Hi. How to do reversible converting between parse tree and AST? My final task can be thought as the following reversible computation: stream of chars <---> stream of tokens <---> parse tree <---> AST On first piece (chars <---> tokens): I will probably just give up on reversible lexing, so the problem is gone. :) I will write separate lexer and printer. On second piece (tokens <---> parse tree): printing is easy. For parsing I will use package "Earley" from Hackage. And I will use brute force to ensure unambiguity. On third piece (parse tree <---> AST): it seems to be last remaining piece. I don't know how to get both translators in the same time. Consider usual arithmetic language, i. e. a * (b + c). This is how parse tree may look: data Term = Id Id | Braces LBrace Expr RBrace data Product = Term Term | Times Product Times Term data Expr = Product Product | Plus Expr Plus Product As you can see this data types are direct mapping of grammar. And this is how corresponding AST looks: data AExpr = AId Id | ATimes AExpr AExpr | APlus AExpr AExpr Is there some way to simultaneously write translators in both directions? == Askar Safin https://github.com/safinaskar