[Haskell-cafe] question on the design of protohaskell (in Write You a Haskell)
tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk
Sat Jan 23 11:00:47 UTC 2016
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
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
> 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.
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