Episode: 3392
Title: HPR3392: Structured error reporting
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr3392/hpr3392.mp3
Transcribed: 2025-10-24 22:35:56

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This is Hacker Public Radio Episode 3392-4 Tuesday, the 3rd of August 2021.
Tid's show is entitled, Structured Air Reporting, and is part of the series' Haskell it is hosted
by Tukuta Oto, and is about 29 minutes long, and carries an explicit flag.
The summary is two turtle talks about how she improved build times, by breaking down air
reporting to smaller parts.
This episode of HPR is brought to you by archive.org.
Support universal access to all knowledge by heading over to archive.org forward slash donate.
Hello, this is Tula Turto, and you are listening to the Hacker Public Radio.
Today's episode is about air reporting.
In Episode 2020, I talked a little bit about validation in Haskell, and then I have been working on
that a little bit more, and improved how the errors are now reported.
So this episode will sort of continue from that episode.
So, Tukuta when I wrote the error reporting, I had a, well, I basically wanted to have a unified
way of reporting errors, so that when the server sends an error report to the client,
it's always in the same format, it's an adjacent response that has a machine-readable code
for the client process, and then it would have some human-readable message that can be shown
to the end user. And how I originally wrote that I had a one-type error code that basically
enumerated all the possible error cases that could happen on the server side.
By the way, a lot of the code I'm talking about in this episode will be in the show notes as
usual, so that error code had long list of possible errors that could happen on the server side.
And then I had four health functions, error code to status code, which takes error code and returns
in there. And this one is for designing what HTTB status code the servers would be returning.
So, for example, if there's a, for example, the client's, in example, if the client tries to
upgrade the intrigue, stats of the person in the game, that defines how good that person is
in the intrigue. And if that upgraded value is too low, then this, we would be raising the,
stat is too low error code. And the error code to status code would turn that to the 400,
so that would indicate that it's a tag request. Another function, status code to text,
takes this integral status code and returns the byte string. This is for the HTTB response again.
So, for the 400, it would return a tag request. And then the actual need of the problem,
error code to text. This takes the error code and returns a textual representation of that,
something that can be shown to the end user. And in this case, it would be stat intrigue is too low.
And then the final function is racist errors. This takes a list of error code and returns handler
for app unit. This is, this is the thing that the handler function, that's the function that
is handling the user request calls when it wants to raise an error. If the, if that list contains
any error codes, then it uses the, these three functions I mentioned earlier and construct HTTB
response. So in our case, it would be HTTB 400, that request, that would have a body response
body of JSON, that will contain an array, that will contain an object, that would have a
code element containing another object that has a tag status to flow and content in rig. This is
machine-readable part. And then it will have an error element that contains stat intrigue is too low,
text, this is the part that gets shown to the user. And this works pretty fine. I was
happy how this worked. But this has a problem. All the, all the possible errors are enumerated in one
type. So that type will grow longer and longer and longer. And every part of the system that wants
to report an error is referring to that type. Meaning whenever I add a new error to the enumeration,
then all the places that refer to that type, that depend on that type, will have to be compiled.
And of course, that triggers a cascading effect because all the modules that refer to those
compiled ones need to be compiled and so on. So the picket type grows and the more places it is
referred, the slower the compile time will be because it triggers quite a big compilation.
And I didn't like that. So I wanted a solution and thought that I could break that error code type
into smaller types. So instead of having one huge error, I would have a, for example, person
creation error type that would list only the errors that would happen when you are creating a new
person. And then there would be an account error that would list all the errors that relate to
the account. And these types will be defined in different modules close to, close to
place where they used, used. Which means that if I add a new person creation error,
then only that module will be compiled and all the modules that depend on that module.
So there's still a compilation coin on, but there's a lot less compilation coin on.
But here's a problem. But I cannot have a, I cannot have a list that contains two different types
in it. Like that. Race if errors takes a list of error code. And if I have multiple types,
I cannot, I cannot, I cannot put multiple different types of values of multiple different types
in the same, the list always has to contain the same, same type. But I decided that I worried
that a little bit later. First, I wanted to split the huge error code type into the smaller types.
So I defined person creation error that is at just a numberation of all the errors relating to
the person creation. And then use the template Haskell to derive to JSON and from JSON functions
for this specific type, these are, these functions can be used to serialize and then serialize
person creation error to and from JSON, which is what I'm sending to sending and receiving from the
client. And then I wanted to have a way of getting the HTTP status code and that error description.
Previously, there were defined as functions that took the error code. But now I have to work on
multiple types. So answer was to define a type class. Type class is sort of kind of similar
than interface in object-oriented programs, but slightly different. Because in object-oriented
world, when you define an interface, you define a set of functions or methods that you have to
implement when you implement that interface in Haskell, that part is the similar. But in the,
for example, in the sheet shop, when you define a class that implements an interface, you have to
implement those functions and methods when you are defining the class. In Haskell world,
when you are defining a data type, you're not actually telling that this has that specific type
class. You are doing that separately. So you can have a data type and then you can, in addition to
that definition, you can define the type class instances for that. Which means that if I come up with a
completely new type class that I need, I can define an instance of that for the already existing
types. I can, for example, take the Boolean type and say that here's the instance of my type class
for the Boolean. So I'm adding new functionality that can be used to handle the Boolean
without having to modify the Boolean. That's the big difference.
In any way, anyway, so I defined a type class called error code class A. And the definition
goes like a class, error code class A, where HDTV status code, a double colon A, a row int,
description, double colon A, a row text. This means that any, any instance of error code class
has to have two functions defined. HDTV status codes, that takes that data type A and turns that
into the int. And then there has to be the description function that takes that data type A
and turns that into text. And these are the ones that I'm using to
turning the person creation error into a HDTV status code and to the user-readable message.
That status code to text that I mentioned earlier, that takes the status code and turns that into
the HDTV message, like that request, for example, that I could use as is. I didn't have to modify that
because that takes an int, returns a binary, yeah, byte string. Sorry.
Okay, so to define the instance of error code class, I just wrote wrote instance error code class,
person creation error, where HDTV status code equals slash case, status to low,
a row 400 and anywhere at all the cases. So HDTV status code is a case analysis basically.
So if it's a, if the, if the error, person creation error is the status to low,
then I'm returning 400. If I have a could not confirm data of third, then that's 500.
That's a, the internal server error, there's something really fishy going on in case if a
person doesn't have a date of third. And then the description is again a,
a description equals slash case, status to low, s, a row, status, plus s, plus is to low.
This is the textual, textual, that's the name of the status, that is to low. So when,
when I'm raising a status to low error somewhere, I'm creating a status to low error somewhere,
I have to, I have to tell what status to low, is it intrigue or is it a
diplomacy or is it a martial skill or something else. So I have to tell that this specific
status to low and then it's reported to the user that, hey, by the way, you try to do something
funny or the program try to do something funny and this status to low, somebody has to fix it.
So, and of course, since I had multiple types of different conscious error, I had to
define this error code class instance to all of those types. It's pretty mechanical work.
You just list all the error cases that you, you can have decide, but HTTP status, they should
result to and what textual representation to the user should be given.
But then the bigger error, so I need to, way to put these different kinds of error into same list,
and that's, that's not something that Haskell can do in Haskell. A list is always
the regular list is always of all the values inside of it has to be of same type.
So, if they have to be a same type, I thought that maybe I could sort of have a helper type and
grab these specific specific types into that type. So, I define the new type as data E code A
equals E code A. So, this is a, this defines a type E code that can, that has a one constructor,
also called E code that takes one parameter A. So, it can wrap anything.
And, but the problem here is that the resulting, resulting type is a E code A. So,
if I'm using it to wrap a person creation error, the resulting type is E code
person creation error. And if I use it wrap common error, then I, the resulting type is E code
common error. And these are different types. So, I need to, I need to, I need it a way that I could
wrap type values of types. And somehow lose the type of the wrapped value in a way that the
resulting, resulting type would always be the same. So, and then of course, the another thing is
that I need that the previous definition that had only the data E code A.
It doesn't have, well, it can be anything. It could be indexer, it could be a boolean,
it could be a some map or any type. So, I cannot, I cannot, I cannot, I cannot turn any type
into a error report. I have to, I, all the data that I wrap needs to, needs to have a
instance for the error report class. And also, I need to have an instance for the two JSON. So,
that I can serialize them to the JSON data and send them to the, send them back to the client.
There's several ways of doing this, but I chose to use general, generalized algebraic data types
against for the short. That's an extension that has to be turned on. That's not in the standard
Haskell. So, I had to have a pragma language cards in the top of the file. And that allowed me
to define the E code as a following data, E code, where E code, double colon, error code class A,
comma, JSON, A, tick, arrow, A, arrow, E code. So, what that, what that means is that, that defines a
type E code that has a one constructor, also called E code, that constructor takes a one
parameter A and returns a E code. And that A is constrained. It has to have an instance for the
error code class and to JSON. So, it has constraints, which is what we wanted. And also,
the resulting type is E code, not E code A. So, we essentially have, with this constructor,
we can wrap suitable types, resulting types will be E code. So, we have lost the type
information there. We know that the value that has been wrapped, it has instances to the error
code class and to JSON. So, we can still use functions from those type classes. But that's
all we need to do with that. So, we still need that A to turn this E code into an error message.
But that, that is, that is, that was easy because everything we wrapped with the E code has
error code class type class defined with. So, I think I could just define
instance error code class E code. So, I'm defining an instance of error code class for the E code,
where the status code is just the, is HTTP status code E R. Sorry, HTTP, HTTP status code R and
description is description R. A, meaning that the, when you are asking HTTP status code from the
E code, it's the HTTP status code of the fractal value and description of the E code is the
description of the fractal value. And then I wrote an instance for the, of the JSON E code.
So, turning this E code into JSON. So, I can serialize it to the client. I wrote that instance
by hand. I could have derived that automatically, but I wanted to have a final control.
I wanted to decide by myself how, what, what is the format and structs of the JSON.
The code, code is on the, on the show notes, but basically it's just the serialized, when,
when you are serializing E code, you are creating a JSON object that has three fields, HTTP code,
that is that status code, fault code that is, that fractal value turned into JSON.
So, in case we have a, just on, for example, error, first name is empty, then that first name is
empty, because it doesn't have any parameters, it's just turned into the string. So, fault code will be
the first name is empty. In case, it's a status tool, that takes a, where the constructor takes
parameters, then the fault code will be an object that has a tag, status tool load and condensed
in trig. Tag is the constructor that was used and condensed is the, all the values that were
touched to that constructor. That is the machine readable part and the error description is the
description A, that's the user readable part in our case, starting trig is too low.
And now I could just define the racist errors in terms of the list of E code,
returning handler for app unit, meaning this is a function that you can call from the handler
function and it doesn't return any values, but if it, if, if the, if the errors, it will send a
status JSON. The code is on the, on the show notes, but basically, first we check if the list contains
any errors to begin with and if it does, then we take the first element of the list,
call HTTP status code function to that, to get the status code. And if for some reason,
even if we checked for the, for the condensed, condensed of the list, if for some reason it would be
end here, then we would be using 500 internal server error. The message is the status code to text
function called with the, that code. So, for example, a factory quest or internal server error.
And then the, we would call send the status JSON, give it the status, with the code and the message,
and body of errors turned into the JSON. And this, this works pretty, pretty neatly. So,
then since all the errors, so, so this use, this uses E code, as a, as a parameter. So,
I cannot give a status to low value to here. I have to wrap it to the E code. So, I have to
basically call it E code, lower status to low, quotation, start in a intrigue, quotation mark.
And having to manually wrap everything to the E code, it's bit tedious.
So, I define the helper function, status to low, that takes a text and returns an E code.
And that basically is just status to low s equals E code, lower status to low s. So,
it doesn't contain much logic at all. It just takes that string that you give to it.
It uses that to create a status to low value and wraps that value into E code.
So, that just makes things a little bit easier for you to less typing, basically. It
saves a little bit of spiking. And another thing is, if I at some point in the future,
sorry, if I, in some point of the future, decided that I want to change how the internal,
internal of this error reporting works. I can just change the internals and then update all the
functions that I'm using, using to create these error values. And hopefully, whatever these
functions are called, don't need to change at all. So, a little bit of future proof in here,
but the main idea is that I'm just, I just wanted to save a little bit of typing. And this works
pretty well. I get a faster compilation times. I got, I get the same error messages on the client
side as before. And this is nice. There's a different solution to this problem, right?
Instead of having those wrapping, wrapping everything into the E code and having all those
different, different types that you use to create those errors, I could have just defined one
record code, E code that would have filled HD deeper code that is in and description that is text.
So, instead of having a typed errors, I would, I would just use record. Whenever I would want to
report an error, I would just create an, I would just create a new E code record,
put a suitable HD deep HD deep code and suitable description on it. And that would, that would
work just fine. The drawback with that approach would have been that I would not have any control
over what gets reported. I could have a HD deep code 1, 2, 3, 4 and description something funny
happened. Or I could make a typo. Somewhere I would be reporting a, I wanted to report with
error code 400 and I could mistype that action to 499. And compiler wouldn't be able to catch that.
Of course, I could still have a helper function that is to low, that takes a parameter of text,
that returns an E code and instead of that, status to, status to low function, construct this E code
record. But I just basically decided to record with the first, first option when I was thinking
about this, there isn't that, there isn't that big difference in the end result. Actually the
end result is the same. It's just a little bit, little bit difference in the internals of the
machinery of the things work. Okay, that's, that's it. So this is, this is how I, I
broke one huge error code type into, into smaller types and improved the compilation times
of my project. If you have any questions, comments or feedback, I can be reached with the email
or at theferiverse, where I am, todurdoattech.lgpt. Have a happy hacking.
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