Some languages, like ML and Haskell, allow the abstract syntax tree to be defined in a way that is virtually identical to syntax rules. Write data type definitions in ML or Haskell that follow the...

Some languages, like ML and Haskell, allow the abstract syntax tree to be defined in a way that is virtually identical to syntax rules. Write data type definitions in ML or Haskell that follow the abstract syntax for expr described in Exercise 6.43.

Exercise 6.43