marcandre

marcandre

I had to write a macro helper which I wanted to return the ast corresponding to a string with the interpolation for a parameter:

def example(some_ast, value)
  quote do
    "...#{unquote(some_ast)}...#{unquote(value)}!"
  end
end

example(quote do foo() end, :bar) # => quote do "...#{foo()}...#{:bar}!" end

This works but I was wondering if there was a simple way to modify the implementation so it returns quote do "...#{foo()}...bar!" end instead?

It’s really just curiosity, as I wouldn’t be surprised if the second interpolation was optimized away.

Showing Posts 1 to 3

kip

kip

ex_cldr Core Team

The AST for a string with interpolation is the AST for string concatenation. The interpolation syntax doesn’t exist in the AST - its syntactic sugar. For example (as you’ve probably seen):

iex> quote do
...> "This is #{some} string"
...> end
{:<<>>, [],
 [
   "This is ",
   {:"::", [],
    [
      {{:., [], [Kernel, :to_string]}, [], [{:some, [], Elixir}]},
      {:binary, [], Elixir}
    ]},
   " string"
 ]}

I’d therefore be structuring my returned AST that way, rather than trying to recreate what is human-readable-only format (the interpolation syntax).

hst337

hst337

So you want to partially evaluate the AST. This is just not possible in elixir out of the box.

marcandre

marcandre OP

Thanks to you both, that’s what I thought, thanks for the confirmations.

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