Comment by rtfeldman

We recently changed Roc's lambda syntax from the syntax that languages like Elm and Haskell use...

    foo = \arg1, arg2 ->
        body

...to this:

    foo = |arg1, arg2|
        body

The reason for this change was that we have a new and extremely well-received language feature (landed but not yet formally announced) which results in `->` and `=>` having different meanings in the type system. This made it confusing to have `->` in the syntax for anonymous functions, because it seemed to suggest a connection with the type-level `->` that wasn't actually there.

The most popular syntax that mainstream languages use today for anonymous functions is something like `(arg1, arg2) => body` but of course that has the same problem with having an arrow in it, so changing to that wouldn't have solved the problem.

Rust uses `|arg1, arg2| body` (and Ruby kinda uses it too for blocks), and we'd all had fine experiences using that syntax in Rust, so we chose it as the new lambda syntax. You can see the new syntax in the code example at the top of roc-lang.org.