[Haskell-cafe] Getting both the value and name of a variable in one expression

[Christopher Done]

I considered an expression-within-an-expression but thought a
stage-restriction would break it. Maybe not! Let me go test it!

On Wed, 21 Aug 2019 at 17:31, Michael Sloan <mgsloan at gmail.com> wrote:

> One potential solution here is to have checkGrammar_take2 return an
> expression which returns an expression.  I haven't tried compiling
> this, but something roughly like:
>
> checkGrammar_2 name = [| if valueFine $(varE name) then varE name else
> error "value isn't fine" |]
>
> Then it'd be used like $($(checkGrammar_2 'thename)).  The th-orphans
> package can also be useful for this because it provides Lift instances
> for the TH AST.
>
> -Michael
>
> On Tue, Aug 20, 2019 at 10:36 AM Christopher Done <chrisdone at gmail.com>
> wrote:
> >
> > Hi all,
> >
> > Do we have already a syntax for ‘foo that also contains the typed value
> like TExp?
> >
> > I have e.g. an AST that I want to do more static checks on it that
> aren’t as convenient to do in the type system. Here’s an example:
> >
> > -- | Check the grammar spec to produce a grammar.
> > checkGrammar :: (SchemaName, [(SchemaName, Schema)]) -> Q Exp
> > checkGrammar (toplevel, rules) =
> >   if M.size rulesMap /= length rules
> >     then error "Duplicate rule names in grammar."
> >     else lift (Grammar {grammarToplevel = toplevel, grammarRules =
> rulesMap})
> >   where
> >     rulesMap = M.fromList rules
> >
> > -- | Grammar for Haskell.
> > grammar :: Grammar
> > grammar = $(checkGrammar $ runDefine $ mdo
> >   -- General expression
> >   expression       <- rule "Expression" (ChoiceSchema [variable,
> constructor, parentheses
> >                                                       ,tuple, let',
> application, string])
> >   application      <- rule "Application" (CompositeSchema [expression,
> expression])
> >   parentheses      <- rule "Parentheses" (CompositeSchema
> [openParenSchema, expression, closeParenSchema])
> >
> > ...
> >  pure expression)
> >
> > Here I do a trivial check for duplicates. After I’ve checked the
> > expression at compile-time, I Lift it so that it can be used at
> > runtime. That’s pretty good. But some types like (a -> b) don’t Lift. So
> > an alternative would be:
> >
> > grammar = $(checkGrammar_take2 thename 'thename)
> >
> > In which checkGrammar_take2 would:
> >
> > Use thename at compile-time for a check.
> > If the check passes, then return (VarE thename)
> >
> > E.g.
> >
> > checkGrammar_take2 value name = if valueFine value then varE name else
> > error "value isn't fine"
> >
> > That’s actually quite a good solution because it avoids a lift, and I
> > didn’t transform the AST. It’s also more efficient than lifting.
> >
> > But there’s no checked relationship between thename and ‘thename.
> > checkGrammar_take2 has no way of knowing that they refer to the same
> > thing. See?
> >
> > Hence, if I could get e.g. `thename to produce both the value and a
> > name for the value, that would be cool. My gist doesn’t go this
> > far. It might look like this:
> >
> > checkGrammar_take2 namedValue = if valueFine (getValue namedValue) then
> getExp namedValue else error "value isn't fine"
> >
> > and call it like:
> >
> > mygrammar = checkGrammar_take2 `thename
> >
> > So the semantics would be roughly similar to
> >
> > [|| thename ||] :: TExp a
> >
> > but you’d get
> >
> > `thename :: Named a
> >
> > where
> >
> > data Named a = { namedThing :: a, nameOfThing :: Name }
> >
> > I feel like the more DSLs I design, the more I’d like something like
> this to perform my static checks.
> >
> > Cheers,
> >
> > Chris
> >
> > _______________________________________________
> > Haskell-Cafe mailing list
> > To (un)subscribe, modify options or view archives go to:
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> > Only members subscribed via the mailman list are allowed to post.
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/haskell-cafe/attachments/20190822/e450405e/attachment.html>