No Erlang compile time errors for missing functions

Question

Why is there no compile time errors or warnings when I call a function in another module that doesn't exist or has the wrong arity?

The compiler has all of the exports information in a module to make this possible. Is it just not implemented yet or is there a technical reason why it is not possible that I am not seeing?

Answer 1

You may use the xref application to check the usage of deprecated, undefined and unused functions (and more!).

Compile the module with debug_info:

Eshell V6.2  (abort with ^G)
1> c(test, debug_info).
{ok,test}

Check the module with xref:m/1:

2> xref:m(test).
[{deprecated,[]},
 {undefined,[{{test,start,0},{erlang,foo,0}}]},
 {unused,[]}]

You may want to check out more about xref here:

Erlang -- Xref - The Cross Reference Tool (Tools User's Guide)

Erlang -- xref (Tools Reference Manual)

Answer 2

It is as others have said. Modules are compiled separately and there is absolutely no guarantee that the environment which exists at compile-time is the same as the one that will exit at run-time. This implies that doing checks at compile-time about the existence of a module, or of a function in it, is basically meaningless. At run-time that module may or may not be loaded, the function you call may or may not be defined in the module, or it may do something completely different from what you expected.

All this is due to the very dynamic nature of Erlang systems. There is no real way as such to define what is in system at run-time. Hot code-loading is a part of this and works properly because of the dynamic nature of the system. It means you can redefine the system at run-time, you can load in new versions of existing modules with a different interface and you can load in completely new modules and remove existing modules.

For this to work all checks about the existence of a module or function must be done at run-time.

Tools like dialyzer can help with this but they do assume that you don't do anything "funny" at run-time and the system you check is the same as the system you run. Which is of course all good, but very static. And against Erlang's nature which is to be dynamic, in everything.

Unfortunately, in this case, you can't both have your cake and eat it.

Answer 3

When you compile e.g. module alpha which has a call to beta:some_function(...), the compiler cannot assume some specific version of beta to be in use at runtime. Maybe you will compile a newer version of beta after you compiled alpha and this will have the correct some_function exported. Maybe you will upload alpha to be used on a different host, which has all the other modules.

The compiler therefore just compiles the remote call and any errors (non-existent module or function) are resolved at run time, when some version of beta will be loaded.

Answer 4

In my opinion most, if not all, compiler does not verify that a function exists at compilation. What it is required in general is a prototype declaration of the function: the type of the return value, the list and type of all arguments. This is done in C/C++ by including some_file.h in each module definition (not the .c or .cpp).

In Erlang this type verification is done dynamically, while the program is running, so it is not necessary to include these definitions. It is even totally useless because Erlang allows to upgrade the application in run, so the function type may change, or the function may disappear, on purpose or by mistake, during application life time; it is why the Erlang designer have chosen to make this verification at run time and not at build time.

The error you speak about generally occurs during the link phase of the code generation, when the "compiler" tries to gather all together some individual pieces of object code to build an executable file or a library, during this phase the linker solves all the external addresses (for shared variable, static call...). This phase does not exist in Erlang, a module is totally self contained; it does no share anything with the rest of the application, no variable nor function address.

Of course, it is mandatory to use some tools and make some test before updating a running production program, but I consider that these verifications have exactly the same level of importance than the correctness of the algorithm itself.

Answer 5

It is due hot code loading. Each module can be loaded in any particular time. So when you have in your module A code which calls function B:F then you can't tell it is wrong in compile time when your source code of module B has no function B:F. Imagine this: You compile module A with call to B:F. You load module B into memory without function B:F. Then you load module A which contain call B:F but don't call it. Then compile new version of module B with B:F. Then load this new module and then you can call B:F and everything is perfectly right. Imagine your module A makes module B on fly and load it. You can't tell in any particular time that it is wrong that module A contain call to nonexistent function B:F.

Answer 6

I don't know why it's missing (probably because modules are completely separate and compilation of one doesn't depend on the other really - but that's just speculation). But I believe you can find problems like this with dialyzer static analysis. Have a look at http://www.erlang.org/doc/man/dialyzer.html

It's part of the system itself, so try including it in your workflow.