Comment by needlesslygrim

Well, ML (or at least the first versions of it) used a λx • x syntax [1] for λ-abstractions, the same (excluding the use of • over .) notation as used with the Lambda Calculus, and I've always assumed \ was an ASCII stand in.

[1]: https://homepages.inf.ed.ac.uk/wadler/papers/papers-we-love/... (can be spotted on page 353)