Question

GHC gives me a parse error on input \~ if I try to put a tilde on the pattern of a lambda expression like I can do with named functions

let step = \~(x,s) -> run (f x) s  -- Parse Error

let step ~(x, s) = run (f x) s -- Works fine

Answer 1

You have to add a space between the lambda and the tilde

\ ~(x,s) -> run (f x) s

The source of the confusion is because \ and ~ are both valid characters for user defined operators so \~ is parsed as one instead of being parsed as the start of a lambda expression:

-- Defining a custom \~ operator is allowed:
let a \~ b = {- ... -}