Make parse round-trip-able

[cstrahan]

There are a number of tools (including one I desperately need to write) that would require the ability to parse a nix expression, transform it in some way, and then write the expression back out - all while keeping unmodified portions unchanged in the output.

I could take a stab at this, but I think I might need some hints/help.

[jwiegley]

That's a great idea @cstrahan. You'll need to plumb attributes through the syntax tree that preserve knowledge like surrounding whitespace, etc. This part should be fairly easy, since we're using a Free representation.

Then, in the evaluator, you only need to preserve the attributes over any transformation (they would be abstract in that case). In the pretty printer, it would use the attributes for laying out the elements to be printed, or any other style decisions.

It's not exactly trivial, but should be straightforward, and I'm sure there are examples of something similar living out on the Web.

[cstrahan]

@jwiegley I don't think trailing white space matters as much, so perhaps just tracking source spans would be sufficient (ala haskell-src-exts)?

If so, do you suppose I should add a monomorphic field (e.g. SrcSpan) to each construct? haskell-src-exts uses a polymorphic field in their annotated AST, but I don't know if we'd benefit from going that route.

[jwiegley]

You shouldn't need to change the AST at all, but just change the Free type synonym.

[cstrahan]

Do you mean the Fix synonym? (grepping for Free doesn't turn up anything)

Are you suggesting something like this?

data SrcSpan = SrcSpan Int Int
data Ann x f a = Ann x (f a)
type NExpr = Fix (Ann SrcSpan NExprF)

[jwiegley]

Oh yeah, sorry, Fix. And yes, what you pasted is exactly what I had meant. That will give you an annotation at every level of the resulting tree.

[expipiplus1]

I've started working towards this in #35

[jwiegley]

@cstrahan We've now merged @expipiplus1 's work, does this help you in building your tools?

[cstrahan]

@jwiegley I believe so, although I've shelved that work for the time being. Thanks!

[expipiplus1]

For an example of this being used you can take a look here: https://github.com/expipiplus1/update-nix-fetchgit

This doens't do a round trip though, It uses the source locations to modify the underlying string.