Disallow non-variable and non-constant patterns in body atoms

[magnus-madsen]

The grammar for TypedAst for constraints has:

    sealed trait Body extends TypedAst.Predicate

    object Body {

      case class Atom(pred: Name.Pred, den: Denotation, polarity: Ast.Polarity, fixity: Ast.Fixity, terms: List[TypedAst.Pattern], tpe: Type, loc: SourceLocation) extends TypedAst.Predicate.Body

      case class Guard(exp: TypedAst.Expression, loc: SourceLocation) extends TypedAst.Predicate.Body

      case class Loop(varSyms: List[Symbol.VarSym], exp: TypedAst.Expression, loc: SourceLocation) extends TypedAst.Predicate.Body

    }

The safety phase should check that the terms passed to Atom are only variables and constants.

If not, it should emit a SafetyError saying that such patterns are not supported.

[magnus-madsen]

The right place to start is around here: https://github.com/flix/flix/blob/master/main/src/ca/uwaterloo/flix/language/phase/Safety.scala#L539

[magnus-madsen]

The goal is to reject programs that contain constraints like:

A(x) :- B(Some(x)).

which currently crash the compiler.

[mlutze]

IMO this should be its own AST node rather than reusing pattern. Resolver is messy as a result of the reuse. See also https://github.com/flix/flix/issues/4854

[magnus-madsen]

@mlutze We want to allow patterns some day, we just dont have engine support yet. In that case, is it ok?

[mlutze]

@mlutze We want to allow patterns some day, we just dont have engine support yet. In that case, is it ok?

Yes, in that case it's worth the slight weirdness in resolver.

[magnus-madsen]

@AStenbaek CC