DidactMacros
Regarding AST form with qualified call
I’ve been reading a great AST guide that I really recommend.
One of the examples given in the first part correctly states that…
quote do
Foo.{Bar, Baz}
end
#=>
{
{:., [], [{:__aliases__, [], [:Foo]}, :{}]},
[],
[{:__aliases__, [], [:Bar]}, {:__aliases__, [], [:Baz]}]
}
There is an explanation given in the guide as to why the form [alias, :{}] is used, but I don’t really understand why there would be ambiguity in using the form below…
quote do
Foo.{Bar, Baz}
end
#=>
{
:.,
[],
[
{:__alias__, [], [:Foo]},
{:{}, [], [{:__alias__, [], [:Bar]}, {:__alias__, [], [:Baz]}]}
]
}
I put the above hypothetical AST in a Macro.to_string call, and I got the following error.
** (CaseClauseError) no case clause matching: :Bar
(elixir 1.16.0) lib/code/formatter.ex:2319: Code.Formatter.force_args?/2
(elixir 1.16.0) lib/code/formatter.ex:1172: Code.Formatter.call_args_to_algebra_no_blocks/6
(elixir 1.16.0) lib/code/formatter.ex:1109: Code.Formatter.call_args_to_algebra/6
(elixir 1.16.0) lib/code/formatter.ex:1056: Code.Formatter.local_to_algebra/5
(elixir 1.16.0) lib/code/formatter.ex:1695: anonymous fn/5 in Code.Formatter.args_to_algebra_with_comments/7
(elixir 1.16.0) lib/code/formatter.ex:1998: Code.Formatter.each_quoted_to_algebra_without_comments/4
(elixir 1.16.0) lib/code/formatter.ex:1984: Code.Formatter.quoted_to_algebra_with_comments/6
iex:33: (file)
Is my AST not correct?
Marked As Solved
al2o3cr
It’s only normally written in the context of an alias statement, like alias Foo.{Bar, Baz}.
Interestingly, the parser won’t let you write a function named {} explicitly, but one can be created by using a macro to build the AST:
defmodule WeirdMacro do
defmacro do_fun(name) do
{:def, [context: Elixir, import: Kernel],
[
{name, [context: Elixir],
[{:x, [if_undefined: :apply], Elixir}, {:y, [if_undefined: :apply], Elixir}]},
[
do: {{:., [], [{:__aliases__, [alias: false], [:IO]}, :inspect]}, [],
[
{{:x, [if_undefined: :apply], Elixir},
{:y, [if_undefined: :apply], Elixir}}
]}
]
]}
end
end
defmodule Foo do
require WeirdMacro
WeirdMacro.do_fun(:{})
end
Foo.{:arg1, :arg2}
I’m unsure when such a function would be useful - maybe in a complex DSL? - but it’s neat to see that it’s possible.
Also Liked
al2o3cr
The AST is expecting :__aliases__; not sure where the singular form has appeared from.
Fixing that in your sample AST and calling Macro.to_string produces:
Foo . {Bar, Baz}
It may help to contemplate this syntax, which expands to identical AST as Foo.{Bar, Baz}:
Foo."{}"(Bar, Baz)
Foo."{}" corresponds to {:., [], [{:__aliases__, [], [:Foo]}, :{}]}.
This is the same shape that a normal function call like Foo.huh(Bar, Baz) would produce, except the function being called has the otherwise-impossible name {}.
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