exercism/elixir/top-secret/HINTS.md

Hints

General

• Read about quoting in the official Getting Started guide.
• Read the introduction to Elixir AST by Lucas San Román.
• Read the official documentation for quote.
• Inspect the output of quote to familiarize yourself with how ASTs look like for specific code snippets.

1. Turn code into data

• There is a built-in function that turns a string with code into an AST.

2. Parse a single AST node

• Inspect the output of quote to familiarize yourself with how ASTs look like for specific code snippets.
• The operations that define a function are :def and :defp.
• The operation is the first element in a three-element AST node tuple.
• You can ignore the second element in the tuple in this exercise completely.
• The third element in the tuple is the argument list of the operation that defines the function.
• The first element on that list is a tuple with the function's name and arguments, and the second element is the function's body.

3. Decode the secret message part from function definition

• Inspect the output of quote to familiarize yourself with how ASTs look like for specific code snippets.
• The AST node that contains the function's name also contains the function's argument list as the third element.
• The arity of a function is the length of its argument list.
• There is a built-in function in the String module that can get the first n characters from a string.
• A function without arguments written without parentheses will not have a list as argument but an atom.

4. Fix the decoding for functions with guards

• Inspect the output of quote to familiarize yourself with how ASTs look like for specific code snippets.
• When a function has a guard, the third element in the tuple for the :def/:defp operation is a bit different.
• That third element is a list with two elements, the first one is the tuple for the :when operation, and the second one is the function's body.
• The :when operation's arguments are a two-element list, where the first argument is the function's name, and the second is the guard expression.

5. Decode the full secret message

• Use the function to_ast/1 that you implemented in the first task to create the AST.
• There is a built-in function that can visit each node in an AST with an accumulator.
• Use the function decode_secret_message_part/2 that you implemented in previous tasks to prewalk the AST.
• To reverse the accumulator at the end and turn it into a string, refresh your knowledge of the Enum module.