1. I know what lambda calculus is, and I didn't even make the connection between \ and lambda. It's pretty tenuous.
2. Most programmers do not know what lambda calculus is. This is supposed to be a friendly language, not an obscure academic one.
3. It's not even the same lambda as in lambda calculus, e.g. it takes multiple arguments.
4. Lambda was a reasonable choice in lambda calculus since it's a very "mathsy" language, and it's pretty much the only symbol in the language. It's a pretty awful choice for a practical programming language though - there's a reason 99% of languages use something like `fn` or `fun` or `function` or `def` instead of lambda to define top-level functions.
I'm not a huge fan of `|foo|` either to be honest - I don't see why you can't simply use the same syntax for anonymous and named functions - but it is at least a little better.
If it were up to me I'd go with something like
fn double(a: list[int]) -> list[int] {
a.map(fn(i) { i * 2 })
}