Elm FAQ

This document is obsolete. It has been moved to elm-community.github.io/elm-faq and is being maintained there.

Here are some common questions that I’ve seen on the Elm IRC channel and on the mailing lists. Contributions to this document are welcome.

Why does elm-repl (or elm-make) report “cannot find module ‘Html’”?

You need to install the Html module:

elm package install evancz/elm-html

Several modules are available by default in the base Elm tools but other common modules like Html have to be installed in the working directory before they can be used in elm-make, elm-repl, and elm-reactor.

Why does elm-repl report a syntax problem for any type annotation, such as for `fib : Int -> Int`?

elm-repl does not support type annotations at all. If you try you’ll see a “syntax problem” message saying something like “I ran into something unexpected when parsing your code!” and pointing to the colon.

To use type annotations you will have to use elm-reactor or elm-lang.org/try, or build and run a complete app.

How do I know what package name to use for `elm package install`?

Search on package.elm-lang.org for the module name and use the package name that you find there.

How can I write debugging information to the console?

Wrap any value with Debug.log “some message” and that message and the value will be written to the javascript console every time the value is evaluated. For example:

case Debug.log "action" action of

If you want to just log a message and value without using that value, try this in a let clause:

_ = Debug.log "my message" someValue

How do I generate an Action as an Effect?

Effects.task (Task.succeed SomeAction)

How do I install an older version of Elm, 0.15.1 for example?

npm install -g elm@0.15.1

If you need to switch between multiple versions of elm, consider elmenv.

What does `=>` mean?

A common idiom is to define the (=>) operator as a synonym for the (,) operator that constructs tuples. This makes a shorthand for long lists of tuple pairs, often used with the Html.style property. So ["color" => "red", "padding" => "2px"] means [("color", "red"), ("padding", "2px")].

How can I output literal HTML and avoid escaping of entities?

Use the innerHTML property. For example:

span [ property "innerHTML" (Json.Encode.string "&copy;") ] []

What does `()` mean?

It is the empty tuple or unit type. It serves much like “void”, “null”, or “None” in other languages.

What good is the `<|` operator if it is just function application?

It has lower precedence than function application expressed by adjacency (e.g. sqrt x) and so it can be used instead of parentheses to group function arguments. For example, a b (c d) is the same as a b <| c d. More concretely, max 3 (sqrt x) can be written as max 3 <| sqrt x.

What are the Elm operator precedences?

See an Elm operator precedence table. See also Basics.elm.

Why isn’t my StartApp-based program running any tasks?

You need to set app.port.

port tasks : Signal (Task.Task Never ())
port tasks =
    app.tasks

Why doesn’t the `<~` operator work?

It was removed in Elm version 0.16. You might use andMap from Signal.Extra instead.

How can I use multiple Elm programs on the same page?

You can compile multiple modules into a single elm.js and then instantiate whatever module you need on the appropriate div. [ref]

elm-make A.elm B.elm --output elm.js

Why does elm-repl (or elm-make) report “cannot find module ‘Html’”?

Why does elm-repl report a syntax problem for any type annotation, such as for fib : Int -> Int?

How do I know what package name to use for elm package install?