tutorials3and4

Tutorials 3 and 4
=================

   Download from
   https://github.com/joearms/erl2

Tutorial 3
==========

Here is an erlang module with embedded expressions:
(I've changed the file extension  to .erld)

$ cat example7.erld

  -module(example7).
  -export([test/1]).
  -local([fac/1]).

  fac(0) -> 1;
  fac(N) -> N*fac(N-1).

  Fac50 = fac(50).

  io:format("Fac50=~p~n",[Fac50]).

  120 = fac(5).

  io:format("Unit test succeeds~n").

  test(X) ->
     X*Fac50.
	
To run the program we say:

$./erld example7.erld
  Fac50=30414093201713378043612608166064768844377641568960512000000000000
  Unit test succeeds
  Success
  Created:gen/example7.erl

erld runs the code in example7.erld and writes an output file example7.erl.
We can see the generated code

 $ cat gen/example7.erl

-module(example7).
-export([test/1]).
-local([{fac,1}]).

test(X) ->
    Fac50 =
        30414093201713378043612608166064768844377641568960512000000000000,
    X * Fac50.

Note: 1) the unit tests vanished
      2) fac/0 vanished
      3) the Fac50 binding was imported into the definition of test/1

Tutorial 4
==========

Now we change the code to the following:

 $ cat example8.erld

  -module(example8).
  -export([test/1]).
  -local([fac/1]).

  fac(0) -> 1;
  fac(N) -> N*fac(N-1).

  Fac50 = fac(50).

  io:format("Fac50=~p~n",[Fac50]).

  120 = fac(4).

  io:format("Unit test succeeds~n").

  test(X) ->
      X*Fac50.
	
This is *identical* to example7.erld
BUT the unit test will fail since fac(5) is NOT 1200

Now we run the program:


  $./erld example8.erld
  Created:"gen/exprs.tmp"
  Fac50=30414093201713378043612608166064768844377641568960512000000000000
  Error:{{badmatch,24},[{erl_eval,expr,3,[]}]}
  Compile failed

Now NO CODE is generated.

So 

  1) If the test fails the code is not compiled.
  2) The unit tests are NOT run after the code is compiled
     but *before*
  3) Compile modules failing the unit tests are *impossible* to create

Cheers

/Joe