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+# Deprecate
+
+This HOWTO shows how to deprecate functionality from Erlang/OTP.
+
+When adding a new *deprecation* or *removal* warning you need to add an attribute to
+the module in question and update [$ERL_TOP/system/doc/general_info/DEPRECATIONS][1]
+with information about when the deprecation was added.
+
+After changing or adding an attribute, and updating the [`DEPRECATIONS`][1],
+you need to update the internal state by running:
+
+    $ ./otp_build update_deprecations
+
+This will update the documentation and the central list of deprecated and removed
+interfaces ([`otp_internal.erl`][2]).
+
+## Attribute format
+
+To mark a function/type as deprecated or removed, point out its name and arity
+together with a suggestion on what the user should do instead:
+
+    -deprecated([{now,0,
+                  "see the \"Time and Time Correction in Erlang\" "
+                  "chapter of the ERTS User's Guide for more information"}]).
+    
+    -deprecated([{cmac, 3, "use crypto:mac/4 instead"},
+                 {cmac, 4, "use crypto:macN/5 instead"}]).
+    
+    -removed([{md5_mac,2,"use crypto:hmac/3 instead"},
+              {md5_mac_96,2,"use crypto:hmac/4 instead"}]).
+    
+    -deprecated_type([{gadget,1,"use widget/1 instead"}]).
+    
+    -removed_type([{column,0,"use erl_anno:column() instead"},
+                   {line,0,"use erl_anno:line() instead"},
+                   {location,0,"use erl_anno:location() instead"}]).
+
+Wildcards can be used to match all names and/or arities:
+
+    -removed([{rsa_sign,'_',"use crypto:sign/4 instead"},
+              {rsa_verify,'_',"use crypto:verify/5 instead"}]).
+    
+    -deprecated([{next_iv, '_',"see the 'New and Old API' chapter of the CRYPTO User's guide"}]).
+    
+    -deprecated([{'_','_',"use the 'rand' module instead"}]).
+    
+    -deprecated_type([{grunka,'_',"use frobnitz/1,2 instead"}]).
+
+You can also use the `Name/Arity` shorthand for all of these variants, e.g.
+`-deprecated([f/1])`, which will result in a generic description to the effect of
+"see the documentation for details."
+
+Note that it is not possible to warn about a module that no longer exists.
+This is to prevent later namespace clashes from raising warnings, like the ones
+we had when the long-removed `net` module reappeared in the socket NIF.
+"Removed" modules should instead have their contents replaced by
+`-removed()` attributes until there's no longer any point in warning
+about their use.
+
+## The DEPRECATIONS file
+
+The [$ERL_TOP/system/doc/general_info/DEPRECATIONS][1] file contains additional
+information about each deprecated function, namely in what release it was deprecated
+and optionally in what release it will be removed. The information in this file will
+be used to generate the [Deprecations](http://erlang.org/doc/general_info/deprecations.html)
+and [Scheduled for Removal](http://erlang.org/doc/general_info/scheduled_for_removal.html)
+pages in the documentation.
+
+Here is how the entry for `erlang:now/0` that was deprecated in OTP 18 looks like:
+
+    erlang:now/0 since=18
+
+Here is an example of a function that was deprecated in OTP 23 and is scheduled for removal in OTP 25:
+
+    filename:safe_relative_path/1 since=23 remove=25
+
+
+ [1]: ../system/doc/general_info/DEPRECATIONS
+ [2]: ../lib/stdlib/src/otp_internal.erl