English for Elm Developers

Elm’s strict architecture and famously friendly compiler produce a distinct vocabulary — “The Elm Architecture,” “message,” “no runtime exceptions” — that’s worth using precisely, since Elm developers treat these as concrete guarantees, not marketing phrases. This guide covers the English used when discussing Elm code with a team.

Key Vocabulary

The Elm Architecture (TEA) — the pattern of Model, Update, View that every Elm application follows: state lives in one Model, changes flow through Update, and View renders purely from Model. “This component is reaching outside The Elm Architecture to mutate a value directly — everything needs to flow through a Msg and the Update function instead.”

Message (Msg) — a value describing something that happened (a button click, a server response), the only way state changes are triggered in Elm; there’s no direct mutation. “Instead of updating the model inline inside the view function — which Elm won’t even let you do — dispatch a Msg and handle the state change in Update.”

No runtime exceptions — Elm’s compiler guarantee that a compiling program cannot crash with a null reference, undefined function, or type mismatch at runtime. “We don’t need defensive null checks scattered through this view — Elm’s compiler already guarantees this value can’t be missing, or it wouldn’t have compiled.”

Decoder — a value describing how to parse and validate untyped JSON into a specific typed Elm value, explicitly handling the boundary between untyped external data and Elm’s type system. “This API response isn’t guaranteed to match our assumed shape — write a proper decoder with fallback handling instead of assuming the field will always be present.”

Command (Cmd) — a value describing a side effect (an HTTP request, generating a random number) that Elm’s runtime executes and reports back as a Msg, keeping Update itself pure. “Don’t try to make an HTTP call directly inside Update — return a Cmd describing the request, and let the runtime report the result back as its own Msg.”

Subscription (Sub) — a declaration of external events (WebSocket messages, timers) an Elm application wants to be notified about, delivered as messages just like user interactions. “Add this as a subscription rather than polling manually — the runtime will deliver each WebSocket message as a Msg exactly when it arrives.”

Common Phrases

• “Is this state change flowing through a proper Msg and Update, or is something bypassing the architecture?”
• “Does this decoder handle the case where the field is missing or has an unexpected shape?”
• “Should this side effect be a Cmd, so Update itself stays pure?”
• “Would a subscription be cleaner here than manually polling for this event?”
• “Does the compiler actually guarantee this value can’t be missing, or are we assuming it based on the API docs?”

Example Sentences

Reviewing a pull request: “This decoder assumes the email field is always present, but the API docs mark it optional — use Json.Decode.maybe here instead of failing the whole decode on one missing field.”

Explaining a design decision: “We modeled the multi-step form as one Msg type with explicit variants per step, so Update stays a single exhaustive pattern match instead of scattered boolean flags.”

Describing an incident: “The unexpected blank screen traced back to a decoder silently failing on an unexpected API shape — since Elm has no runtime exceptions, the whole page just showed the initial model instead of crashing.”

Professional Tips

• Say “no runtime exceptions” precisely when explaining why Elm skips certain defensive checks — it’s a compiler guarantee, not an assumption, and worth stating as such.
• Name “The Elm Architecture” explicitly when a change seems to bypass expected data flow — it signals exactly which convention is being violated.
• Use “decoder” rather than “parser” when discussing JSON boundaries — it’s Elm’s precise term and distinguishes it from general string parsing.
• Distinguish Cmd from Sub clearly — one is a one-off side effect request, the other is an ongoing subscription to external events; conflating them causes design confusion.

Practice Exercise

1. Explain in two sentences why Elm has no runtime exceptions if a program compiles successfully.
2. Write a one-sentence code review comment flagging a decoder that assumes a field is always present.
3. Describe, in your own words, the difference between a Cmd and a Sub.