Erlang -- Functions

User's Guide
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Efficiency Guide
User's Guide
Version 5.8.3

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Chapters

- Introduction
  - Top of chapter
  - Purpose
  - Prerequisites
- The Eight Myths of Erlang Performance
  - Top of chapter
  - Myth: Funs are slow
  - Myth: List comprehensions are slow
  - Myth: Tail-recursive functions are MUCH faster than recursive functions
  - Myth: '++' is always bad
  - Myth: Strings are slow
  - Myth: Repairing a Dets file is very slow
  - Myth: BEAM is a stack-based byte-code virtual machine (and therefore slow)
  - Myth: Use '_' to speed up your program when a variable is not used
- Common Caveats
  - Top of chapter
  - The regexp module
  - The timer module
  - list_to_atom/1
  - length/1
  - setelement/3
  - size/1
  - split_binary/2
  - The '--' operator
- Constructing and matching binaries
  - Top of chapter
  - How binaries are implemented
  - Constructing binaries
  - Matching binaries
- List handling
  - Top of chapter
  - Creating a list
  - List comprehensions
  - Deep and flat lists
  - Why you should not worry about recursive lists functions
- Functions
  - Top of chapter
  - Pattern matching
  - Function Calls
  - Memory usage in recursion
- Tables and databases
  - Top of chapter
  - Ets, Dets and Mnesia
  - Ets specific
  - Mnesia specific
- Processes
  - Top of chapter
  - Creation of an Erlang process
  - Process messages
  - The SMP emulator
- Drivers
  - Top of chapter
  - Drivers and concurrency
  - Avoiding copying of binaries when calling a driver
  - Returning small binaries from a driver
  - Returning big binaries without copying from a driver
- Advanced
  - Top of chapter
  - Memory
  - System limits
- Profiling
  - Top of chapter
  - Do not guess about performance - profile
  - Big systems
  - What to look for
  - Tools
  - Benchmarking

# 6 Functions

### 6.1 Pattern matching

Pattern matching in function head and in case and receive clauses are optimized by the compiler. With a few exceptions, there is nothing to gain by rearranging clauses.

One exception is pattern matching of binaries. The compiler will not rearrange clauses that match binaries. Placing the clause that matches against the empty binary last will usually be slightly faster than placing it first.

Here is a rather contrived example to show another exception:

DO NOT

    atom_map1(one) -> 1;
    atom_map1(two) -> 2;
    atom_map1(three) -> 3;
    atom_map1(Int) when is_integer(Int) -> Int;
    atom_map1(four) -> 4;
    atom_map1(five) -> 5;
    atom_map1(six) -> 6.

The problem is the clause with the variable Int. Since a variable can match anything, including the atoms four, five, and six that the following clauses also will match, the compiler must generate sub-optimal code that will execute as follows:

First the input value is compared to one, two, and three (using a single instruction that does a binary search; thus, quite efficient even if there are many values) to select which one of the first three clauses to execute (if any).

If none of the first three clauses matched, the fourth clause will match since a variable always matches. If the guard test is_integer(Int) succeeds, the fourth clause will be executed.

If the guard test failed, the input value is compared to four, five, and six, and the appropriate clause is selected. (There will be a function_clause exception if none of the values matched.)

Rewriting to either

DO

    atom_map2(one) -> 1;
    atom_map2(two) -> 2;
    atom_map2(three) -> 3;
    atom_map2(four) -> 4;
    atom_map2(five) -> 5;
    atom_map2(six) -> 6;
    atom_map2(Int) when is_integer(Int) -> Int.

or

DO

    atom_map3(Int) when is_integer(Int) -> Int;
    atom_map3(one) -> 1;
    atom_map3(two) -> 2;
    atom_map3(three) -> 3;
    atom_map3(four) -> 4;
    atom_map3(five) -> 5;
    atom_map3(six) -> 6.

will give slightly more efficient matching code.

Here is a less contrived example:

DO NOT

    map_pairs1(_Map, [], Ys) ->
        Ys;
    map_pairs1(_Map, Xs, [] ) ->
        Xs;
    map_pairs1(Map, [X|Xs], [Y|Ys]) ->
        [Map(X, Y)|map_pairs1(Map, Xs, Ys)].

The first argument is not a problem. It is variable, but it is a variable in all clauses. The problem is the variable in the second argument, Xs, in the middle clause. Because the variable can match anything, the compiler is not allowed to rearrange the clauses, but must generate code that matches them in the order written.

If the function is rewritten like this

DO

    map_pairs2(_Map, [], Ys) ->
        Ys;
    map_pairs2(_Map, [_|_]=Xs, [] ) ->
        Xs;
    map_pairs2(Map, [X|Xs], [Y|Ys]) ->
        [Map(X, Y)|map_pairs2(Map, Xs, Ys)].

the compiler is free rearrange the clauses. It will generate code similar to this

DO NOT (already done by the compiler)

    explicit_map_pairs(Map, Xs0, Ys0) ->
        case Xs0 of
    	[X|Xs] ->
    	    case Ys0 of
    		[Y|Ys] ->
    		    [Map(X, Y)|explicit_map_pairs(Map, Xs, Ys)];
    		[] ->
    		    Xs0
    	    end;
    	[] ->
    	    Ys0
        end.

which should be slightly faster for presumably the most common case that the input lists are not empty or very short. (Another advantage is that Dialyzer is able to deduce a better type for the variable Xs.)

### 6.2 Function Calls

Here is an intentionally rough guide to the relative costs of different kinds of calls. It is based on benchmark figures run on Solaris/Sparc:

- Calls to local or external functions (foo(), m:foo()) are the fastest kind of calls.
- Calling or applying a fun (Fun(), apply(Fun, [])) is about three times as expensive as calling a local function.
- Applying an exported function (Mod:Name(), apply(Mod, Name, [])) is about twice as expensive as calling a fun, or about six times as expensive as calling a local function.

#### Notes and implementation details

Calling and applying a fun does not involve any hash-table lookup. A fun contains an (indirect) pointer to the function that implements the fun.

Warning

Tuples are not fun(s). A "tuple fun", {Module,Function}, is not a fun. The cost for calling a "tuple fun" is similar to that of apply/3 or worse. Using "tuple funs" is strongly discouraged, as they may not be supported in a future release, and because there exists a superior alternative since the R10B release, namely the fun Module:Function/Arity syntax.

apply/3 must look up the code for the function to execute in a hash table. Therefore, it will always be slower than a direct call or a fun call.

It no longer matters (from a performance point of view) whether you write

    Module:Function(Arg1, Arg2)

or

    apply(Module, Function, [Arg1,Arg2])

(The compiler internally rewrites the latter code into the former.)

The following code

    apply(Module, Function, Arguments)

is slightly slower because the shape of the list of arguments is not known at compile time.

### 6.3 Memory usage in recursion

When writing recursive functions it is preferable to make them tail-recursive so that they can execute in constant memory space.

DO

    list_length(List) ->
        list_length(List, 0).

    list_length([], AccLen) ->
        AccLen; % Base case

    list_length([_|Tail], AccLen) ->
        list_length(Tail, AccLen + 1). % Tail-recursive

DO NOT

    list_length([]) ->
        0. % Base case
    list_length([_ | Tail]) ->
        list_length(Tail) + 1. % Not tail-recursive