macro troubles, trying to produce multiple expressions (class def + block)

[Rob Nikander]

Hi,

I want a macro used like `query("select * from foo")` to generate code
like:

case class Row( /* fields from DB */)
val buf = new ListBuffer[Row]
val rs = conn.prepareStatement(sql).executeQuery()
while (rs.next) {
buf += Row(rs.getString(1), rs.getInt(2), ... fields from
DB ... ) // ***
}
lb.toList

But from what I know so far, I must put that all in a single block
expression, and that is generating type errors that I don't
understand. The compilation failed with:

MacroTests.scala:25: error: type mismatch;
found : Row
required: Row.type

I can deduce that this comes from the line marked with *** above. So
it didn't like my ListBuffer[Row] declaration. My tree looks like:

Apply(
Select(
New(
AppliedTypeTree(
Ident(listBufferSym),
List(Ident("Row"))
// List(Ident(typeOf[Any].typeSymbol)) -- this
get's it to run, but without desired type
)
),
nme.CONSTRUCTOR
),
List()
)

To understand why it failed I tried to reify this:

reify({ case class R(id: Int); val lb = new ListBuffer[R](); lb
+= R(1) })

In response, It printed a short story...

<console>:12: error: Cannot materialize {
[ lots of internally generated scala code ]
} because:
scala.reflect.macros.TypecheckException: type mismatch;
found : $u.TypeTag[(some other)lb.type(in value res4)
forSome { type (some other)lb.type(in value res4) <:
scala.collection.mutable.ListBuffer[(some other)R(in value res4)] with
Singleton; type (some other)R(in value res4) <: Product with
Serializable{val id: Int; def copy(id: Int): (some other)R(in value
res4); def copy$default$1: Int} }]
required: $u.WeakTypeTag[lb.type(in value res4) forSome
{ type lb.type(in value res4) <:
scala.collection.mutable.ListBuffer[R(in value res4)] with Singleton;
type R(in value res4) <: Product with Serializable{val id: Int; def
copy(id: Int): R(in value res4); def copy$default$1: Int} }]

I don't understand that. I haven't learned about 'forSome' yet.

Any suggestions? Is there a way for the macro to introduce multiple
expressions without wrapping them in a block? (And let me know if
there a more appropriate mailing list for this.)

thank you,
Rob

[Eugene Burmako]

Try Ident(newTypeName("Row"))

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[Eugene Burmako]

Answering your other questions:

1) Macros can only expand into multiple expressions if those are wrapped in a block.

2) Please submit the problem with reify to JIRA. This failure indicates a compiler bug.

On 13 May 2013 22:12, Rob Nikander <rob.ni...@gmail.com> wrote:

[Eugene Burmako]

Oh btw this works:

scala> reify({ case class R(id: Int); val lb = new ListBuffer[R](); lb += R(1); () })
res4: reflect.runtime.universe.Expr[Unit] =
Expr[Unit]({
  case class R extends Product with Serializable {
    <caseaccessor> <paramaccessor> val id: Int = _;
    def <init>(id: Int) = {
      super.<init>();
      ()
    }
  };

  val lb = new ListBuffer[R]();

  lb.$plus$eq(R.apply(1));
  ()
})

On 13 May 2013 22:12, Rob Nikander <rob.ni...@gmail.com> wrote:

[Rob Nikander]

On May 13, 4:16 pm, Eugene Burmako <eugene.burm...@epfl.ch> wrote:
> Try Ident(newTypeName("Row"))
>

That works. Thanks!

[Rob Nikander]

On May 13, 4:18 pm, Eugene Burmako <eugene.burm...@epfl.ch> wrote:
> 2) Please submit the problem with reify to JIRA. This failure indicates a
> compiler bug.

Done.

https://issues.scala-lang.org/browse/SI-7478

[Rob Nikander]

On May 13, 4:18 pm, Eugene Burmako <eugene.burm...@epfl.ch> wrote:

> 1) Macros can only expand into multiple expressions if those are wrapped in
> a block.

Is something like this possible?

class/object X {
val sql = "select * from foo"
m(sql)
}

where:
1. macro m expands to a class definition
2. m's implementation can see the constant string value of sql,
"select * from foo".

Rob

[Eugene Burmako]

1) m can expand to a class definition, but it won't be visible from outside of the expansion.

2) unfortunately, in general case this is either very hard or impossible.

Rob

[Rob Nikander]

Okay, it sounds like I won't be able to create what I was imagining --
a DB query macro that prepared the JDBC statement at compile time and
generated a row class with typesafe fields. I got it working with
tuples, but to access the columns with named fields it sounds like I
must build a separate tool that generates .scala files.

Thanks again for the helpful responses.

Rob

On May 13, 5:21 pm, Eugene Burmako <eugene.burm...@epfl.ch> wrote:
> 1) m can expand to a class definition, but it won't be visible from outside
> of the expansion.
> 2) unfortunately, in general case this is either very hard or impossible.
>

[Eugene Burmako]

Do you have to you use Scala 2.10.x, or an experimental version of scalac will do?

[Rob Nikander]

I can use experimental.

Rob

On May 13, 5:54 pm, Eugene Burmako <eugene.burm...@epfl.ch> wrote:
> Do you have to you use Scala 2.10.x, or an experimental version of scalac
> will do?
>

[Eugene Burmako]

Take a look at http://docs.scala-lang.org/overviews/macros/typemacros.html and https://github.com/xeno-by/typemacros-h2db.

[Simon Ochsenreither]

https://github.com/jonifreeman/sqltyped might be of interest, too.

[Rob Nikander]

Will I be able to use the Eclipse Scala IDE with this macro paradise compiler? 

[Eugene Burmako]

Good question. I haven't done anything special to enable that, so if it breaks I wouldn't be surprised. Are you having problems?

[Rob Nikander]

On Tue, May 14, 2013 at 3:46 PM, Eugene Burmako <xen...@gmail.com> wrote:

Good question. I haven't done anything special to enable that, so if it breaks I wouldn't be surprised. Are you having problems?

It appears I'll need to build a custom Scala IDE, because it comes with a baked-in compiler and it can't switch to others. This may stop me on this path for now.

[Adriaan Moors]

You should be able to swap out the compiler jar when using an IDE version that's "close enough"

http://scala-ide.org/docs/tutorials/scalac-trunk/index.html#Swapping_the_Scala_compiler_inside_Eclipse__risky_