Functions — Erlang System Documentation v27.3.4.7

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# Functions

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## Pattern Matching

Pattern matching in function head as well as 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 does not rearrange clauses that match binaries. Placing the clause that matches against the empty binary last is usually slightly faster than placing it first.

The following is a rather unnatural example to show another exception where rearranging clauses is beneficial:

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`. As a variable can match anything, including the atoms `four`, `five`, and `six`, which the following clauses also match, the compiler must generate suboptimal code that executes 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 match, the fourth clause match as a variable always matches.
- If the guard test `is_integer(Int)` succeeds, the fourth clause is executed.
- If the guard test fails, the input value is compared to `four`, `five`, and `six`, and the appropriate clause is selected. (There is 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.

gives slightly more efficient matching code.

Another 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 as follows, the compiler is free to rearrange the clauses:

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 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.

This is slightly faster for probably the most common case that the input lists are not empty or very short. (Another advantage is that Dialyzer can deduce a better type for the `Xs` variable.)

## Function Calls

This is a rough hierarchy of the performance of the different types of function calls:

- Calls to local or external functions (`foo()`, `m:foo()`) are the fastest calls.
- Calling or applying a fun (`Fun()`, `apply(Fun, [])`) is just a little slower than external calls.
- Applying an exported function (`Mod:Name()`, `apply(Mod, Name, [])`) where the number of arguments is known at compile time is next.
- Applying an exported function (`apply(Mod, Name, Args)`) where the number of arguments is not known at compile time is the least efficient.

### 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.

`apply/3` must look up the code for the function to execute in a hash table. It is therefore always slower than a direct call or a fun call.

Caching callback functions into funs may be more efficient in the long run than apply calls for frequently-used callbacks.

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