Is it possible to emit raw source code with Template Haskell?

Question

Say I have a String (or Text or whatever) containing valid Haskell code. Is there a way to convert it into a [Dec] with Template Haskell?

I'm pretty sure the AST doesn't directly go to GHC so there's going to be a printing and then a parsing stage anyways.

This would be great to have since it would allow different "backends" for TH. For example you could use the AST from haskell-src-exts which supports more Haskell syntax than TH does.

Answer 1

I'm pretty sure the AST doesn't directly go to GHC so there's going to be a printing and then a parsing stage anyways.

Why would you think that? That isn’t the case, the TH AST is converted to GHC’s internal AST directly; it never gets converted back to text at any point in that process. (If it did, that would be pretty strange.)

Still, it would be somewhat nice if Template Haskell exposed a way to parse Haskell source to expressions, types, and declarations, basically exposing the parsers behind various e, t, and d quoters that are built in to Template Haskell. Unfortunately, it does not, and I don’t believe there are currently any plans to change that.

Currently, you need to go through haskell-src-exts instead. This is somewhat less than ideal, since there are differences between haskell-src-exts’s parser and GHCs, but it’s as good as you’re currently going to get. To lessen the pain, there is a package called haskell-src-meta that bridges haskell-src-exts and template-haskell.

For your use case, you can use the parseDecs function from Language.Haskell.Meta.Parse, which has the type String -> Either String [Dec], which is what you’re looking for.