ~condition)
     (do ~@body)))

(unless false
  (println "This runs"))

Elm:

-- No macros - use types and functions instead
unless : Bool -> (() -> a) -> Maybe a
unless condition thunk =
    if not condition then
        Just (thunk ())
    else
        Nothing

-- Or just use if directly (more idiomatic)
if not condition then
    Debug.log "This runs"
else
    ()

Why this translation:

• Elm has no macros - all code is explicit
• Use higher-order functions for abstraction
• Phantom types encode compile-time constraints
• Code generation (elm-codegen) for repetitive code

---

Pattern: Destructuring

Clojure:

;; Map destructuring
(let [{:keys [name age]} user]
  (str name " is " age))

;; Vector destructuring
(let [[first & rest] coll]
  (process first rest))

Elm:

-- Record destructuring
let
    { name, age } = user
in
    name ++ " is " ++ String.fromInt age

-- List pattern matching (not destructuring)
case list of
    first :: rest ->
        process first rest

    [] ->
        -- Must handle empty list
        defaultValue

Why this translation:

• Elm's record destructuring is similar to Clojure's map destructuring
• List destructuring requires pattern matching with case
• Must handle all cases - compiler enforces exhaustiveness
• Can destructure in function parameters: greet { name } = ...

---

Error Handling

Clojure Exception Model → Elm Result Model

Clojure uses exceptions:

(defn parse-int [s]
  (try
    (Integer/parseInt s)
    (catch NumberFormatException e
      (throw (ex-info "Invalid number" {:input s})))))

(defn divide [a b]
  (if (zero? b)
    (throw (ex-info "Division by zero" {:a a :b b}))
    (/ a b)))

Elm uses Result type:

parseInt : String -> Result String Int
parseInt s =
    String.toInt s
        |> Result.fromMaybe ("Invalid number: " ++ s)

divide : Float -> Float -> Result String Float
divide a b =
    if b == 0 then
        Err "Division by zero"
    else
        Ok (a / b)

-- Chaining Results
parseAndDivide : String -> String -> Result String Float
parseAndDivide numStr denomStr =
    Result.map2 divide
        (parseInt numStr)
        (parseInt denomStr)
        |> Result.andThen identity

Why this approach:

• Elm has no exceptions - all errors are values
• Result type makes error handling explicit
• Compiler forces you to handle Err case
• No try/catch needed - pattern match on Result

Error Propagation

Clojure:

;; Exceptions bubble up automatically
(defn process-user [id]
  (let [user (fetch-user id)        ;; May throw
        validated (validate user)   ;; May throw
        saved (save-user validated)] ;; May throw
    saved))

Elm:

-- Results must be explicitly chained
processUser : Int -> Cmd Msg
processUser id =
    fetchUser id
        |> Task.andThen validateUser
        |> Task.andThen saveUser
        |> Task.attempt ProcessUserComplete

-- Or with Result in update:
case fetchUser id of
    Ok user ->
        case validateUser user of
            Ok validated ->
                case saveUser validated of
                    Ok saved ->
                        ( { model | user = Just saved }, Cmd.none )

                    Err saveError ->
                        ( { model | error = Just saveError }, Cmd.none )

            Err validationError ->
                ( { model | error = Just validationError }, Cmd.none )

    Err fetchError ->
        ( { model | error = Just fetchError }, Cmd.none)

-- Better: use Result helpers
processUser : Int -> Result String User
processUser id =
    fetchUser id
        |> Result.andThen validateUser
        |> Result.andThen saveUser

Translation strategy:

1. Map Clojure exceptions to Elm Result types
2. Use Result.andThen for sequential error handling
3. Use Result.map2/map3 for combining multiple Results
4. Pattern match in update function to handle errors

---

Architecture Translation

REPL-Driven Development → The Elm Architecture (TEA)

Clojure approach:

;; Direct interaction with state
(def app-state (atom {:count 0 :users []}))

;; Functions mutate state directly
(defn increment! []
  (swap! app-state update :count inc))

(defn add-user! [user]
  (swap! app-state update :users conj user))

;; REPL experimentation
(increment!)
@app-state ;; => {:count 1 :users []}

Elm approach (TEA):

-- 1. Define Model (all state)
type alias Model =
    { count : Int
    , users : List User
    }

-- 2. Define Msg (all possible actions)
type Msg
    = Increment
    | AddUser User

-- 3. Define Update (pure state transitions)
update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        Increment ->
            ( { model | count = model.count + 1 }, Cmd.none )

        AddUser user ->
            ( { model | users = model.users ++ [ user ] }, Cmd.none )

-- 4. Define View (pure render function)
view : Model -> Html Msg
view model =
    div []
        [ div [] [ text ("Count: " ++ String.fromInt model.count) ]
        , button [ onClick Increment ] [ text "+" ]
        , div [] (List.map viewUser model.users)
        ]

Translation strategy:

1. Identify Clojure atoms/refs → Elm Model fields
2. Map mutation functions → Msg constructors
3. Extract pure logic into update branches
4. Build view as pure function of Model

---

Side Effects → Cmd and Sub

Clojure (side effects anywhere):

(defn fetch-and-save-user [id]
  (let [user (http/get (str "/api/users/" id))  ;; Side effect
        parsed (json/parse-string (:body user))] ;; Pure
    (db/save! parsed)  ;; Side effect
    parsed))

Elm (managed effects):

-- Side effects ONLY through Cmd
type Msg
    = FetchUser Int
    | GotUser (Result Http.Error User)
    | SaveUser User
    | UserSaved (Result Http.Error ())

update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        FetchUser id ->
            ( { model | loading = True }
            , Http.get
                { url = "/api/users/" ++ String.fromInt id
                , expect = Http.expectJson GotUser userDecoder
                }
            )

        GotUser (Ok user) ->
            ( model, saveUserCmd user )

        GotUser (Err error) ->
            ( { model | error = Just error }, Cmd.none )

        SaveUser user ->
            ( model, saveUserCmd user )

        UserSaved (Ok ()) ->
            ( { model | saved = True }, Cmd.none )

        UserSaved (Err error) ->
            ( { model | error = Just error }, Cmd.none )

-- Cmd constructors
saveUserCmd : User -> Cmd Msg
saveUserCmd user =
    Http.post
        { url = "/api/users"
        , body = Http.jsonBody (encodeUser user)
        , expect = Http.expectWhatever UserSaved
        }

Translation strategy:

1. Extract all side effects → Cmd in update
2. Define Msg for each async result
3. Chain effects through Msg flow
4. Use Task for sequential async operations

---

Common Pitfalls

1. Assuming Dynamic Typing

Problem: Treating Elm like dynamically-typed Clojure

-- ❌ WRONG: Can't have heterogeneous lists
users = [ { name = "Alice" }, { name = "Bob", age = 30 } ]
-- ERROR: Record fields must match

-- ✓ CORRECT: Define explicit type with Maybe for optional fields
type alias User =
    { name : String
    , age : Maybe Int
    }

users =
    [ { name = "Alice", age = Nothing }
    , { name = "Bob", age = Just 30 }
    ]

2. Forgetting to Handle All Cases

Problem: Incomplete pattern matching

-- ❌ WRONG: Missing Nothing case
getName : Maybe User -> String
getName maybeUser =
    case maybeUser of
        Just user ->
            user.name
-- ERROR: Missing pattern: Nothing

-- ✓ CORRECT: Handle all cases
getName : Maybe User -> String
getName maybeUser =
    case maybeUser of
        Just user ->
            user.name

        Nothing ->
            "Anonymous"

3. Trying to Mutate State

Problem: Thinking of Elm Model like Clojure atom

-- ❌ WRONG: Can't mutate model
update msg model =
    model.count = model.count + 1  -- ERROR: No assignment in Elm
    ( model, Cmd.none )

-- ✓ CORRECT: Create new record with updated field
update msg model =
    ( { model | count = model.count + 1 }, Cmd.none )

4. Expecting Macros

Problem: Looking for Clojure-style macros

-- ❌ WRONG: No macros in Elm
-- Can't write:
-- defmacro myWhen [condition & body] ...

-- ✓ CORRECT: Use functions or code generation
myWhen : Bool -> (() -> a) -> Maybe a
myWhen condition thunk =
    if condition then
        Just (thunk ())
    else
        Nothing

-- Or just use if directly (more idiomatic)

5. Missing Type Annotations

Problem: Relying too much on type inference

-- ❌ BAD: No type signature
add x y =
    x + y
-- Inferred type might not be what you want

-- ✓ GOOD: Explicit type signature
add : Int -> Int -> Int
add x y =
    x + y

6. Forgetting Else Branch

Problem: Clojure when has no else; Elm if requires it

;; Clojure: when has implicit nil
(when condition
  (do-thing))

-- ❌ WRONG: Missing else
if condition then
    doThing
-- ERROR: if needs else branch

-- ✓ CORRECT: Provide else (even if unit)
if condition then
    doThing
else
    ()

---

Tooling

Tool	Purpose	Notes
elm-format	Auto-format Elm code	Like cljfmt
elm-test	Unit and property testing	Similar to test.check
elm-review	Custom linting rules	Like clj-kondo
elm-json	Package manager	Like lein or deps.edn
elm-live	Development server with hot reload	Like figwheel
elm-analyse (deprecated)	Code analysis	Use elm-review instead
elm-codegen	Generate Elm code	For repetitive boilerplate

---

Examples

Example 1: Simple - Data Transformation

Before (Clojure):

;; Transform list of maps
(defn process-users [users]
  (->> users
       (filter :active)
       (map :name)
       (map str/upper-case)))

(process-users [{:name "alice" :active true}
                {:name "bob" :active false}
                {:name "charlie" :active true}])
;; => ("ALICE" "CHARLIE")

After (Elm):

-- Transform list of records
type alias User =
    { name : String
    , active : Bool
    }

processUsers : List User -> List String
processUsers users =
    users
        |> List.filter .active
        |> List.map .name
        |> List.map String.toUpper

-- Usage
processUsers
    [ { name = "alice", active = True }
    , { name = "bob", active = False }
    , { name = "charlie", active = True }
    ]
-- => [ "ALICE", "CHARLIE" ]

---

Example 2: Medium - Error Handling with HTTP

Before (Clojure):

(require '[clj-http.client :as http]
         '[cheshire.core :as json])

(defn fetch-user [id]
  (try
    (-> (http/get (str "https://api.example.com/users/" id))
        :body
        (json/parse-string true))
    (catch Exception e
      (println "Error fetching user:" (.getMessage e))
      nil)))

(defn display-user [id]
  (if-let [user (fetch-user id)]
    (str "User: " (:name user))
    "User not found"))

After (Elm):

import Http
import Json.Decode as Decode exposing (Decoder)

-- Model includes loading states
type alias Model =
    { user : RemoteData Http.Error User
    }

type RemoteData error value
    = NotAsked
    | Loading
    | Success value
    | Failure error

-- Messages for async flow
type Msg
    = FetchUser Int
    | GotUser (Result Http.Error User)

-- User decoder
userDecoder : Decoder User
userDecoder =
    Decode.map2 User
        (Decode.field "name" Decode.string)
        (Decode.field "email" Decode.string)

-- Update handles side effects
update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        FetchUser id ->
            ( { model | user = Loading }
            , Http.get
                { url = "https://api.example.com/users/" ++ String.fromInt id
                , expect = Http.expectJson GotUser userDecoder
                }
            )

        GotUser (Ok user) ->
            ( { model | user = Success user }, Cmd.none )

        GotUser (Err error) ->
            ( { model | user = Failure error }, Cmd.none )

-- View renders based on state
view : Model -> Html Msg
view model =
    case model.user of
        NotAsked ->
            button [ onClick (FetchUser 1) ] [ text "Load User" ]

        Loading ->
            div [] [ text "Loading..." ]

        Success user ->
            div [] [ text ("User: " ++ user.name) ]

        Failure error ->
            div [] [ text "User not found" ]

---

Example 3: Complex - Full Application with State Management

Before (Clojure):

(ns app.core
  (:require [clojure.string :as str]))

;; Application state
(def state (atom {:users []
                  :search ""
                  :filter :all}))

;; State mutations
(defn add-user! [user]
  (swap! state update :users conj user))

(defn set-search! [query]
  (swap! state assoc :search query))

(defn set-filter! [filter-type]
  (swap! state assoc :filter filter-type))

;; Pure logic
(defn matches-search? [user query]
  (str/includes? (str/lower-case (:name user))
                 (str/lower-case query)))

(defn matches-filter? [user filter-type]
  (case filter-type
    :all true
    :active (:active user)
    :inactive (not (:active user))))

(defn filtered-users [users search filter-type]
  (->> users
       (filter #(matches-search? % search))
       (filter #(matches-filter? % filter-type))))

;; Usage
(add-user! {:name "Alice" :active true})
(add-user! {:name "Bob" :active false})
(set-search! "ali")
(set-filter! :active)

(let [{:keys [users search filter]} @state]
  (filtered-users users search filter))
;; => ({:name "Alice" :active true})

After (Elm):

module Main exposing (main)

import Browser
import Html exposing (Html, button, div, input, text)
import Html.Attributes exposing (placeholder, value)
import Html.Events exposing (onClick, onInput)

-- MODEL

type alias User =
    { name : String
    , active : Bool
    }

type FilterType
    = All
    | Active
    | Inactive

type alias Model =
    { users : List User
    , search : String
    , filter : FilterType
    }

init : () -> ( Model, Cmd Msg )
init _ =
    ( { users = []
      , search = ""
      , filter = All
      }
    , Cmd.none
    )

-- UPDATE

type Msg
    = AddUser User
    | SetSearch String
    | SetFilter FilterType

update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        AddUser user ->
            ( { model | users = model.users ++ [ user ] }, Cmd.none )

        SetSearch query ->
            ( { model | search = query }, Cmd.none )

        SetFilter filterType ->
            ( { model | filter = filterType }, Cmd.none )

-- VIEW HELPERS (Pure logic)

matchesSearch : String -> User -> Bool
matchesSearch query user =
    String.contains
        (String.toLower query)
        (String.toLower user.name)

matchesFilter : FilterType -> User -> Bool
matchesFilter filterType user =
    case filterType of
        All ->
            True

        Active ->
            user.active

        Inactive ->
            not user.active

filteredUsers : Model -> List User
filteredUsers model =
    model.users
        |> List.filter (matchesSearch model.search)
        |> List.filter (matchesFilter model.filter)

-- VIEW

view : Model -> Html Msg
view model =
    div []
        [ div []
            [ input
                [ placeholder "Search users..."
                , value model.search
                , onInput SetSearch
                ]
                []
            ]
        , div []
            [ button [ onClick (SetFilter All) ] [ text "All" ]
            , button [ onClick (SetFilter Active) ] [ text "Active" ]
            , button [ onClick (SetFilter Inactive) ] [ text "Inactive" ]
            ]
        , div []
            [ button
                [ onClick (AddUser { name = "Alice", active = True }) ]
                [ text "Add Alice" ]
            , button
                [ onClick (AddUser { name = "Bob", active = False }) ]
                [ text "Add Bob" ]
            ]
        , div []
            (filteredUsers model
                |> List.map viewUser
            )
        ]

viewUser : User -> Html Msg
viewUser user =
    div []
        [ text (user.name ++ if user.active then " ✓" else " ✗")
        ]

-- MAIN

main : Program () Model Msg
main =
    Browser.element
        { init = init
        , update = update
        , view = view
        , subscriptions = \_ -> Sub.none
        }

---

See Also

For more examples and patterns, see:

• meta-convert-dev - Foundational patterns with cross-language examples
• convert-elm-clojure - Reverse conversion (Elm → Clojure)
• lang-clojure-dev - Clojure development patterns
• lang-elm-dev - Elm development patterns

Cross-cutting pattern skills:

• patterns-concurrency-dev - Async, state management across languages
• patterns-serialization-dev - JSON, validation, encoding/decoding
• patterns-metaprogramming-dev - Compile-time abstractions across languages