[mtlynch]

I get a lot of value from this rule even without those language features.

I follow "Parse, Don't Validate" consistently in Go. For example, if I need to parse a JSON payload from an end-user for Foo, I define a struct called FooRequest, and I have exactly one function that creates a FooRequest instance, given a JSON stream.

Anywhere else in my application, if I have a FooRequest instance, I know that it's validated and well-formed because it had to have come from my FooRequest parsing function. I don't need sum types or any special language features beyond typing.

[jotaen]

My main take-away is the same, I wonder though whether “parse, don’t validate” is the right term for it. To me, “parse, don’t validate” somehow suggests that you should do parsing instead of validation, but the real point for me is that I still validate (as before), plus I “capture”/preserve validation success by means of a type.

[dangets]

A similar saying and popular blog post title is "Make illegal states unrepresentable"

https://fsharpforfunandprofit.com/posts/designing-with-types...

https://ybogomolov.me/making-illegal-states-unrepresentable/

[qsort]

It's in the same sense of "whitelist, don't blacklist", or "by the love of god it's 2023, do not escape SQL".

Don't define reasons why the input is invalid, instead have a target struct/object, and parse the input into that object.

[blincoln]

I like this explanation and approach, but how does it solve the first problem described in the article - the case where there's an array being processed that might be empty?

There are plenty of cases in real-world code where an array that's part of a struct or object may or may not contain any elements. If you're just parsing input into that, it seems like you'd either still end up doing an equivalent of checking whether the array is empty or not everywhere the array might be used later, even if that check is looking at an "array has elements" type flag in the struct/object, and so you're still maintaining a description of ways that the input may be invalid. But I'm not a world-class programmer, so maybe I'm missing something. Maybe you mean something like for branches of the code that require a non-empty array, you have a second struct/object and parser that's more strict and errors out if the array is empty?

[strgcmc]

Remember, the author of the article constructed a scenario where, it was expected that the "main" function ends up treating an empty "CONFIG_DIRS" input as an uncatchable IOError; in other words, an empty array was invalid/not-allowed, per the rules of this program. Depending on the context in which you are operating, you may or may not have similar rules or requirements to follow.

Empty lists are actually generally not a big deal - they are just lists of size 0, and they generally follow all the same things you can do with non-empty lists. The fact that a "head" function throws an error on an empty list, is really just a specific form of the more general observation that: any array would throw an index-out-of-bounds exception when given an index that's... out of of bounds. So any time you are dealing with arrays, you probably need to think about, "what happens if I try to index something that's out of bounds? is that possible?"

In this particular contrived example, all that mattered was the head of the array. But what if you wanted to pick out the 3rd argument in a list of command line arguments, and instead the user only gave you 2 inputs? If 3 arguments are required, then throw an IOError as early as possible after failing to parse out 3 arguments; but once you pass the point of parsing the input into a valid object/struct/whatever, from that point forward you no longer care about checking whether the 3rd input is empty or not.

So again, it depends on your scenario. Actually the more interesting variant of this issue (in OO languages at least) is probably handling nulls, as empty lists are valid lists, but nulls are not lists, and requires some different logic usually (and hence why NullPointerExceptions aka NPEs are such a common failure mode).

[blincoln]

I see what you're saying, but I'm still not understanding how it becomes a generalizable rule for real-world code without adding a lot of exceptions to the rule, or doing something over-engineered like parsing into an increasingly specific number of customized structs/objects in different branches of the code.

Just to be clear, I actually really like the idea of parsing the input into a structure. I do the same thing in a lot of my code. I just don't see how it removes the need to also perform validation in many (maybe most) cases as soon as one gets beyond contrived examples.

The empty array example seems to be a can of worms. Maybe it's specific to the kinds of software that I've written, but in most of the cases I can think of, I wouldn't know if it was OK for a particular array within a structure to be empty until after the code had made some other determinations and branched based on them. And yet, like the example, once it got to the real handling for that case, it would be a problem if the array were empty. So the image in my mind is many layers of parsing that are much more complicated and confusing to read than validating the length of the array.

I still think it's a great idea for a lot of things, just that the "parse, don't validate" name seems really misleading. I might go with something like "parse first, validate where necessary".

[lmm]

> There are plenty of cases in real-world code where an array that's part of a struct or object may or may not contain any elements. If you're just parsing input into that, it seems like you'd either still end up doing an equivalent of checking whether the array is empty or not everywhere the array might be used later, even if that check is looking at an "array has elements" type flag in the struct/object, and so you're still maintaining a description of ways that the input may be invalid.

You only check it if it makes a difference to validity or not. There's no scenario where you keep the array and a parallel flag - either an empty array is invalid in which case you refuse to construct if it's empty, or an empty array is valid in which case you don't even check. Same thing for if you're checking whether it's got an even number of elements, got less than five elements, etc. - you don't keep the flag around, you refuse to construct your validated structure (even if that "validated structure" is actually just a marker wrapper around the raw array, if your type system isn't good enough to express the real constraint directly).

[bcrosby95]

Depending upon language, and what you're using to hold the array, inheritance.

A 'NotEmpty a' is just a subclass of a potentially empty 'a'. You also get the desirable behavior, in this scenario, of automatic upcasting of a 'NotEmpty a' into a regular old 'a'.

[secdeal]

Not quite, 'a' is the type of the elements 'NonEmpty a' contains.

It is rather the subclass of some kind of 'Iterable a'.

[masklinn]

TFA does explain what they mean:

> in my mind, the difference between validation and parsing lies almost entirely in how information is preserved

“parse don’t validate” is a pithy and easy to remember maxim for this preservation.

Because validation is implicitly necessary for parsing to a representation which captures your invariants anyway, by banning validation as a separate concept you ensure sole validation doesn’t get reintroduced, because any validation step outside of a wider parsing process is considered incorrect.

[lkitching]

The post is suggesting that parsing and validation are different things, since the output of a parser captures the properties being checked in the type, and validation does not. Downstream consumers of validated input cannot rely on the properties that were validated since the representation type doesn't encode them e.g. the non-emptiness of a list.

[autophagian]

I suppose that every successful parse also has an implicit validation step in it. Like you said, for me the principle is more about embedding that validation step into an actual type rather than crossing my fingers and hoping that whatever's coming into my function is what I expect it to be.

[ghusbands]

It's not just about the validation success, but about having only one bit of code consuming the looser input and producing a definitely-correct output. If you simply validate and preserve success, you still later need to produce the output you need, and it's hard to be sure that the earlier validation and the later parsing actually agree on what is valid.

If you're talking about consuming the looser input and producing a definitely-correct output, already, then you're talking about parsing, not validation. Most validation occurs naturally during parsing.

[friendzis]

> suggests that you should do parsing instead of validation

Kind of yes, but this discussion is much dependent on definitions of `parse` and `validate`, which the article does not explicitly elaborate on. The chapter "The power of parsing" captures this difference implicitly "validateNonEmpty always returns (), the type that contains no information". Validation, in the context of all of this, can be defined as "checking conformance to a set of rules" while parsing is mostly synonymous with deserialization.

In most practical application you explicitly do not want to only validate inputs as in you have no need to perform any computation on invalid input anyway. Sometimes you explicitly want to analyze invalid inputs, maybe try and recover some information or do some other magic. Sure then, go and validate input and do that magic on invalid input. In most cases you want to simply reject invalid inputs.

However, when you think about it, that is what parsing does. Validation happens during parsing implicitly: parser will either return a valid object or throw an error, but parsing has an added benefit that the end result is a datum of a known datatype. Of course it only really works in statically typed languages.

The thing is that it is rather easy to conflate the two. Take for example the following JSON `{"foo": "bar", "baz": 3}`. A "parser" can return 1) a list of `(String, String, String)` 3-tuples for (type, key, value) that downstream has to process again 2) full blown `FoobarizerParams` object or something in between.

[b0afc375b5]

How about "Parsing IS validation"?