Fragment args 2024 amendments

[JoviDeCroock]

This spec contains amendments to #1010, a diffed view is available at mjmahone#3.

These amendments are made from comments on the implementation PR and alterations from the new implementation

• Changing fragment-key to just be fragment-name Fragment Arguments Reference Implementation graphql-js#3835 (comment)
• Removing the substitution logic and going closer to the coerce logic we use in the general flow

In general the biggest changes are that we introduce fragmentVariableValues which will be on the groupedFieldSet, these are derived from the arguments in scope of the fragmentDefinition where this field is used.

We introduce localFragmentVariables which as we are traversing down fragment-spreads are a coerced set of variables i.e.

query($b: String!, $c: String!) {
  field3(b: $b) ## uses b from variable-values
  ...A(a: "A", b: "B")
}

fragment A($a: String!, $b: String!) on X {
  field(a: $a) ## uses $a from localVariableValues
  field2(c: $c) ## we have access to variableValues so we take c from there
  ...B(b: $b) ## uses $b from localVariableValues
}

fragment B($b: String!) on X {
  ## This fragment has access to $b and $c but not to $a
  field4(b: $b) ## uses $b from localVariableValues
}

Last but not least we introduce getArgumentValuesFromSpread which looks at the spread and fragment-definition and establishes a coerced set of localVariableValues.

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[mjmahone]

This is clear to me :)

Very excited to see progress, and thank you for cleaning up and clarifying the PR!

[mjmahone]

Huh I had the thought "we should just always make this invalid" but realized that's not really any different from making it invalid to have a fragment spread anywhere with a different variable name as the argument.

This being allowed is a weird consequence of how fragment arguments compose. We maybe should disallow spreading the same fragment at the same level within the same fragment but different argument values (or variables), but only in the exact same way that we disallow field usage with different arguments on the same level.

[JoviDeCroock]

I think it's good to allow for ...commandFragment ...commandFragment (two argumentless fragment-spreads) for backwards compatability and ...commandFragment(a: $a) ...commandFragment(a: $a) because they are the same which in my mind ties intuitively into the former case while the different arguments should be disallowed for ease of reasoning otherwise we'd have to go into the fragment-spreads and analyse whether this changed variable results in conflicting fields due to arguments on the field or @include/@skip/...

[benjie]

I agree with Matt - this should use the same logic as field usage. This would thus be invalid for the same reason that this query (and a query using any two of these three fields) would be invalid:

query ($id1: ID! = "ZmlsbXM6MQ==", $id2: ID! = "ZmlsbXM6MQ==") {
  node(id: "ZmlsbXM6MQ==") { id }
  node(id: $id1) { id }
  node(id: $id2) { id }
}

Example on SWAPI

[JoviDeCroock]

Made it more clear in 03ba255

[mjmahone]

nit: maybe say "for a fragment to define a variable with a name that is also defined on an operation" to disambiguate that we don't mean fragments and operations can define two of the same variable within a single definition

[JoviDeCroock]

Addressed in 48fa841, does that sound better?

[mjmahone]

ah this is a very good catch

[Keweiqu]

The sentence would flow better if we do something like this --

Operation-defined variables have... Fragment-defined variables have ... So a variable used within a fragment must ...

Thoughts?

[JoviDeCroock]

That sounds great, what do you think about 5a73543

[Keweiqu]

Should we specify "must have identical sets of arugments and values?"

[Keweiqu]

and directives?

[JoviDeCroock]

I actually hadn't thought about directives, that's a great point! Addressed in 48fa841

[JoviDeCroock]

@graphql/tsc is there anything I can do to move this forward?

[benjie]

It feels like section 2 should comment on the fact that shadowed variables may not be referenced implicitly by child fragment spreads. (This is probably not the right way of saying that!) E.g. in the following query, Forbidden is spread inside of F and attempts to use variable $a which is ambiguous, and hence not permitted.

query Q($a: Int!, $b: Int!) {
  ...F(a: 7)
}
fragment F($a: Int!) {
  ...Fine
  ...Forbidden
}
fragment Fine {
  b: echo(input: $b)
}
fragment Forbidden {
  a: echo(input: $a)
}

[JoviDeCroock]

@benjie from a purely technical perspective I agree, I was approaching that more from a backwards compatibility perspective. The Fragments variables are only applicable within the context of a FragmentDefinitiion so we don't carry nested values. A fragment is always looked at in isolation and not from a call-stack point of view.

i.e.

query ($a: String!) {
  operation {
    ...fields(a: 'some-value')
  }
}

fragment fields($a: String) on Type {
  field(x: $a) ## this carries 'some-value'
  ...moreFields
}

fragment moreFields on Type {
  field2(x: $a) ## this would carry the value passed into the operation
}

This also makes Fragment-Arguments easier to reason about for me atleast as it's either the Variables passed into the definition or from the operation itself.

[benjie]

I haven't got as far as section 6 yet.

[benjie]

This sentence is doing a lot. Maybe an algorithm would be more appropriate here? Also note that "applying arguments" is under-defined, in particular because validation does not have access to variable values.

[benjie]

This is doing a lot; I would prefer to clarify it with an algorithm:

- Let {spreadsForName} be {GroupFragmentSpreadsByName(visitedSelections)}.

...

GroupFragmentSpreadsByName(selections):

- Let {groupedFragmentSpreads} be a map.
- For each {selection} in {selections}:
  - If {selection} is a {FragmentSpread}:
    - Let {name} be the name of {selection}.
    - Let {fragmentSpreadsWithName} be the set in {groupedFragmentSpreads} for {name}; if no such set exists, create it as an empty set.
    - Add {selection} to {fragmentSpreadsWithName}.
- Return {groups}.

but I see this is based on existing spec text which does similar for fields, so I guess this isn't needed.

[benjie]

@JoviDeCroock Ah! I thought we landed on the other side with this one, this does simplify things significantly because we can look at fragments in isolation 👍