Type instantiation in result function types when using constructors as functions

[tvdstorm]

Let's say we have the following module:

module Curry

int sum(int x, int y) = x + y;

data Expr
  = add(Expr lhs, Expr rhs)
  | lit(int x);

&S(&U) curry(&S(&T, &U) f, &T t)
  = &S(&U u) { return f(t, u); }; 

At the prompt I can do f = curry(sum, 1), which results in:

&S(&U): function(...)

The return type, should be int(int) I guess, but calling f(2) correctly evaluates to 3.

But calling curry on a constructor, leads to an exception:

rascal>g = curry(add, lit(1));
&S (&U): function(|project://TwoStones/src/Curry.rsc|(139,28,<11,4>,<11,32>))
rascal>g(lit(2))
io.usethesource.vallang.exceptions.IllegalOperationException: Operation getTypeParameters not allowed on value
(internal error)
        at $root$(|main://$root$|)

io.usethesource.vallang.exceptions.IllegalOperationException: Operation getTypeParameters not allowed on value
        at io.usethesource.vallang.type.Type.getTypeParameters(Type.java:595)
        at io.usethesource.vallang.type.ParameterType.getTypeParameters(ParameterType.java:92)
        at io.usethesource.vallang.type.AbstractDataType.match(AbstractDataType.java:418)
        at io.usethesource.vallang.type.TupleType.match(TupleType.java:414)
        at org.rascalmpl.interpreter.result.ConstructorFunction.call(ConstructorFunction.java:230)
        at org.rascalmpl.semantics.dynamic.Expression$CallOrTree.interpret(Expression.java:536)
        at org.rascalmpl.semantics.dynamic.Statement$Expression.interpret(Statement.java:365)
        at org.rascalmpl.semantics.dynamic.Statement$Return.interpret(Statement.java:782)
        at org.rascalmpl.interpreter.result.RascalFunction.runBody(RascalFunction.java:388)
        at org.rascalmpl.interpreter.result.RascalFunction.call(RascalFunction.java:329)
        at org.rascalmpl.semantics.dynamic.Expression$CallOrTree.interpret(Expression.java:536)
        at org.rascalmpl.semantics.dynamic.Command$Expression.interpret(Command.java:61)
        at org.rascalmpl.interpreter.Evaluator.eval(Evaluator.java:1157)
        at org.rascalmpl.interpreter.Evaluator.eval(Evaluator.java:1027)
        at org.rascalmpl.interpreter.Evaluator.eval(Evaluator.java:982)
        at org.rascalmpl.repl.RascalInterpreterREPL.evalStatement(RascalInterpreterREPL.java:136)
        at org.rascalmpl.eclipse.repl.RascalTerminalConnector$2.evalStatement(RascalTerminalConnector.java:342)
        at org.rascalmpl.repl.BaseRascalREPL.handleInput(BaseRascalREPL.java:108)
        at org.rascalmpl.eclipse.repl.RascalTerminalConnector$2.handleInput(RascalTerminalConnector.java:297)
        at org.rascalmpl.repl.BaseREPL.handleInput(BaseREPL.java:172)
        at org.rascalmpl.repl.BaseREPL.run(BaseREPL.java:335)

So I think there are two problems, but it's unclear to me if and if so, how, they are related:

• unbound type variables in generic function results
• constructors are not treated the same as functions everywhere.

[tvdstorm]

void main() {
  int(int) f = curry(sum, 1);
  
  Expr(Expr) g = curry(add, lit(1));
  
  println(f(2));
  
  println(g(lit(2))); 
}

This passed the compiler's type checker.

The interpreter, however, complains when I try to run it:

rascal>main()
|project://TwoStones/src/Curry.rsc|(210,17,<16,11>,<16,28>): Expected int (int), but got &S (&U)
Advice: |http://tutor.rascal-mpl.org/Errors/Static/UnexpectedType/UnexpectedType.html|
ok

[jurgenvinju]

Interesting bug. Will have a look in January; can you manage a workaround in the mean time?

[jurgenvinju]

The bug is caused by this line right here:

rascal/src/org/rascalmpl/interpreter/result/ResultFactory.java

Line 203 in 437a60a

return (AbstractFunction) value;

The code correctly returns the function as a result (this is a quick implementation), but in case of an instantiated abstract function, the type will not be instantiated in this case resulting in incorrect information loss.

[jurgenvinju]

io.usethesource.vallang.exceptions.IllegalOperationException: Operation getTypeParameters not allowed on value still happens

[jurgenvinju]

The second bug is caused by this line in vallang: https://github.com/usethesource/vallang/blob/f1a159feda1e13e09d9e581d17f29f1513917717/src/main/java/io/usethesource/vallang/type/AbstractDataType.java#L418

[tvdstorm]

More fun: self-applying curry doesn't terminate.

curry(curry, sum)

[jurgenvinju]

The current state is that self-application of curry is not possible yet. The root cause is a very basic (and wrong) assumption in the code of the interpreter: for functions (closures and/or generic or otherwise) the static type is equal to their dynamic type. This is because each function instance extends Result<IValue> (which tuples a static type with a dynamic value), as well as implements IFunction and ICalleableValue which are dynamic value implementations. Both Result and IValue implement getType() but their intended semantics is quite different. Fixing this entails a significant rewrite of the function calling semantics in the interpreter.