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From Dániel Dékány (Jira) <j...@apache.org>
Subject [jira] [Comment Edited] (FREEMARKER-107) Hash expansion to macro arguments (Python **kwargs style)
Date Fri, 11 Oct 2019 13:32:00 GMT

    [ https://issues.apache.org/jira/browse/FREEMARKER-107?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16949448#comment-16949448
] 

Dániel Dékány edited comment on FREEMARKER-107 at 10/11/19 1:31 PM:
--------------------------------------------------------------------

Aha, I see. That seems to be something that increases the value of {{spread_args}} significantly. I
will see if there are any major technical gotchas with implementing {{.args}}.

As you are saying, {{.args}} has to contain the catch-all parameters "exploded". One problematic
edge case is, what if the call to {{m2}} passed arguments by positional, as in {{<@m2
1 2 3 4/>}}, and {{m2}} has a catch-all arguments. While repeating the positional call
on {{m3}} is possible technically, people could burn themselves too easily (not expecting
that arguments are sometimes aren't matched by name, but by order, when {{?spread_args(.args)}}
is applied). So, I think that inside a macro, {{.args}} will always return an "FTL hash"
(a map really), and so using it for a positional call that has non-empty catch-all argument
is always an error, because we can't associate names to the catch-all arguments then. We could
just return a sequence for the whole {{.args}} in that case, and it's even logical to do
as the call was positional, but I think it's too dangerous/surprising in practice to do so.
Inside functions, however, {{.args}} will always return an "FTL sequnce" (a list).

 


was (Author: ddekany):
Aha, I see. That seems to be something that increases the value of {{spread_args}} significantly. I
will see if there are any major technical gotchas in implement {{.args}}.

As you are saying, {{.args}} has to contain the catch-all parameters "exploded". One problematic
edge case is, what if the call to {{m2}} passed arguments by positional, as in {{<@m2
1 2 3 4/>}}, and {{m2}} has a catch-all arguments. While repeating the positional call
on {{m3}} is possible technically, people could burn themselves too easily (not expecting
that arguments are sometimes aren't matched by name, but by order, when {{?spread_args(.args)}}
is applied). So, I think that inside a macro, {{.args}} will always return an "FTL hash"
(a map really), and so using it for a positional call that has non-empty catch-all argument
is always an error, because we can't associate names to the catch-all arguments then. We could
just return a sequence for the whole {{.args}} in that case, and it's even logical to do
as the call was positional, but I think it's too dangerous/surprising in practice to do so.
Inside functions, however, {{.args}} will always return an "FTL sequnce" (a list).

 

> Hash expansion to macro arguments (Python **kwargs style)
> ---------------------------------------------------------
>
>                 Key: FREEMARKER-107
>                 URL: https://issues.apache.org/jira/browse/FREEMARKER-107
>             Project: Apache Freemarker
>          Issue Type: New Feature
>          Components: engine
>    Affects Versions: 2.3.28
>            Reporter: Pascal Proulx
>            Priority: Major
>
> Hello,
> We heavily rely on Freemarker macros to build a helper template API, but have had to
make large workarounds for passing contents of hashes as macro arguments, for several years.
(In truth I should have made this ticket much sooner!)
> It would help greatly simplify our work to have hash expansion to macro arguments, like
this:
> {code:java}
> <#macro myMacro arg1 arg2 arg3="value3">...</#macro>
> <#assign myHash = {"arg1":"value1", "arg2":"value2"}>
> <@myMacro **myHash/><#-- the hash contents are passed as parameters, instead
of the hash itself -->
> {code}
> This exists in Python:
> {code:java}
> def test_var_args_call(arg1, arg2, arg3):
>     pass
> kwargs = {"arg3": 3, "arg2": "two"}
> test_var_args_call(1, **kwargs)
> {code}
> Essentially the hash contents fill in any arguments not explicitly specified.
> For the case where arguments are specified in addition to the hash, you may need to decide
on a good syntax, e.g.:
> {code:java}
> <@myMacro arg1="value1" **myHash/>{code}
> This example doesn't have much precedent in freemarker syntax but is fairly understandable.
> Although we don't need it nearly as much, the same could be done with lists and function
arguments:
> {code:java}
> <#function myFunc arg1 arg2>...</#function>
> <#assign myList = ["val1", "val2"]>
> ${myFunc(*myList)}
> <#assign myList = ["val2"]>
> ${myFunc("val1", *myList)}
> {code}
> Again similar to Python:
> {code:java}
> def test_var_args_call(arg1, arg2, arg3):
>     pass
> args = ("two", 3)
> test_var_args_call(1, *args)
> {code}
> You might want this for consistency, although in practice the hash expansion will be
many times more useful to us.
> If there's a lack a manpower I could try to see what I can do digging into the source,
but wanted to bring this up for discussion first. It doesn't appear hard to implement to dump
a hash into the macro args map, but there is defining the syntax.
> We use Freemarker 2.3.28 at the moment.
> Thank you



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