Great questions! Let’s break down the two separate roles of the | symbol in your Elm code—they look the same but serve completely different purposes.

1. | in Case Expressions: Pattern Matching Branch Separator #

In Elm’s case expression, the | acts as a separator for pattern-matching branches. It lets you list all possible patterns your value could match, each paired with the code to execute when that pattern is hit.

In your snippet:

case msg of
    Change newContent -> { model | content = newContent }

If you had more Msg variants (like a Submit action), you’d add another branch with |:

case msg of
    Change newContent -> { model | content = newContent }
    Submit -> { model | content = "Submitted!" }

Each | signals "here’s another possible pattern to check against the value (msg in this case)". Elm evaluates branches in order, runs the code for the first matching pattern, and stops.

2. { model | content = newContent }: Immutable Record Update Syntax #

This is Elm’s record update syntax—and despite sharing the | symbol, it has no connection to the case expression’s use of the character.

Here’s exactly what this line does:

• It creates a new copy of the model record (Elm is purely functional, so all data is immutable—you can’t modify the original model directly).
• It preserves all existing fields of model exactly as they are.
• It only overwrites the content field with the value of newContent.

To address your specific follow-ups:

• It does not bind newContent to both model and content. The original model stays unchanged; this syntax generates a brand-new Model instance with an updated content field.
• The | here is just part of the record update syntax—it separates the original record (model) from the list of fields you want to update. It’s purely a syntax choice, unrelated to the case branch separator.

For example, if your Model had extra fields like id : Int, this syntax would keep the id intact while updating content:

type alias Model = { id : Int, content : String }
-- Creates new record with same id, updated content
{ model | content = newContent } 

内容的提问来源于stack exchange，提问作者Saurabh kukade