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Diffstat (limited to 'system/doc/reference_manual')
| -rw-r--r-- | system/doc/reference_manual/expressions.xml | 97 |
1 files changed, 85 insertions, 12 deletions
diff --git a/system/doc/reference_manual/expressions.xml b/system/doc/reference_manual/expressions.xml index b56b8acbf0..714ecccaf6 100644 --- a/system/doc/reference_manual/expressions.xml +++ b/system/doc/reference_manual/expressions.xml @@ -217,12 +217,16 @@ lists:keysearch(Name, 1, List)</code> <p>In the second form of function calls, <c>ExprF(Expr1,...,ExprN)</c>, <c>ExprF</c> must be an atom or evaluate to a fun.</p> + <p>If <c>ExprF</c> is an atom the function is said to be called by - using the <em>implicitly qualified function name</em>. If - <c>ExprF/N</c> is the name of a function explicitly or - automatically imported from module <c>M</c>, then the call is - short for <c>M:ExprF(Expr1,...,ExprN)</c>. Otherwise, - <c>ExprF/N</c> must be a locally defined function. Examples:</p> + using the <em>implicitly qualified function name</em>. If the + function <c>ExprF</c> is locally defined, it is called. + Alternatively if <c>ExprF</c> is explicitly imported from module + <c>M</c>, <c>M:ExprF(Expr1,...,ExprN)</c> is called. If + <c>ExprF</c> is neither declared locally nor explicitly + imported, <c>ExprF</c> must be the name of an automatically + imported BIF. Examples:</p> + <code type="none"> handle(Msg, State) spawn(m, init, [])</code> @@ -238,16 +242,85 @@ Fun2([1,2], [3,4]) fun lists:append/2([1,2], [3,4]) => [1,2,3,4]</code> - <p>To avoid possible ambiguities, the fully qualified function - name must be used when calling a function with the same name as - a BIF, and the compiler does not allow defining a function with - the same name as an explicitly imported function.</p> + <p>Note that when calling a local function, there is a difference - between using the implicitly or fully qualified function name, as - the latter always refers to the latest version of the module. See - <seealso marker="code_loading">Compilation and Code Loading</seealso>.</p> + between using the implicitly or fully qualified function name, as + the latter always refers to the latest version of the module. See + <seealso marker="code_loading">Compilation and Code Loading</seealso>.</p> + <p>See also the chapter about <seealso marker="functions#eval">Function Evaluation</seealso>.</p> + + <section> + <title>Local Function Names Clashing With Auto-imported BIFs</title> + <p>If a local function has the same name as an auto-imported BIF, + the semantics is that implicitly qualified function calls are + directed to the locally defined function, not to the BIF. To avoid + confusion, there is a compiler directive available, + <c>-compile({no_auto_import,[F/A]})</c>, that makes a BIF not + being auto-imported. In certain situations, such a compile-directive + is mandatory.</p> + + <warning><p>Before OTP R14A (ERTS version 5.8), an implicitly + qualified function call to a function having the same name as an + auto-imported BIF always resulted in the BIF being called. In + newer versions of the compiler the local function is instead + called. The change is there to avoid that future additions to the + set of auto-imported BIFs does not silently change the behavior + of old code.</p> + + <p>However, to avoid that old (pre R14) code changed it's + behavior when compiled with OTP version R14A or later, the + following restriction applies: If you override the name of a BIF + that was auto-imported in OTP versions prior to R14A (ERTS version + 5.8) and have an implicitly qualified call to that function in + your code, you either need to explicitly remove the auto-import + using a compiler directive, or replace the call with a fully + qualified function call, otherwise you will get a compilation + error. See example below:</p> </warning> + + <code type="none"> +-export([length/1,f/1]). + +-compile({no_auto_import,[length/1]}). % erlang:length/1 no longer autoimported + +length([]) -> + 0; +length([H|T]) -> + 1 + length(T). %% Calls the local funtion length/1 + +f(X) when erlang:length(X) > 3 -> %% Calls erlang:length/1, + %% which is allowed in guards + long.</code> + + <p>The same logic applies to explicitly imported functions from + other modules as to locally defined functions. To both import a + function from another module and have the function declared in the + module at the same time is not allowed.</p> + + <code type="none"> +-export([f/1]). + +-compile({no_auto_import,[length/1]}). % erlang:length/1 no longer autoimported + +-import(mod,[length/1]). + +f(X) when erlang:length(X) > 33 -> %% Calls erlang:lenght/1, + %% which is allowed in guards + + erlang:length(X); %% Explicit call to erlang:length in body + +f(X) -> + length(X). %% mod:length/1 is called</code> + + + <p>For auto-imported BIFs added to Erlang in release R14A and thereafter, + overriding the name with a local function or explicit import is always + allowed. However, if the <c>-compile({no_auto_import,[F/A])</c> + directive is not used, the compiler will issue a warning whenever + the function is called in the module using the implicitly qualified + function name.</p> + </section> </section> <section> |