roehst
Hi, I have been playing around with the Elixir AST (trying to help with an Issue in Credo) and I found two things that I would like to know more about.
The issue was that Credo has an ABC complexity metric check and it has to detect function calls, but the AST (such as the one that Code.string_to_quoted gives) has very little syntatic information, i.e., I expected that terms would be marked with their syntatic category (variables, functions, etc).
I tried reading the Elixir compiler source or the Code and Macro modules to find if there was a way to get that information but I could not find it.
I also failed to find how to maintain a stack using the Macro.traverse/prewalk/postwalk calls (to keep information about the current lexical scope).
The book on Elixir Metaprogramming is wonderful on writing macros, but does anyone know a resource about analyzing the AST, even if that means diving deeper in the compiler and using lower-level representations? I would be immensely grateful for that!
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OvermindDL1
You can do it via the accumulator argument. Honestly though I just encode my own processing as just normal function calls, Elixir’s AST is really simple.
roehst
I could push values to the accumulator in the prewalk function, but how can I pop them at the right time? Because for example in the prewalk it will read a node and then its children, and then it will go to its sibling node. How can I tell I’m at the last child so that I can pop information from the stack? For example suppose the stack is tracking nested function calls
OvermindDL1
Same way as you would anywhere else, just pop the values and return the new structure with the popped values removed.
Use traverse so you get both ‘up’ and ‘down’ calls.
roehst
Are pre and post arguments in traverse the push/pop down/up calls?
OvermindDL1
Yep,
premeans it is run ‘before’ recursing down,postmeans it is run ‘after’ recursing down, so you will push environment in onpreand pop the environment off inpost.OvermindDL1
Also, the
prewalkandpostwalkare just like usingtraversebut doing nothing in the other one, soprewalkis like using theprein traverse while doing nothing inpost(fn x -> x end) and likewise for the other way.roehst
Cool thanks! But you roll your own traversals usually?
OvermindDL1
I tend to do slightly more complex work than most though… ^.^;