Some Elixir functions have 2 variants for indicating error
Return a tuple e.g. File.open which returns something like {:ok, io_device} or {:error, posix}
Raise exception e.g. File.open!
My questions are:
What's the intention of having two ways?
Is one preferred over the other (like best practices)?
There are two ways of handling errors, because there are two types of errors:
the expected errors - like user providing bad data, etc. In that case you use the tuple-style return values to handle the error. This also forces the caller to consider the error case and handle it properly.
the truly unexpected exceptions - like a configuration file suddenly disappearing, that can't be recovered from and that there's not much to do beside crashing. In that case you raise an exception.
Because of those two ways, you extremely rarely find yourself in need of rescuing exceptions - where in other languages you would rescue an exception, in Elixir you avoid raising it in the first place, and rather return an ok/error tuple instead.
I'd say the tuple-style is superior, as it gives control to the caller - the caller can decide what to do with the error by either pattern matching on the return value in a case expression and handling both possibilities, or, ignoring the erroneous one, pattern matching directly on the ok tuple. The second one will convert the return value to a MatchError exception, should the unexpected error occur. You can see how the first style can be easily converted to the second one. That said many libraries provide "bang" functions that raise the error for ease of use and ability to provide better error messages than a plain MatchError does allow.
While the {:ok, value} is often paired with {:error, reason}, it's merely a convention. There are many APIs that return only :error without a reason, where the reason is obvious, there are also some that return something different in the successful case. The rule here is to provide an easy pattern match that is not order dependent. Let's see some examples:
{value, rest} | :error
That's a good choice, since the cases are easily distinguishable - this style is used, for example by Integer.parse/2. If the success condition has two return values and there's only one reason for failure, this style is recommended.
string | :error
This doesn't seem like a good idea, you'd either need to have a guard in the pattern match or be careful to match the :error atom first. Instead, one would wrap the success value in a {:ok, string} tuple for ease of use.
Related
This code works:
(3,6...66).contains( 9|21 ).say # OUTPUT: «any(True, True)»
And returns a Junction. It's also tested, but not documented.
The problem is I can't find its implementation anywhere. The Str code, which is also called from Cool, never returns a Junction (it does not take a Junction, either). There are no other methods contain in source.
Since it's autothreaded, it's probably specially defined somewhere. I have no idea where, though. Any help?
TL;DR Junction autothreading is handled by a single central mechanism. I have a go at explaining it below.
(The body of your question starts with you falling into a trap, one I think you documented a year or two back. It seems pretty irrelevant to what you're really asking but I cover that too.)
How junctions get handled
Where is contains( Junction) defined? ... The problem is I can't find [the Junctional] implementation anywhere. ... Since it's autothreaded, it's probably specially defined somewhere.
Yes. There's a generic mechanism that automatically applies autothreading to all P6 routines (methods, operators etc.) that don't have signatures that explicitly control what happens with Junction arguments.
Only a tiny handful of built in routines have these explicit Junction handling signatures -- print is perhaps the most notable. The same is true of user defined routines.
.contains does not have any special handling. So it is handled automatically by the generic mechanism.
Perhaps the section The magic of Junctions of my answer to an earlier SO Filtering elements matching two regexes will be helpful as a high level description of the low level details that follow below. Just substitute your 9|21 for the foo & bar in that SO, and your .contains for the grep, and it hopefully makes sense.
Spelunking the code
I'll focus on methods. Other routines are handled in a similar fashion.
method AUTOTHREAD does the work for full P6 methods.
This is setup in this code that sets up handling for both nqp and full P6 code.
The above linked P6 setup code in turn calls setup_junction_fallback.
When a method call occurs in a user's program, it involves calling find_method (modulo cache hits as explained in the comment above that code; note that the use of the word "fallback" in that comment is about a cache miss -- which is technically unrelated to the other fallback mechanisms evident in this code we're spelunking thru).
The bit of code near the end of this find_method handles (non-cache-miss) fallbacks.
Which arrives at find_method_fallback which starts off with the actual junction handling stuff.
A trap
This code works:
(3,6...66).contains( 9|21 ).say # OUTPUT: «any(True, True)»
It "works" to the degree this does too:
(3,6...66).contains( 2 | '9 1' ).say # OUTPUT: «any(True, True)»
See Lists become strings, so beware .contains() and/or discussion of the underlying issues such as pmichaud's comment.
Routines like print, put, infix ~, and .contains are string routines. That means they coerce their arguments to Str. By default the .Str coercion of a listy value is its elements separated by spaces:
put 3,6...18; # 3 6 9 12 15 18
put (3,6...18).contains: '9 1'; # True
It's also tested
Presumably you mean the two tests with a *.contains argument passed to classify:
my $m := #l.classify: *.contains: any 'a'..'f';
my $s := classify *.contains( any 'a'..'f'), #l;
Routines like classify are list routines. While some list routines do a single operation on their list argument/invocant, eg push, most of them, including classify, iterate over their list doing something with/to each element within the list.
Given a sequence invocant/argument, classify will iterate it and pass each element to the test, in this case a *.contains.
The latter will then coerce individual elements to Str. This is a fundamental difference compared to your example which coerces a sequence to Str in one go.
The large majority of SonarLint rules that I've come across in Java seemed plausible and justified. However, ever since I've started using SonarLint for VB.NET, I've come across several rules that left me questioning their usefulness or even whether or not they are working correctly.
I'd like to know if this is simply a problem of me using some VB.NET constructs in a suboptimal way or whether the rule really is flawed.
(Apologies if this question is a little longer. I didn't know if I should create a separate question for each individual rule.)
The following rules I found to leave some cases unconsidered that would actually turn up as false-positives:
S1871: Two branches in the same conditional structure should not have exactly the same implementation
I found this one to bring up a lot of false-positives for me, because sometimes the order in which the conditions are checked actually does matter. Take the following pseudo code as example:
If conditionA() Then
doSomething()
ElseIf conditionB() AndAlso conditionC() Then
doSomethingElse()
ElseIf conditionD() OrElse conditionE() Then
doYetAnotherThing()
'... feel free to have even more cases in between here
Else Then
doSomething() 'Non-compliant
End If
If I wanted to follow this Sonar rule and still make the code behave the same way, I'd have to add the negated version of each ElseIf-condition to the first If-condition.
Another example would be the following switch:
Select Case i
Case 0 To 40
value = 0
Case 41 To 60
value = 1
Case 61 To 80
value = 3
Case 81 To 100
value = 5
Case Else
value = 0 'Non-compliant
There shouldn't be anything wrong with having that last case in a switch. True, I could have initialized value beforehand to 0 and ignored that last case, but then I'd have one more assignment operation than necessary. And the Java ruleset has conditioned me to always put a default case in every switch.
S1764: Identical expressions should not be used on both sides of a binary operator
This rule does not seem to take into account that some functions may return different values every time you call them, for instance collections where accessing an element removes it from the collection:
stack.Push(stack.Pop() / stack.Pop()) 'Non-compliant
I understand if this is too much of an edge case to make special exceptions for it, though.
The following rules I am not actually sure about:
S3385: "Exit" statements should not be used
While I agree that Return is more readable than Exit Sub, is it really bad to use a single Exit For to break out of a For or a For Each loop? The SonarLint rule for Java permits the use of a single break; in a loop before flagging it as an issue. Is there a reason why the default in VB.NET is more strict in that regard? Or is the rule built on the assumption that you can solve nearly all your loop problems with LINQ extension methods and lambdas?
S2374: Signed types should be preferred to unsigned ones
This rule basically states that unsigned types should not be used at all because they "have different arithmetic operators than signed ones - operators that few developers understand". In my code I am only using UInteger for ID values (because I don't need negative values and a Long would be a waste of memory in my case). They are stored in List(Of UInteger) and only ever compared to other UIntegers. Is this rule even relevant to my case (are comparisons part of these "arithmetic operators" mentioned by the rule) and what exactly would be the pitfall? And if not, wouldn't it be better to make that rule apply to arithmetic operations involving unsigned types, rather than their declaration?
S2355: Array literals should be used instead of array creation expressions
Maybe I don't know VB.NET well enough, but how exactly would I satisfy this rule in the following case where I want to create a fixed-size array where the initialization length is only known at runtime? Is this a false-positive?
Dim myObjects As Object() = New Object(someOtherList.Count - 3) {} 'Non-compliant
Sure, I could probably just use a List(Of Object). But I am curious anyway.
Thanks for raising these points. Note that not all rules apply every time. There are cases when we need to balance between false positives/false negatives/real cases. For example with identical expressions on both sides of an operator rule. Is it a bug to have the same operands? No it's not. If it was, then the compiler would report it. Is it a bad smell, is it usually a mistake? Yes in many cases. See this for example in Roslyn. Should we tune this rule to exclude some cases? Yes we should, there's nothing wrong with 2 << 2. So there's a lot of balancing that needs to happen, and we try to settle for an implementation that brings the most value for the users.
For the points you raised:
Two branches in the same conditional structure should not have exactly the same implementation
This rule generally states that having two blocks of code match exactly is a bad sign. Copy-pasted code should be avoided for many reasons, for example if you need to fix the code in one place, you'll need to fix it in the other too. You're right that adding negated conditions would be a mess, but if you extract each condition into its own method (and call the negated methods inside them) with proper names, then it would probably improves the readability of your code.
For the Select Case, again, copy pasted code is always a bad sign. In this case you could do this:
Select Case i
...
Case 0 To 40
Case Else
value = 0 ' Compliant
End Select
Or simply remove the 0-40 case.
Identical expressions should not be used on both sides of a binary operator
I think this is a corner case. See the first paragraph of the answer.
"Exit" statements should not be used
It's almost always true that by choosing another type of loop, or changing the stop condition, you can get away without using any "Exit" statements. It's good practice to have a single exit point from loops.
Signed types should be preferred to unsigned ones
This is a legacy rule from SonarQube VB.NET, and I agree with you that it shouldn't be enabled by default in SonarLint. I created the following ticket in our JIRA: https://jira.sonarsource.com/browse/SLVS-1074
Array literals should be used instead of array creation expressions
Yes, it seems to be a false positive, we shouldn't report on array creations when the size is explicitly specified. https://jira.sonarsource.com/browse/SLVS-1075
I am looking for a tool (may be build time or eclipse plugin) that can help me to identify if I am not logging the Exception trace/message.
We have a legacy application that has try catch block in which a custom error message is logged. The exception is not logged and is not thrown. So, when a problem occurs, there is no stack trace in the log files that would help to debug the issue. An example of this is:
try {
do something....
} catch (Throwable exception) {
Log.log("<<custom message>>");
}
I need a tool like Coverity or Checkstyle that can help me to identify all such occurrences in my code base.
Thanks and Regards
I'd expect you to be able to do a decent job with any tool that can search text using regular expressions (e.g, grep).
The regex would be something like this:
"catch\W*\(.*\)\W*{\W*Log\.log"
where W stands for some whitespace recognizer that picks up blank and newline.
Your pattern is unique enough I'd expect very few false hits, if the programmers
were consistent with the convention you showed.
[EDIT] OP indicates
I am looking for catch blocks where I am NOT doing the following - '+ exception':
try { do something.... }
catch (Throwable exception)
{ Log.log("<<custom message>>" + exception)
We're back to a regular expression as a pretty decent hack. You need hunt for anyplace that doesn't call Log.log("<<....>", or if it does, doesn't have a following "+exception".
This is awkward to code as a regexp without a "not" operator, but possible. Assuming
the catch clause exists (a different regexp test), and the Log.log call exists, this will probably do it:
"catch\W*\(.*\)\W*{\W*Log\.log\(\".*\+^[\+]"
The last check looks to see if "+" is there. Anything matching this doesn't have the "+".
Our Source Code Search Engine (SCSE) uses the lexemes of the language rather than regexes to enable straightforward searches, so it has a slightly unusual query language written in terms of language lexemes. It also allows "negation" on a larger scale; you can subtract hits in two regions, and that's really useful. This means the following query would do the trick:
'catch' '(' I I ')' '{' I - I=Log '.' I=log '(' S '+' I ')'
This finds hits for all "catch" clauses and the start of the block (assuming it starts with "Log", and subtracts away any matches to the logging idiom. Quoted terms are language atoms. I stands for "I(dentifier)"; it can be any identifier (just I) or constrained to a particular regex for the identifier (of which "Log" is a particularly simple regex). S stands for "S(tring)", also allowing constraints which we don't need for this query. This query has two sub-queries, one part before the minus sign that finds "catch" clauses and a prefix of the catch body, and one part after the minus sign that looks for that idiom OP insists he wants. Any overlaps of results of the second subquery with the first cause the overlapped queries to be "subtracted" (the minus sign) from the result. So the final result are "catch clauses that don't start with a logging step".
A more sophisticated check requires finding the catch clauses, and logging clauses, and verifying that the logging clauses do not occur anywhere in the catch block. The SCSE can't do this by itself. More sophisticated engines that parse and build ASTs can be used to determine this. I know of tools that can do, this too, if OP wants further elaboration.
Imagine you have a function that converts ints to roman string:
public String roman(int)
Only numbers from 1 to 3999 (inclusive) are valid for conversion.
So what do you do if someone passes 4000 in any OO language?
raise an exception
return “” or some other special string
write an assert
…
Number 1: raise an exception. That's what ArgumentOutOfRangeException is for (at least in .NET):
if (intToConvert >= 4000)
{
throw new ArgumentOutOfRangeException("intToConvert ", "Only numbers 1-3000 are valid for conversion.");
}
I find the validation topic very interesting in general. In my opinion option 2 (returning a special value) is not a good one, since you are forcing the client to do if/case to check for the returned value and that code must be repeated everywhere. Also, unlike exceptions that propagate through the calling stack, in this scenario the caller is almost always the one that has to handle that special value.
In the context of OOP raising an exception or having an assertion is, IMO, a more elegant way to cope with it. However i find that inlining verification code in every method doesn't scale well for some reasons:
Many times your validation logic ends up being greater than the method logic itself, so you end up cluttering your code with things that are not entirely relevant to it.
There is no proper validation code reuse (e.g. range validation, e-mail validation, etc).
This one depends on your tastes, but you will be doing defensive programming.
Some years ago I attended to a talk about validators (a similar talk slide's are here. The document explaining it used to be in http://www.caesarsystems.com/resources/caesarsystems/files/Extreme_Validation.pdf but now its a 404 :( ) and totally like the concept. IMHO having a validation framework that adopts the OO philosophy is the way to go. In case you want to read about it I've written a couple of posts about it here and here (disclaimer: the posts are part of the blog of the company I work for).
HTH
This might be an odd question, but I'm looking for a word to use in a function name. I'm normally good at coming up with succinct, meaningful function names, but this one has me stumped so I thought I'd appeal for help.
The function will take some desired state as an argument and compare it to the current state. If no change is needed, the function will exit normally without doing anything. Otherwise, the function will take some action to achieve the desired state.
For example, if wanted to make sure the front door was closed, i might say:
my_house.<something>_front_door('closed')
What word or term should use in place of the something? I'd like it to be short, readable, and minimize the astonishment factor.
A couple clarifying points...
I would want someone calling the function to intuitively know they didn't need to wrap the function an 'if' that checks the current state. For example, this would be bad:
if my_house.front_door_is_open():
my_house.<something>_front_door('closed')
Also, they should know that the function won't throw an exception if the desired state matches the current state. So this should never happen:
try:
my_house.<something>_front_door('closed')
except DoorWasAlreadyClosedException:
pass
Here are some options I've considered:
my_house.set_front_door('closed')
my_house.setne_front_door('closed') # ne=not equal, from the setne x86 instruction
my_house.ensure_front_door('closed')
my_house.configure_front_door('closed')
my_house.update_front_door('closed')
my_house.make_front_door('closed')
my_house.remediate_front_door('closed')
And I'm open to other forms, but most I've thought of don't improve readability. Such as...
my_house.ensure_front_door_is('closed')
my_house.conditionally_update_front_door('closed')
my_house.change_front_door_if_needed('closed')
Thanks for any input!
I would use "ensure" as its succinct, descriptive and to the point:
EnsureCustomerExists(CustomerID)
EnsureDoorState(DoorStates.Closed)
EnsureUserInterface(GUIStates.Disabled)
Interesting question!
From the info that you have supplied, it seems to me that setstate (or simply set, if you are setting other things than states) would be fine, though ensure is good if you want to really emphasize the redundancy of an if.
To me it is however perfectly intuitive that setting a state does not throw an exception, or require an if. Think of setting the state of any other variable:
In C:
int i;
i = 5; // Would you expect this to throw an exception if i was already 5?
// Would you write
if (i != 5)
i = 5;
// ?
Also it only takes about one sentence to document this behaviour:
The function does nothing if the
current state equals the requested
state.
EDIT: Actually, thinking about it, if it is really important to you (for some reason) that the user is not confused about this, I would in fact pick ensure (or some other non-standard name). Why? Because as a user, a name like that would make me scratch my head a bit and look up the documentation ("This is more than just an ordinary set-function, apparently").
EDIT 2: Only you know how you design your programs, and which function name fits in best. From what you are saying, it seems like your setting functions sometimes throw exceptions, and you need to name a setting function that doesn't - e.g. set_missile_target. If that is the case, I think you should consider the set_if, set_when, set_cond or cond_set names. Which one would kind of depend on the rest of your code. I would also add that one line of documentation (or two, if you're generous), which clarifies the whole thing.
For example:
// Sets missile target if current target is not already the requested target,
// in which case it does nothing. No exceptions are thrown.
function cond_set_missile_target ()
or function cond_set_MissileTarget ()
or function condSet_MissileTarget ()
or function condSetMissileTarget ()
ensure is not so bad, but to me it implies only that there is additional logic required to set the state (e.g. multiple states tied together, or other complications). It helps to make the user avoid adding unnecessary ifs, but it does not help much with the exception issue. I would expect an ensure function to throw an exception sooner than a set function, since the ensure function clearly has more responsibilities for, well, ensuring that this setting operation is in fact done right.
I'd go for ensure for the function you describe. I'd also use camelCase, but I suppose you may be in a language that prefers underscores.
You could always document (shock!) your API so that others don't make the mistakes you describe.