I have been trying to understand the following function and I think i am interpreting it incorrectly. I am trying to convert it to Python3
type UITreeNodeChild
= UITreeNodeChild UITreeNode
unwrapUITreeNodeChild : UITreeNodeChild -> UITreeNode
unwrapUITreeNodeChild child =
case child of
UITreeNodeChild node ->
node
I see the bottom part is that there is a function called unwrapUITreeNodeChild which has a arg called child which is of UITreeNodeChild and the function returns a UITreeNode.
In Python i see it as:
class UITreeNodeChild(Enum):
UITreeNodeChild=UITreeNode
def unwrapUITreeNodeChild(child: UITreeNodeChild) -> UITreeNode:
if child is UITreeNodeChild: #
pass # I have no idea what it really means.
I dont understand the rest of the function signature. After looking at the ELM docs, i was confused as the switch but it looks like it is passing node into the definition of UITreeNodeChild.
I know im wrong here, but i would love to be both correct and proper explained to me.
EDIT
So it is looking for a typecheck essentially in the sample, so what you would want to do is confirm that it is that type and then use this new property in underlying code. It would look somewhat similar to:
class UITreeNodeChild:
pass
def unwrapUITreeNodeChild(child: UITreeNodeChild) -> UITreeNode:
if type(child) == UITreeNodeChild:
pass
Note you want to check type not check the exact same object.
You are looking at a tagged union and a simple pattern match against that tagged union.
In type UITreeNodeChild = UITreeNodeChild UITreeNode, the UITreeNodeChild on the right side is the tag. UITreeNode is the data type that is tagged.
When you say:
case child of
UITreeNodeChild node ->
node
You are pattern matching child against a pattern that begins with UITreeNodeChild. The tagged data type value is bound to the name node and this is also what it is returned from the case expression for this patten match.
Since this tagged union has only one variant you also have the option to unpack it in the parameter declaration:
unwrapUITreeNodeChild : UITreeNodeChild -> UITreeNode
unwrapUITreeNodeChild (UITreeNodeChild node) =
node
The way you do this in python depends on the way you chose to implement the tagged union. The general method is the same. You have to have some kind of tag and you have to have a way to determine the tag of a value. You can then return the embedded data type based on the tag. With a single tag this becomes just returning the embedded data type.
Related
Given a list of arbitrary objects
input = listOf(a, b, c, a)
... is there a function (with one non-collection argument) in the Kotlin standard library that I can use to make a copy of this list, removing all instances of ONE object?
Something like:
val filtered = input.removeAllInstancesOf(a)
To clarify, I'm aware of other (potential) solutions to this task:
Using the filter function to do this. → val output = input.filterNot { it == a }
Using the minus function with a collection → val output = input.minus(listOf(a))
Using the minus function with a non-collection argument → val output = input.minus(a) ← Only removes the first instance of a!
Removing all instances from a mutable list.
Writing such a function. → Wrap any of the above.
... but I'm wondering why I can't find a function which takes just ONE, non-collection value.
but I'm wondering why I can't find a function which takes just ONE, non-collection value.
Because that's a hyper-specific use-case of the already existing filter function. As you yourself showed it can be done in one line, and is probably the first thing a Kotlin dev would try to do (at least I would). So adding new function to the standard library probably doesn't add much value.
I am trying to abstract inserting objects of different types into sql tables of similar structure. Here's what I'm trying to do:
class TableAccess[A : Meta](table: String) {
def insert(key: String, a: A): ConnectionIO[Unit] = {
(fr"insert into " ++ Fragment.const(table) ++ fr" values ($key, $a);").update.run.map(_ => ())
}
}
But I get this compile error:
[error] diverging implicit expansion for type doobie.util.param.Param[A]
[error] starting with method fromMeta in object Param
[error] (fr"insert into " ++ Fragment.const(table) ++ fr" values ($key, $a);").update.run.map(_ => ())
All I can find in the documentation is:
doobie allows you to interpolate values of any type (and options
thereof) with an Meta instance, which includes...
But it seems that is not enough in this case; what's the right typeclass/imports/conversions I need?
I'll go ahead an answer my own question, almost a year later. I never fully understood what was happening, and I have since updated to a newer version of doobie, so I am not sure how relevant this is. But now the documentation contains this clue:
Note: it is important to understand that Meta exists only to introduce
Get/Put pairs into implicit scope. You should never demand Meta as
evidence in user code: instead demand Get, Put, or both.
def foo[A: Meta](...) // don't do this
def foo[A: Get: Put](...) // ok
And indeed, between that change and the new version, this now compiles just fine for me:
class TableAccess[A: Get: Put](table: String) {
When the compiler is resolving implicit its searches for one of a specific type in the current scope. Here it seems like his finding more than one in his tree search.
It's not a matter of a missing typeclass or imports, it's more like you have too many of them and the compiler cant figure the right one.
Try removing some implicit and see how that works or pass them explicitly.
One way I resolved this was to localize the type parameters (and their evidence) onto the method (on a static/companion object), and then it compiled.
Something like
object MinimalGood {
def good[A: Meta, B: Meta](a: A, b: B): Update0 =
sql"""$a $b""".update
}
I'm working on a Function Module to assist with dealing with included text with logic embedded. While looking into the way SAP handles SAPScript files and parses the logic I found a structure that is declared as so:
DATA BEGIN OF events OCCURS 100.
INCLUDE STRUCTURE ITCCA.
DATA: command LIKE BOOLEAN,
template LIKE BOOLEAN,
mask LIKE BOOLEAN,
END OF events.
This obviously works, as I can trace through it while it is running a print program. So I thought I would try a similar structure in my own code but even when I copied the code 1 for 1 like above I get an error during activation. The error is
"BOOLEAN" must be a flat structure. Internal tables, references,
strings and structures are forbidden as components.
Can someone explain to me why this structure is valid in one program and not mine?
To explain the actual effect: LIKE usually refers to a data object (an actual variable) on the right-hand side, not a data type. As you rightly discovered, once you provide a data object named BOOLEAN, that is used to construct the type. If a data object of that name is not present and you're not within a class or an interface, an obsolete variant of the LIKE statement will be triggered that also takes data types into account, but only allows for certain elements on the right-hand side - namely only flat structured objects or their components. LIKE DATATYPE-BOOLEAN should have worked. As usual, the error message is somewhat less than helpful.
It seems during my initial investigation I missed a declaration for the BOOLEAN type. In the STXC function group SAP decided to declare their own variable for boolean in a different include file like this:
data: boolean(1) type c.
I had originally assumed that they were doing this with the dictionary defined type which has a similar name and is a 1 character long string. What I also found is that if I were to change my structure declaration like this:
DATA BEGIN OF events OCCURS 100.
INCLUDE STRUCTURE ITCCA.
DATA: command TYPE BOOLEAN,
template TYPE BOOLEAN,
mask TYPE BOOLEAN,
END OF events.
My code would be valid because it would then be using the dictionary defined value. So either I have to add a declaration for my own definition of boolean so that I can use the LIKE keyword or I have to use the TYPE keyword to use the dictionary definition.
Is there a way to pass the path to a simplexml node as a variable?
This is what I tried:
//set the path to the node in a variable
$comp = 'component->structuredBody->component';
echo count($xml->component->structuredBody->component); //=== 13
echo count($xml->$comp); //===0
echo count($xml->{$comp});//===0
What you need is XPath, and more specifically SimpleXML's xpath() method. Instead of traversing using PHP's -> operator, you would traverse using XPath's / operator, but otherwise, could achieve exactly the effect you wanted:
$comp = 'component[1]/structuredBody[1]/component';
echo count( $xml->xpath($comp) );
You might think that this could be simplified to 'component/structuredBody/component', but that would find all possible paths matching the expression - that is if there are multiple structuredBody elements, it will search all of them. That might actually be useful, but it is not equivalent to $xml->component->structuredBody->component, which is actually shorthand for $xml->component[0]->structuredBody[0]->component.
A few things to note:
Unlike most operations with SimpleXML, the result is an array, not another SimpleXML object (think of it as a set of search results). It is therefore vital that you check it is not empty before accessing element 0 as an object.
As you can see in the example above, XPath counts from 1, not 0. I don't know why, but it's caught me out before, so I thought I'd warn you.
The library SimpleXML is built on supports only XPath 1.0;
examples you see online for XPath 2.0 may not work.
XPath 1.0 has no notion of a "default namespace", and SimpleXML doesn't automatically register namespace prefixes for use in XPath, so if you're working with namespaces, you need to use registerXPathNamespace().
Consider the following line of code:
private void DoThis() {
int i = 5;
var repo = new ReportsRepository<RptCriteriaHint>();
// This does NOT work
var query1 = repo.Find(x => x.CriteriaTypeID == i).ToList<RptCriteriaHint>();
// This DOES work
var query1 = repo.Find(x => x.CriteriaTypeID == 5).ToList<RptCriteriaHint>();
}
So when I hardwire an actual number into the lambda function, it works fine. When I use a captured variable into the expression it comes back with the following error:
No mapping exists from object type
ReportBuilder.Reporter+<>c__DisplayClass0
to a known managed provider native
type.
Why? How can I fix it?
Technically, the correct way to fix this is for the framework that is accepting the expression tree from your lambda to evaluate the i reference; in other words, it's a LINQ framework limitation for some specific framework. What it is currently trying to do is interpret the i as a member access on some type known to it (the provider) from the database. Because of the way lambda variable capture works, the i local variable is actually a field on a hidden class, the one with the funny name, that the provider doesn't recognize.
So, it's a framework problem.
If you really must get by, you could construct the expression manually, like this:
ParameterExpression x = Expression.Parameter(typeof(RptCriteriaHint), "x");
var query = repo.Find(
Expression.Lambda<Func<RptCriteriaHint,bool>>(
Expression.Equal(
Expression.MakeMemberAccess(
x,
typeof(RptCriteriaHint).GetProperty("CriteriaTypeID")),
Expression.Constant(i)),
x)).ToList();
... but that's just masochism.
Your comment on this entry prompts me to explain further.
Lambdas are convertible into one of two types: a delegate with the correct signature, or an Expression<TDelegate> of the correct signature. LINQ to external databases (as opposed to any kind of in-memory query) works using the second kind of conversion.
The compiler converts lambda expressions into expression trees, roughly speaking, by:
The syntax tree is parsed by the compiler - this happens for all code.
The syntax tree is rewritten after taking into account variable capture. Capturing variables is just like in a normal delegate or lambda - so display classes get created, and captured locals get moved into them (this is the same behaviour as variable capture in C# 2.0 anonymous delegates).
The new syntax tree is converted into a series of calls to the Expression class so that, at runtime, an object tree is created that faithfully represents the parsed text.
LINQ to external data sources is supposed to take this expression tree and interpret it for its semantic content, and interpret symbolic expressions inside the tree as either referring to things specific to its context (e.g. columns in the DB), or immediate values to convert. Usually, System.Reflection is used to look for framework-specific attributes to guide this conversion.
However, it looks like SubSonic is not properly treating symbolic references that it cannot find domain-specific correspondences for; rather than evaluating the symbolic references, it's just punting. Thus, it's a SubSonic problem.