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Diffstat (limited to 'system/doc/reference_manual/expressions.xml')
| -rw-r--r-- | system/doc/reference_manual/expressions.xml | 106 |
1 files changed, 91 insertions, 15 deletions
diff --git a/system/doc/reference_manual/expressions.xml b/system/doc/reference_manual/expressions.xml index fa7870d96c..497d7eb464 100644 --- a/system/doc/reference_manual/expressions.xml +++ b/system/doc/reference_manual/expressions.xml @@ -4,7 +4,7 @@ <chapter> <header> <copyright> - <year>2003</year><year>2009</year> + <year>2003</year><year>2011</year> <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> @@ -13,12 +13,12 @@ compliance with the License. You should have received a copy of the Erlang Public License along with this software. If not, it can be retrieved online at http://www.erlang.org/. - + Software distributed under the License is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License for the specific language governing rights and limitations under the License. - + </legalnotice> <title>Expressions</title> @@ -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 its + 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> @@ -1257,6 +1330,9 @@ end</pre> <cell align="left" valign="middle"><c>is_bitstring/1</c></cell> </row> <row> + <cell align="left" valign="middle"><c>is_boolean/1</c></cell> + </row> + <row> <cell align="left" valign="middle"><c>is_float/1</c></cell> </row> <row> |