Not able to iterate over List[value]

[djalberts37]

After using getChildren on a node, which consists of a list and a location, I would expect to get the list with the following statements:

list[value] nodes = getChildren(statement);
list[value] nodesList = nodes[0];

Output of nodes[0]

[if(methodCall(false,"isNull",[],src=|project://smallsql/src/smallsql/database/ExpressionFunctionDegrees.java|(1566,8,<42,5>,<42,13>),typ=unresolved()),return(number("0",src=|project://smallsql/src/smallsql/database/ExpressionFunctionDegrees.java|(1583,1,<42,22>,<42,23>),typ=unresolved()),src=|project://smallsql/src/smallsql/database/ExpressionFunctionDegrees.java|(1576,9,<42,15>,<42,24>)),src=|project://smallsql/src/smallsql/database/ExpressionFunctionDegrees.java|(1563,22,<42,2>,<42,24>)),declarationStatement(variables(int(),[variable("i",0,infix(number("1",src=|project://smallsql/src/smallsql/database/ExpressionFunctionDegrees.java|(1597,1,<43,10>,<43,11>),typ=unresolved()),"+",number("1",src=|project://smallsql/src/smallsql/database/ExpressionFunctionDegrees.java|(1601,1,<43,14>,<43,15>),typ=unresolved()),src=|project://smallsql/src/smallsql/database/ExpressionFunctionDegrees.java|(1597,5,<43,10>,<43,15>),typ=unresolved()),src=|project://smallsql/src/smallsql/database/ExpressionFunctionDegrees.java|(1593,9,<43,6>,<43,15>))]),src=|project://smallsql/src/smallsql/database/ExpressionFunctionDegrees.java|(1589,14,<43,2>,<43,16>)),return(methodCall(false,simpleName("Math",src=|project://smallsql/src/smallsql/database/ExpressionFunctionDegrees.java|(1620,4,<44,15>,<44,19>),typ=unresolved()),"toDegrees",[methodCall(false,simpleName("param1",src=|project://smallsql/src/smallsql/database/ExpressionFunctionDegrees.java|(1636,6,<44,31>,<44,37>),typ=unresolved()),"getDouble",[],src=|project://smallsql/src/smallsql/database/ExpressionFunctionDegrees.java|(1636,18,<44,31>,<44,49>),typ=unresolved())],src=|project://smallsql/src/smallsql/database/ExpressionFunctionDegrees.java|(1620,36,<44,15>,<44,51>),typ=unresolved()),src=|project://smallsql/src/smallsql/database/ExpressionFunctionDegrees.java|(1613,44,<44,8>,<44,52>))]

type of nodes[0]
list(adt("Statement",[]))

I would expect this to be a list of nodes, but this seems to be a list[value]. I am trying to write a iterator over the list, but I constantly get parse errors, because this is not a list? I am also not able to use size() over the list, because there is no function with a corresponding parameter type.

[jurgenvinju]

I think I understand the situation as follows:

1. you gave an AST from the Java m3 front-end, stored in variable statement
2. you call getChildren on it an get a list
3. the first element in this list happens to be a list again, containing a single element which is the AST node for the Java "if" statement.

This is all as it should be. It looks as though there are some minor misconceptions about the way getChildren works. Also there is no need to write "iterators" in Rascal and your example code has a type error.

I hope this information helps:

• the size function on list you can find in the List module: int size(list[value] l)
• list indexing like nodes[0] returns a value of the static type of the list elements, so nodes[0] should be of type value and I would suspect an error in list[value] nodesist = nodes[0], namely " Expected list[value], but got value"
• Rascal has a type system with value as the most generic type, and void as the most specific type. ADT types like Statement are somewhere in the middle. Those are all subtypes of node. The size function because it is defined on list[value] works also on list[node] or list[Statement] and on the empty list of type list[void].
• If you want to iterate over a list, you write for loops or comprehensions like so: for (myElement <- myList) { ... } or [ myFunction(myElement) | myElement <- myList]
• So iterating over the children of a node generically could be done like so: for (child <- getChildren(ast)) { ... }
• If you have a node MyNode([1],"hello", |home:///.bashrc|) and call getChildren on it, the result would be: [[1], "hello", |home:///.bashrc|]. The fields of the node are collected in order from left to right into a list and the static type of this list is always list[value] while the dynamic type of this list could be more specific. The first element of this list above is [1] and is a list itself accidentally.
• If the static type of a variable is value then Rascal will not allow you to iterate over it using the <- notation. So when you call nodes[0] you get a value which is not iterable.
• If you want to know if a value is a list, you can pattern match it like so: if (list[value] l := element) { ... } and then l is something you could apply <- to.
• more succinctly: for(list[value] l <- getChildren(ast), elem <- l) would filter the list of children and execute the second iterator only for the elements which happen to be lists, and then execute the body of the loop for all elements of those lists.
• It is also possible to use dynamic dispatch for this, like so:

value recurse(list[value] l) = [recurse(e) | e<- l]; // if the first parameter is a list, then iterate
default value recurse(value x) = x; // otherwise do not iterate

• Rascal also has a visit statement and deep match / which both automate recursion over AST nodes for you.

[jurgenvinju]

@djalberts37 can I close this issue?