code: func EvalType(expr Expr, sources Sources, typmap TypeMapper) DataType {
  	if typmap == nil {
  		typmap = nilTypeMapper{}
  	}
  
  	valuer := TypeValuerEval{
  		TypeMapper: typmap,
  		Sources:    sources,
  	}
  	typ, _ := valuer.EvalType(expr)
  	return typ
  }
docs: // EvalType evaluates the expression's type.