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From Woonsan Ko <woon...@apache.org>
Subject Re: [FM3] Basic treatment of null VS missing
Date Tue, 06 Mar 2018 02:59:22 GMT
On Sat, Mar 3, 2018 at 11:22 AM, Daniel Dekany <ddekany@apache.org> wrote:
> Below I propose a solution for the basic treatment of null VS
> "missing" in FM3. Please tell me what you think!
>
> The way FM2 deals with null-s has quite severe limitations (and
> glitches). The root of the problems is that null and missing is the
> same for the template language. It's not aware of the concept of null;
> for example a variable has either non-null value, or the variable is
> considered to be nonexistent. So <#assign x = obj.m()> fails when
> obj.m() returns null, for the same reason <#assign x = noSuchVariable>
> does. (Note that even if we store null into x on the implementation
> level, reading x will fall back to higher scopes to find an x there,
> instead of returning null.) Another problem is with
> macro/function/method arguments; f(noSuchVariable) will not fail if f
> provides a default for the parameter (as in <#function f(x=0)>), on
> the basis that noSuchVariable is missing, so the parameter was
> practically omitted. Not good, as thus we have suppressed an error
> that tells us that there's no noSuchVariable. Also, if in f you want
> to call a obj.javaMethod(x), where x is the parameter of f, and you
> want to allow x to be null, because obj.javaMethod(x) allows that, you
> can't. Because, if you don't provide a default, f(m.willReturnNull())
> will be an error for using a null/missing value for a required
> parameter, and if you do provide a default, then since null is not a
> thing in the template language, you can't specify null as the default
> value of the parameter.
>
> So, here's my plan for FM3.
>
> The template language will know null, and it's different that from
> "missing". I think so far everyone who has used FM2 heavily will
> agree.
>
> On the API and implementation level we will have a singleton,
> TemplateNullModel.INSTANCE, that stands for things that do exist but
> has null value, while a plain null value indicates that the thing is
> missing. For example, Environment.getVariable(name),
> TempateHashModel.get(key), and TemplateSequenceModel.get(index)
> returns null if the subvariable is missing, and
> TemplateNullModel.INSTANCE if it's present but stores a null value.
> Also in TemplateDirectiveModel and TemplateFunctionModel the
> `TemplateModel[] args` argument of execute(...) will contain null if
> the argument was omitted on the caller side (like in f() the 1st
> parameter is omitted), but TemplateNullModel.INSTANCE if it the
> argument was present, but has null value on the template language
> level (like in f(null) or f(obj.willReturnNull())).
>
> The above can be confusing, as null on the template language level is
> TemplateNullModel.INSTANCE on the API level, and null on the API level
> is "missing" on the template language level... So when I write null,
> pay attention to if it's null on the template language level, or null
> on the API/implementation level.

Could it be an option to use either TemplateModel.NULL_VALUE or
TemplateModel.MISSING_VALUE, whether in Environment.getVariable(name)
or in TemplateCallableModel#...)?
It might be more readable if doable.
I guess there should be quite a change even with
TemplateNullModel.INSTANCE or null in CallableUtils.java (e.g,
#getOptionalStringArgument(...)) as it should be determined how to
handle null or missing value there.

Regards,

Woonsan

>
> On the template language level we should have this:
>
> - null is a keyword. "missing" (aka "undefined") is not a value on the
>   template language level, and there's no keyword for it.
>
> - <#assign x = null> is legal. <#assign x = noSuchVariable> is an
>   error, as noSuchVariable is "missing", and not null.
>
> - Reading x doesn't fall back to higher variable scopes if x is null,
>   only if x is missing.
>
> - javaObject.someMethod() evaluates to null when the Java someMethod
>   method returns null in Java. But if someMethod() has void return
>   type, javaObject.someMethod() evaluates to "missing" in the
>   template. (These two cases were indistinguishable in FM2 templates.)
>
> - bean.noSuchProperty (where you don't have a getNoSuchProperty()
>   method in the bean class) is "missing", while bean.nullProperty
>   (where you do have a getNullProperty() method in the bean class, but
>   it returns null) is null.
>
> - javaMap.noSuchKey is null, similarly as Map.get("noSuchKey") returns
>   null in Java. So we don't differentiate the case when the Map
>   doesn't contain the key from the case when the Map contains the key
>   and the associated value is null. While we could do that using
>   Map.contains(key), I believe that would be highly impractical, as
>   you rarely put null values into to Map-s to ensure that all possible
>   keys are present. Compare that with beans, where the "keys" are
>   defined by the bean class statically.
>
> - importNamespace.noSuchVar is "missing". This will make more sense
>   after #var/#set was introduced, and so variables always will be
>   declared on parse time (not just on runtime), so the set of
>   variables in a namespace is fixed.
>
> - [#var x] (future feature) initializes x to null.
>
> It's hard to evaluate the above if you don't known how the
> null/missing handler operators
> (https://freemarker.apache.org/docs/dgui_template_exp.html#dgui_template_exp_missing)
> will react to "missing". That was touched in
> https://lists.apache.org/thread.html/f520220aefc5064030f8674a1cd482b6469797ff74ad2bdd3c0a1b9c@<dev.freemarker.apache.org>,
> but going into details would derail discussion. But the basic idea is
> that exp!default only handles null, not "missing". Why? Let's say you
> write ${user.realNaem!'Not provided'}. As you see you expect the
> realName to be null, and also you managed to write realNaem instead of
> realName (a typo). If exp!default handles "missing", then this mistake
> is hidden by it, while if it only handles null then you get an error
> (as there's no getRealNaem() method in that bean) as desirable. Now,
> there are some rough edges in this idea, like what if you write
> user.phoneNumber, and while getPhoneNumber() dies exists in class
> EmployeeUser, it doesn't exist in CustomerUser... so in case user in
> the data-model is a customer, ${user.phoneNumber!'Not provided'} will
> not save you. If that can't be addressed on the data-model level (like
> abstract class User is annotated with @ValidPropertyNamesFrom({
> EmployeeUser.class, CustomerUser.class })), there's the possibility
> that there will be something like exp!!default that handless both null
> and "missing", or that the right compromise will be if exp!default
> handle both after all. But, I can proceed without deciding this part
> right now.
>
> Another interesting aspect is parameter defaults in macro/function.
> Clearly, an argument that's "missing" is immediately an error (unlike
> in FM2), so defaults are irrelevant in that case. But what will happen
> if you have <#function f(x=0)>, and then you do f(null)? The parameter
> was not omitted, so will x be 0 or null? I think parameters should
> have an attribute that tells if it should accept null-s or not (kind
> of like "not null" constraint in databases). If we accept that idea,
> then the answer to the question is that if x has a "not null"
> constraint then x will be 0, but if x is nullable then x will be null
> in the example case (because the default value is not applied). BTW, I
> believe the best would be if macro/function parameters by default has
> "not null" constraint, and you explicitly indicate if want to allow
> null-s (e.g. `<#function f(x{nullable}=0)>`).
>
> --
> Thanks,
>  Daniel Dekany
>

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