Erlang Solutions Ltd.
© 1999-2011 Erlang Solutions Ltd.
Sequential Erlang
© 1999-2011 Erlang Solutions Ltd.
Overview: sequential Erlang I
• Sequential Erlang I
- Conditional Evaluation
- Guards
- Recursion
• Sequential Erlang II
• Sequential Erlang III
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Conditional Evaluation: case
case lists:member(foo, List) of
true -> ok;
false -> {error, unknown}
end
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© 1999-2011 Erlang Solutions Ltd.
Conditional Evaluation: case
case lists:member(foo, List) of
true -> ok;
false -> {error, unknown}
end
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case
clauses
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Conditional Evaluation: case
case lists:member(foo, List) of
true -> ok;
false -> {error, unknown}
end
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expression
© 1999-2011 Erlang Solutions Ltd.
Conditional Evaluation: case
case lists:member(foo, List) of
true -> ok;
false -> {error, unknown}
end
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patterns
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Conditional Evaluation: case
case lists:member(foo, List) of
true -> ok;
false -> {error, unknown}
end
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clause
separator
© 1999-2011 Erlang Solutions Ltd.
Conditional Evaluation: case
case lists:member(foo, List) of
true -> ok;
false -> {error, unknown}
end
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no separator
in last clause
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case of
Pattern1 ->
,
,
...
;
Pattern2 ->
,
,
...
;
_ ->
,
...
end
Conditional evaluation: case
•One branch should always
succeed
•Using an unbound
variable or '_' ensures that
the clause will always
match
•The _ clause is not
mandatory
•An exception is raised if
no clause matches
•Returns the value of the
last executed expression
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© 1999-2011 Erlang Solutions Ltd.
convert(Day) ->
case Day of
monday -> 1;
tuesday -> 2;
wednesday -> 3;
thursday -> 4;
friday -> 5;
saturday -> 6;
sunday -> 7;
Other ->
{error, unknown_day}
end.
Defensive Programming
•Defensive programming:
program in the convert
function for the error case
or ...
•... let it fail here by
deleting the Other clause.
•This will raise an
exception
•The caller will have to
handle the error that they
have caused.
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Conditional Evaluation: if
if
X < 1 -> smaller;
X > 1 -> greater;
X == 1 -> equal
end
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© 1999-2011 Erlang Solutions Ltd.
Conditional Evaluation: if
if
X < 1 -> smaller;
X > 1 -> greater;
X == 1 -> equal
end
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if clauses
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Conditional Evaluation: if
if
X < 1 -> smaller;
X > 1 -> greater;
X == 1 -> equal
end
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guard expressions
© 1999-2011 Erlang Solutions Ltd.
Conditional Evaluation: if
if
X < 1 -> smaller;
X > 1 -> greater;
X == 1 -> equal
end
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clause
separator
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Conditional Evaluation: if
if
X < 1 -> smaller;
X > 1 -> greater;
X == 1 -> equal
end
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no separator
in last clause
© 1999-2011 Erlang Solutions Ltd.
if Guard1 ->
,
,
...
;
Guard2 ->
,
,
...
;
...
true ->
,
...
end
Conditional Evaluation: if
•One branch must always
succeed
•By using true as the last
guard, we ensure that a
clause will always succeed
•The true guard is not
mandatory
•An exception is raised if
no clause succeeds
•Returns the value of the
last executed expression
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factorial(N) when N > 0 ->
N * factorial(N - 1);
factorial(0) -> 1.
This is NOT the same as...
factorial(0) -> 1;
factorial(N) ->
N * factorial(N - 1).
Guards
•The reserved word when
introduces a guard
•Fully guarded clauses can
be re-ordered
•Guards can be used in
function heads, case
clauses, receive and if
expressions.
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number(Num) when is_integer(Num) -> integer;
number(Num) when is_float(Num) -> float;
number(_Other) -> false.
Guards: examples
• is_number(X), is_integer(X), is_float(X)
- X is a number
• is_atom(X), is_pid(X), is_tuple(X), is_list(X)
- X is the specified datatype
• length(List) == Int, tuple_size(Tuple) == Size, X > Y + Z
- Some BIFs and mathematical applications can be applied in guards
• X == Y X /= Y X =:= Y X =/= Y
- X is (not) equal to Y, X is exactly (not) equal to Y (1==1.0 ✓, 1=:=1.0 !)
• X =< Y X >= Y
- NB, not <= or =>
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valid_age(Age) when Age >= 18, Age =< 99 ->
true;
valid_age(_) ->
false.
Guards
•All variables in guards have to be bound
•Guards have to be free of side effects
• If all the guards have to succeed, use , to separate them
• If one guard has to succeed, use ; to separate them
• There are restrictions on BIFs and expressions in guards
- See the Erlang reference manual for complete details
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if f(Args) -> ok;
true -> error
end
case f(Args) of
true -> ok;
false -> error
end
General Switch
• The if construct fails because it involves a user-defined function,
which are forbidden in guards
• The case construct succeeds because it accepts user-defined
functions.
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X
✓
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average(X) -> sum(X) / len(X).
sum([H|T]) -> H + Sum(T);
sum([]) -> 0.
len([_|T]) -> 1 + len(T);
len([]) -> 0.
Recursion: traversing lists
•Note the pattern of
recursion is the same in
both cases
•Taking a list and
evaluating an element is a
very common pattern
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© 1999-2011 Erlang Solutions Ltd.
sum([]) -> 0;
sum([H|T]) -> H + sum(T).
Recursion: self-describing code
•You can read the
programs as an
executable description:
•"The sum of an empty list
is 0."
•"The sum of a non-empty
list is the head of the list
added to the sum of the
tail"
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printAll([]) ->
io:format("~n",[]);
printAll([X|Xs]) ->
io:format("~p ",[X]),
printAll(Xs).
Recursion: traversing lists
•Here we're traversing the
list imperatively:
•"If there are no more
elements to process, stop"
•"If there are further
elements, process the
head, and then call the
function recursively on
the tail."
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© 1999-2011 Erlang Solutions Ltd.
printAll(Ys) ->
case Ys of
[] ->
io:format("~n",[]);
[X|Xs] ->
io:format("~p ",[X]),
printAll(Xs)
end.
Recursion: traversing lists
•Same function again:
shows the loop clearly.
The call to printAll(Xs) is
like a jump back to the
top of the loop.
•This is a tail recursive
function: the only
recursive calls come at the
end of the bodies of the
clauses.
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double([H|T])-> [2*H|double(T)];
double([]) -> [].
member(H, [H|_]) -> true;
member(H, [_|T]) -> member(H,T);
member(_, []) -> false.
even([H|T]) when H rem 2 == 0 ->
[H|even(T)];
even([_|T]) ->
even(T);
even([]) ->
[].
Recursion: more patterns
•double/1 maps elements
in a list and returns a new
list
•member/2 is a predicate
looking for an element in
a list
•even/1 filters a list of
integers and returns the
subset of even numbers
•The function member/2
is the only one which is
tail recursive
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© 1999-2011 Erlang Solutions Ltd.
average(X) -> average(X,0,0).
average([H|T], Length, Sum) ->
average(T, Length+1, Sum+H);
average([], Length, Sum) ->
Sum/Length.
Recursion: accumulators
•Only traverses the list
once.
•Executes in constant
space (tail recursive)
•Length and Sum play the
role of accumulators
•average([]) is not defined
•Evaluating average([])
would cause a run time
error.
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© 1999-2011 Erlang Solutions Ltd.
Summary: sequential Erlang I
• Sequential Erlang I
- Conditional evaluation
- Guards
- Recursion
• Sequential Erlang II
• Sequential Erlang III
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© 1999-2011 Erlang Solutions Ltd.
Overview: sequential Erlang II
• Sequential Erlang I
• Sequential Erlang II
- BIFs
- Libraries
- Manual Pages
- The Debugger
• Sequential Erlang III
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date()
time()
length(List)
size(Tuple)
atom_to_list(Atom)
list_to_tuple(List)
integer_to_list(2235)
tuple_to_list(Tuple)
Built-in Functions
•Do what you cannot do (or
is difficult to do) in Erlang
•Mostly written in C for
fast execution
•BIFs are by convention
regarded as being in the
erlang module.
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© 1999-2011 Erlang Solutions Ltd.
Built-in Functions
•There are BIFs for:
- Process and port handling
- Object access and examination
- Meta programming
- Type conversion
- System information
- Distribution
- Others
• For a complete list, see the manual page for the
erlang module.
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Built-in Functions
•Built-in functions can
modify the real time
properties of the system
•A process executing a BIF
will not be suspended
until the BIF has
completed executing
•Other processes will thus
not be allowed to execute
on the same scheduler
•Use BIFs with care!
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Use BIFs
with care!
© 1999-2011 Erlang Solutions Ltd.
Built-in Functions: examples
1> date().
{2010,9,25}
2> atom_to_list(abcd).
"abcd"
3> tuple_to_list(list_to_tuple([1,2,3,4])).
[1,2,3,4]
4> length([1,2,3,4,5]).
5
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apply(Module, Function, Arguments)
M:function(Args)
M:F(Args)
Built-in Functions: meta calls
• apply/3 is a BIF used to dynamically evaluate functions
• The function must be exported
• The arguments can possibly be an empty list
•All the arguments can be established at runtime
• Extremely powerful when implementing generic code
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Built-in Functions: meta calls
1> Module = io.
io
2> Function = format.
format
3> Arguments = ["Hello World~n", []].
["Hello World~n",[]]
4> apply(Module, Function, Arguments).
Hello World
ok
8> io:Function("Hello World ",[]).
Hello World ok
9> Module:Function("Hello World ", []).
Hello World ok
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The arguments to
apply could have
been evaluated
during runtime
The arities of the
M:func(Args)
and M:F(Args)
forms are static
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Libraries
io.erl
generalised input/output functionality
file.erl
generalised interface towards the file system
lists.erl
standard list processing functions
code.erl
functionality to load, test and manipulate code.
math.erl
mathematical functions
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© 1999-2011 Erlang Solutions Ltd.
Libraries
• Erlang has a set of libraries where functionality
useful to the software designer has been placed
•The previous list of modules are all part of the
standard Erlang/OTP distribution
•Many more libraries and modules are available:
- They are referenced in the official documentation
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lists:append(List1, List2) -> NewList.
lists:delete(Element, List) -> NewList.
lists:last(List) -> Element.
lists:reverse(List) -> ReversedList.
lists:sort(List) -> SortedList.
lists:keysort(Pos, TupleList) -> SortedList.
lists:keydelete(Key, Pos, TupleList) -> NewList.
lists:keysearch(Key, Pos, TupleList) ->
false | {value, Tuple}
Libraries
• The lists module is the most used and one of the most useful
ones
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© 1999-2011 Erlang Solutions Ltd.
Manual Pages
In the UNIX shell
$ erl -man Module
In HTML
By accessing file://$ERL_ROOT/doc/index.html
In Emacs
Picking one of the entries under the Erlang menu
In General
Manual pages for all the modules can be read online,
from the shell, in emacs or in the OTP reference manual.
Take a look at the available modules to get an idea of
the existing functionality
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The Debugger
•The Erlang debugger is a graphical tool providing
mechanisms to debug code and influence
program execution
- Allows the user to insert break points
- Step through the code
- Inspect and manipulate variables
- Inspecting the recursive stack
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© 1999-2011 Erlang Solutions Ltd.
The Debugger
•debugger:start()
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The windows
version is
known to be
unstable
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The Debugger
•Interpret the code
- Must be compiled with the debug_info flag
- use c(Module, [debug_info])
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© 1999-2011 Erlang Solutions Ltd.
The Debugger
•Stepping through the
code
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Summary: sequential Erlang II
• Sequential Erlang I
• Sequential Erlang II
- BIFs
- Libraries
- Manual Pages
- The Debugger
• Sequential Erlang III
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© 1999-2011 Erlang Solutions Ltd.
Overview: sequential Erlang III
• Sequential Erlang I
• Sequential Erlang II
• Sequential Erlang III
- Run Time Errors
- Try ... catch
- Throw
- Catch
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•function_clause is returned
when none of the existing
function patterns matchesfactorial(N) when N > 0 ->
N * factorial(N - 1);
factorial(0) -> 1.
Run Time Errors: match
1> math:factorial(-1).
** exception error: no function clause matching
math:factorial(-1)
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© 1999-2011 Erlang Solutions Ltd.
•case_clause is returned
when none of the existing
patterns in the case
statement matches
test(N) ->
case N of
-1 -> false;
1 -> true
end.
Run Time Errors: match
1> test:test(0).
** exception error: no case clause matching 0
in function test:test/1
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•if_clause is returned when
none of the existing
expressions in the if
statement evaluates to true
test(N) ->
if
N < 0 -> false;
N > 0 -> true
end.
Run Time Errors
1> test:test(0).
** exception error: no true branch found when evaluating
an if expression
in function test:test/1
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© 1999-2011 Erlang Solutions Ltd.
1> Tuple = {1, two, 3}.
{1,two,3}
2> {1, two, 3, Four} = Tuple.
** exception error: no match of right hand side value
{1,two,3}
Run Time Errors: match
• badmatch errors occur in situations when pattern matching fails
and there are no other alternative clauses to choose from.
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1> length(helloWorld).
** exception error: bad argument in function length/1
called as length(helloWorld)
Run Time Errors: others
• badarg is returned when a BIF with wrong arguments is called.
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1> test:hello().
** exception error: undefined function test:hello/0
Run Time Errors: others
• undef will be returned if the global function being called is not
defined or exported
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1> 1+a.
** exception error: bad argument in an arithmetic
expression
in operator +/2
called as 1 + a
Run Time Errors: others
• badarith is returned when arithmetical operations are executed
with values that are neither integers or floats.
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© 1999-2011 Erlang Solutions Ltd.
try Expression of
Pattern1 [when Guard1] ->
ExpressionBody1;
Pattern2 [when Guard2] ->
ExpressionBody2
catch
[Class1:]ExceptionPattern1
[when ExceptionGuardSeq1] ->
ExceptionBody1;
[Class2:]ExceptionPattern2
[when ExceptionGuardSeq2] ->
ExceptionBody2
end
Try ... catch
•try ... catch provides a
mechanism for
monitoring the evaluation
of an expression
•It will trap exits caused by
expected run time errors
•The patterns Class1: and
Class2: can define the
type of exception handled
•The ExceptionPatterns
can restrict the reason
why an exception is
raised.
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1> self().
<0.53.0>
2> X=2, X=3.
** exception error: no match of
right hand side value 3
4> self().
<0.57.0>
5> try (X=3) of
5> Val -> {normal, Val}
5> catch
5> _:_ -> 43
5> end.
43
6> self().
<0.57.0>
Try ... catch
•_:_ allows to match on all
errors no matter what
they are.
•The error is caught and
the process doesn't crash
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© 1999-2011 Erlang Solutions Ltd.
1> X=2.
2
2> try (X=3) of
2> Val -> {normal, Val}
2> catch
2> error:Error -> {error,
Error}
2> end.
{error,{badmatch,3}}
3> try (X=3) of
3> Val -> {normal, Val}
3> catch
3> error:{badmatch,_} -> 42
3> end.
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Try ... catch
•The error:Error pattern
allows to bind the error
reason to a variable and
match on it
•error:{badmatch,_} allows
to match only errors
caused by erroneous
pattern matching
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throw()
Throw
•throw is used for non-
local returns in deep
recursive function calls.
•The execution flow jumps
to the first catch in the
execution stack
•Useful for handling
exceptions in deeply
nested code when you do
not want to handle
possible errors.
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WARNING!!
use with care as
it makes the code
hard to debug
and understand
© 1999-2011 Erlang Solutions Ltd.
add(X, Y) ->
test(Y),
test(X),
X + Y.
test(X) when is_integer(X) -> ok;
test(X) -> throw({error, {non_integer, X}}).
1> math:add(1, one).
** exception throw: {error,{non_integer,one}}
2> try math:add(1, one) of
2> _ -> ok
2> catch
2> Class:Reason -> {Class, Reason}
2> end.
{throw,{error,{non_integer,one}}}
Throw
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Try ... catch: examples
-module(exception).
-export([try_wildcard/1]).
try_wildcard(X) when is_integer(X) ->
try return_error(X)
catch
throw:Throw -> {throw, Throw};
error:_ -> error;
Type:Error -> {Type, Error};
_ -> other; %% Will never be returned
_:_ -> other %% Will never be returned
end.
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Try ... catch: examples
return_error(X) when X < 0 ->
throw({'EXIT',{badarith,[{exception,return_error,1},
{erl_eval,do_apply,5},
{shell,exprs,6},
{shell,eval_exprs,6},
{shell,eval_loop,3}]}});
return_error(X) when X == 0 -> 1/X;
return_error(X) when X > 0 ->
{'EXIT',{badarith,[{exception,return_error,1},
{erl_eval,do_apply,5},
{shell,exprs,6},
{shell,eval_exprs,6},
{shell,eval_loop,3}]}}.
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Try ... catch: examples
1> exception:try_wildcard(-1).
{throw,{'EXIT',{badarith,[{exception,return_error,1},
{erl_eval,do_apply,5},
{shell,exprs,6},
{shell,eval_exprs,6},
{shell,eval_loop,3}]}}}
2> exception:try_wildcard(0).
error
3> exception:try_wildcard(1).
{'EXIT',{badarith,[{exception,return_error,1},
{erl_eval,do_apply,5},
...
{shell,eval_loop,3}]}}
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catch
Catch
• catch provides a mechanism for monitoring the evaluation of an
expression
• It will trap
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