[Haskell-cafe] question on the design of protohaskell (in Write You a Haskell)

[Tom Ellis]

On Fri, Jan 22, 2016 at 02:38:38PM -0500, Maurizio Vitale wrote:
> In http://dev.stephendiehl.com/fun/007_path.html compiler steps are piped
> together using the Kleisli (>=>) operator, presumably the one from
> Control.Monad.

Yes, it uses the Monad instance of CompilerMonad, which is

    type CompilerMonad = ExceptT Msg (StateT CompilerState IO)

so the monad can do IO, read and modify the CompilerState, and throw Msg
exceptions.

> they also return some flavour of AST (typically Syn.Module or Core.Module).
> Assuming that the >=> is the one from Control.Monad, what is the purpose of
> this returned value (all the steps are in a compilerMonad and presumably
> would have to update the state w/ the new AST anyhow)?

In CompilerState there is a field

    _ast      :: Maybe Syn.Module    -- ^ Frontend AST

>From the comment I presume this is the AST taken directly from the source
file, to be used for error-reporting and such.  I would guess after the
parseP stage it doesn't change, and the result of each stage is the
Syn.Module returned from that stage not something set in the monad.

Tom