How to get Elixir AST from local function - function

Question

I want to get the AST from a local function that is passed as an argument like doSomething(fn a -> a * 10 end)
So I tried
def test do
inspectAST(fn a,b -> a + b + 42 end)
inspectAST(quote do: fn a,b -> a + b + 42 end)
inspectFunctionInfo(fn a,b -> a + b + 42 end)
:ok
end
def inspectAST(this_is_AST) do
IO.inspect(this_is_AST)
IO.inspect("--------------------------------")
end
def inspectFunctionInfo(fun) do
IO.inspect(Function.info(fun))
IO.inspect("--------------------------------")
end
Results :
iex(3)> Utils.test
#Function<0.30424669/2 in Utils.test/0>
"--------------------------------"
{:fn, [],
[
{:->, [],
[
[{:a, [], Utils}, {:b, [], Utils}],
{:+, [context: Utils, import: Kernel],
[
{:+, [context: Utils, import: Kernel],
[{:a, [], Utils}, {:b, [], Utils}]},
42
]}
]}
]}
"--------------------------------"
[
pid: #PID<0.485.0>,
module: Utils,
new_index: 1,
new_uniq: <<58, 7, 203, 172, 99, 108, 54, 80, 24, 151, 75, 56, 73, 174, 138,
177>>,
index: 1,
uniq: 30424669,
name: :"-test/0-fun-1-",
arity: 2,
env: [],
type: :local
]
"--------------------------------"
What I want is the result from inspectAST(quote do: fn a,b -> a + b + 42 end) ( the AST ) but I would like to send the function like inspectAST(fn a,b -> a + b + 42 end) without the quote do:
If anyone has some idea about this, your help would be really appreciated :)

Answer 1

If you want a "function" to be called on the AST, not values, it should be a macro, not a function.
The following should be a good base for what you want to achieve:
defmacro inspectAST(ast) do
quote do
IO.inspect(unquote(Macro.escape(ast)))
:ok
end
end
def test do
inspectAST(fn a,b -> a + b + 42 end)
end
you need to use defmacro to treat your parameter as AST, not a regular value
we want IO.inspect to happen at runtime, not compile time, so we need to return the AST for it using quote
what we want to inspect is the content of the ast variable, so we are using unquote
but unquote will unescape ast as well, so we need to escape it once more using Macro.escape/2
Please note that the macro should be defined before calling it, so defmacro has to be before test, not after.
For more advanced explanations about macros, you should check this serie of articles which is very detailed.