ALIAS.md

The reference files for this example are in the alias directory.

The reference takes place in /tmp/docs/methods/code_constructs, which is a copy of the code_constructs directory. Reported locations may differ depending on the location of the source files.

The compilation command is :

make -C alias build

The analysis command is :

make -C alias analyze

The compile + analyze command is :

make -C alias

First run

Code:

(* alias_lib.mli *)
val original :
  < used : unit
  ; used_by_alias : unit
  ; unused : unit
  >

val alias :
  < used : unit
  ; used_by_alias : unit
  ; unused : unit
  >

(* alias_lib.ml *)
let original =
  object
    method used = ()
    method used_by_alias = ()
    method unused = ()
  end

let alias = original

(* alias_bin.ml *)
open Alias_lib

let () =
  original#used;
  alias#used_by_alias

Before looking at the analysis results, let's look at the code.

The Alias_lib declares and exports 2 objects : original, and alias. They both have the same type, and alias is actually defined as equal to original. Thus, alias is an alias for original. Their methods are used in Alias_bin. The method used is used via original, and used_by_alias is via alias. Because alias is the same object as original, the 2 methods are actually used for both objects.

Compile and analyze:

$ make -C alias
make: Entering directory '/tmp/docs/methods/code_constructs/alias'
ocamlopt -bin-annot alias_lib.mli alias_lib.ml alias_bin.ml
dead_code_analyzer --nothing -M all .
Scanning files...
 [DONE]

.> UNUSED METHODS:
=================
/tmp/docs/methods/code_constructs/alias/alias_lib.mli:2: original#unused

Nothing else to report in this section
--------------------------------------------------------------------------------


make: Leaving directory '/tmp/docs/methods/code_constructs/alias'

As expected, only the method unused is reported. However, it is only reported for original. Due to the dynamic nature of objects, the analyzer reports the methods of the objects that actually define them. Thus, because alias is an alias for original, it does not define any new method, and the analyzer would not report anything for it.

Removing the unused methods

If we only remove original#unused from alias_lib.ml and alias_lib.mli, we get the following error:

Error: The implementation alias_lib.ml
       does not match the interface alias_lib.mli:
       Values do not match:
         val alias : < used : unit; used_by_alias : unit >
       is not included in
         val alias : < unused : unit; used : unit; used_by_alias : unit >
       The type < used : unit; used_by_alias : unit >
       is not compatible with the type
         < unused : unit; used : unit; used_by_alias : unit >
       Type <  > is not compatible with type < unused : unit >
       File "alias_lib.mli", lines 7-11, characters 0-3: Expected declaration
       File "alias_lib.ml", line 8, characters 4-9: Actual declaration

This is because the .mli still exports alias#unused, although it is not defined. The easy fix is to also remove alias#unused from the .mli.

Note

Alternatively, if we wanted to keep alias#unused, we could swap the definitions, making original the alias (with a coercion to discard unused : let original = (alias :> <used: unit; used_by_alias: unit>)). The analyzer would then report alias#unused as unused.

We could also define alias as a new object with methods used and used_by_alias equal to those of original. Then, alias would not longer be an alias for original and their methods would be tracked and reported independently. Thus, the analyzer would report alias#unused , and alias#used. It would not report any method for original because they are all used by alias.

Code:

(* alias_lib.mli *)
val original :
  < used : unit
  ; used_by_alias : unit
  >

val alias :
  < used : unit
  ; used_by_alias : unit
  >

(* alias_lib.ml *)
let original =
  object
    method used = ()
    method used_by_alias = ()
  end

let alias = original

(* alias_bin.ml *)
open Alias_lib

let () =
  original#used;
  alias#used_by_alias

Compile and analyze:

$ make -C alias
make: Entering directory '/tmp/docs/methods/code_constructs/alias'
ocamlopt -bin-annot alias_lib.mli alias_lib.ml alias_bin.ml
dead_code_analyzer --nothing -M all .
Scanning files...
 [DONE]

.> UNUSED METHODS:
=================

Nothing else to report in this section
--------------------------------------------------------------------------------


make: Leaving directory '/tmp/docs/methods/code_constructs/alias'

There is no more unused method. Our work here is done.