Other Ways to Run Code Fragments

Code.eval_quoted

We can use the function Code.eval_quoted to evaluate code fragments, such as those returned by quote.

iex> fragment = quote do: IO.puts("hello") 
{{:.,[],[{:__aliases__,[alias: false],[:IO]},:puts]},[],["hello"]} 

iex> Code.eval_quoted fragment
hello
{:ok,[]}

By default, the quoted fragment is hygienic and therefore doesn’t have access to variables outside its scope. Using var!(:name), we can disable this feature and allow a quoted block to access variables in the containing scope. In this case, we pass the binding to eval_quoted as a keyword list.

iex> fragment = quote do: IO.puts(var!(a))
{{:., [], [{:__aliases__, [alias: false], [:IO]}, :puts]}, [],
[{:var!, [context: Elixir, import: Kernel], [{:a, [], Elixir}]}]} 

iex> Code.eval_quoted fragment, [a: "cat"]
cat
{:ok,[a: "cat"]}

Code.string_to_quoted

The Code.string_to_quoted function converts a string containing code to its quoted form. Macro.to_string converts a code fragment back into a string.

iex> fragment = Code.string_to_quoted("defmodule A do def b(c) do c+1 end end") 
{:ok,{:defmodule,[line: 1],[{:__aliases__,[line: 1],[:A]},
[do: {:def,[line: 1],[{:b,[line: 1],[{:c,[line: 1],nil}]},
[do: {:+,[line: 1],[{:c,[line: 1],nil},1]}]]}]]}}

iex> Macro.to_string(fragment)
"{:ok, defmodule(A) do\n def(b(c)) do\n c + 1\n end\nend}"

Code.eval_string

We can also evaluate a string directly using Code.eval_string.

iex> Code.eval_string("[a, a*b, c]", [a: 2, b: 3, c: 4])
{[2,6,4],[a: 2, b: 3, c: 4]}

Run the commands given in the snippets in the terminal below: