without lambda keywords?

[abersheeran]

Just like:

def_([x, y], [
    return_[x + y]
])

[hsfzxjy]

Unfortunately this is impossible, since [x, y] and return_[x + y] will be evaluated eagerly and raise a NameError. The lambda can prevent the body being evaluated before transpiling.

Besides, the syntax def_([x, y], ...) is hard to represent advanced parameter specification like keyword-only arguments or default arguments. lambda instead has a consistent parameter specification like ordinary functions. See https://github.com/hsfzxjy/lambdex#parameters