markdev
I am watching this talk by Jesse Anderson at ElixirConf about how macros work. He creates a macro (if statement), then contrasts it with a functional example.
Macro Version:
defmodule MyMacros do
defmacro my_if(condition, do: do_clause, else: else_clause) do
quote do
case unquote(condition) do
true -> unquote(do_clause)
false -> unquote(else_clause)
end
end
end
end
Function version:
defmodule MyFunctions do
def my_if(condition, do: do_clause, else: else_clause) do
case condition do
true -> do_clause
false -> else_clause
end
end
end
If you were to call my_if 1 + 2 == 3, do: "this", else: "that", you get the same result from either module. But if evaluation order matters in the do/else blocks, the behavior is different.
iex(17)> MyMacros.my_if 1 + 2 == 3, do: IO.puts("this"), else: IO.puts("that")
this
:ok
iex(18)> MyFunctions.my_if 1 + 2 == 3, do: IO.puts("this"), else: IO.puts("that")
this
that
:ok
Is this considered an example of lazy evaluation, or is this deferred evaluation typically described some other way?
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tmbb
Your macro isn’t perdorming any kind of evaluation, lazy or strict. The macro works because it expands into a
casespecial form, which does allow you to pick branches. In a sense I guess thecasemacros is doing something that can be considered lazy evaluation.But your
ifmacro is not performing lazy evaluation. It’s just expanding into another syntax form.blatyo
A macro is replaced with the AST that it returns. So, this:
Turns into this at compile time:
Like @tmbb said,
caseis lazy. Not the macro.The function appears non-lazy, because the arguments have to be evaluated before they are passed to
MyFunctions.my_if/2.