[erlang-questions] How do funs work ?

[黃耀賢 (Yau-Hsien Huang)]

On Wed, Mar 28, 2012 at 4:51 PM, Yoshihiro Tanaka <hirotnkg@REDACTED>wrote:

> Hello,
>
> I have a function which returns fun as below:
>
> make_fun(_, [], F) -> F;
> make_fun(Operator, [Operand1, Operand2|Operands], F) ->
>  F2 = case Operator of
>         plus  -> fun() -> Operand1 + Operand2 + F() end;
>         minus -> fun() -> Operand1 - Operand2 + F() end
>       end,
>  make_fun(Operator, Operands, F2).
>
> When I call it as:
> F1 = make_fun(plus, lists:seq(1,10), fun() -> 0 end).
>
> Is F1 same as F2 below ?
> F2 = fun() -> 1 + 2 +
>       fun() -> 3 + 4 +
>         fun() -> 5 + 6 +
>           fun() -> 7 + 8 +
>             fun() -> 9 + 10 +
>               fun() -> 0 end()
>             end()
>           end()
>         end()
>       end()
>     end.
>

No, F1 is not the same as F2. A function F is a function definition, then
F() is a function application. The F1 shell be expand as

(fun() ->
    9 + 10 +
   (fun() ->
        7 + 8 +
       (fun() ->
           5 + 6 +
          (fun() ->
              3 + 4 +
             (fun() ->
                1 + 2 + (fun() -> 0 end)()
              end)()
          end)()
      end)()
  end)()
end)

All F() evaluates itself, but F doesn't. Functions applied can be reduced,
and F1 is reduced as a form

(fun() ->
    9 + 10 +
     (7 + 8 +
        (5 + 6 +
           (3 + 4 +
              (1 + 2 + 0)
     )  )  )
end)

Note that it cannot be a function with any binary operator + accompanied
by a function definition. For example, try to write a program with you
make_fun/3 definition, and make the F1 function in Erlang shell. And
give following instructions:

          F3 = fun()-> 1 + 2 + F1 end, F3().

You will get a bad argument error in arithmetic expression. Obviously
in your make_fun/3, every time it make a fun() -> end expression,
it evaluates a preceding function and puts the result as an operand of +,
and the preceding function is not a function definition any more.


> Also, is there any difference between funs that are defined at runtime
> and funs that are defined at compile time in terms of how they are
> executed ?
>
> Thank you
> Yoshihiro
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>



-- 

Best Regards.

--- Y-H. H.
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