I've also seen another interesting feature in anot...

# language-proposals

[kevinmost]

I've also seen another interesting feature in another in-progress language (the language Jai, being worked on by Jonathan Blow), where you can pass parameters to your lambda, and only those parameters can be modified. Ex:

[fooField, barField] {
    /* only fooField and barField, but no other fields, may be used in this lambda */
}