Integers
10
234
16#AB10F
2#110111010
$A
Floats
17.368
56.654
12.34E10.
 B#Val is used to store numbers in base
< B >.
 $Char is used for ascii values
(example $A instead of 65).
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abcef
start_with_a_lower_case_letter
'Blanks can be quoted'
'Anything inside quotes \n\012'
 Indefinite length atoms are allowed.
 Any character code is allowed within an
atom.
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{123, bcd}
{123, def, abc}
{person, 'Joe', 'Armstrong'}
{abc, {def, 123}, jkl}
{}
 Used to store a fixed number of items.
 Tuples of any size are allowed.
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[123, xyz]
[123, def, abc]
[{person, 'Joe', 'Armstrong'},
{person, 'Robert', 'Virding'},
{person, 'Mike', 'Williams'}
]
"abcdefghi"
becomes  [97,98,99,100,101,102,103,104,105]
""
becomes  []
 Used to store a variable number of items.
 Lists are dynamically sized.
 "..." is short for the list of integers
representing the ascii character codes of the
enclosed within the quotes.
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Abc
A_long_variable_name
AnObjectOrientatedVariableName
 Start with an Upper Case Letter.
 No "funny characters".
 Variables are used to store values of data
structures.
 Variables can only be bound once! The
value of a variable can never be changed
once it has been set (bound).
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[{{person,'Joe', 'Armstrong'},
{telephoneNumber, [3,5,9,7]},
{shoeSize, 42},
{pets, [{cat, tubby},{cat, tiger}]},
{children,[{thomas, 5},{claire,1}]}},
{{person,'Mike','Williams'},
{shoeSize,41},
{likes,[boats, beer]},
...
 Arbitrary complex structures can be cre
ated.
 Data structures are created by writing
them down (no explicit memory alloca
tion or deallocation is needed etc.).
 Data structures may contain bound vari
ables.
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A = 10
Succeeds  binds A to 10
{B, C, D} = {10, foo, bar}
Succeeds  binds B to 10, C to foo and D
to bar
{A, A, B} = {abc, abc, foo}
Succeeds  binds A to abc, B to foo
{A, A, B} = {abc, def, 123}
Fails
[A,B,C] = [1,2,3]
Succeeds  binds A to 1, B to 2, C to 3
[A,B,C,D] = [1,2,3]
Fails
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Pattern Matching (Cont)
[A,BC] = [1,2,3,4,5,6,7]
Succeeds  binds A = 1, B = 2,
C = [3,4,5,6,7]
[HT] = [1,2,3,4]
Succeeds  binds H = 1, T = [2,3,4]
[HT] = [abc]
Succeeds  binds H = abc, T = []
[HT] = []
Fails
{A,_, [B_],{B}} = {abc,23,[22,x],{22}}
Succeeds  binds A = abc, B = 22
 Note the use of "_", the anonymous (don't
care) variable.
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module:func(Arg1, Arg2, ... Argn)
func(Arg1, Arg2, .. Argn)
 Arg1 .. Argn are any Erlang data struc
tures.
 The function and module names (func
and module in the above) must be atoms.
 A function can have zero arguments.
(e.g. date()  returns the current date).
 Functions are defined within Modules.
 Functions must be exported before they
can be called from outside the module
where they are defined.
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module(demo).
export([double/1]).
double(X) >
times(X, 2).
times(X, N) >
X * N.
 double can be called from outside the
module, times is local to the module.
 double/1 means the function double
with one argument (Note that double/1
and double/2 are two different functions).
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unix> erl
Eshell V2.0
1> c(demo).
double/1 times/2 module_info/0
compilation_succeeded
2> demo:double(25).
50
3> demo:times(4,3).
** undefined function:demo:times[4,3] **
** exited: {undef,{demo,times,[4,3]}} **
4> 10 + 25.
35
5>
 c(File) compiles the file File.erl.
 1> , 2> ... are the shell prompts.
 The shell sits in a readevalprint loop.
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date()
time()
length([1,2,3,4,5])
size({a,b,c})
atom_to_list(an_atom)
list_to_tuple([1,2,3,4])
integer_to_list(2234)
tuple_to_list({})
 Are in the module erlang.
 Do what you cannot do (or is difficult to
do) in Erlang.
 Modify the behaviour of the system.
 Described in the BIFs manual.
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Is defined as a collection of clauses.
func(Pattern1, Pattern2, ...) >
... ;
func(Pattern1, Pattern2, ...) >
... ;
...
func(Pattern1, Pattern2, ...) >
... .
Evaluation Rules
 Clauses are scanned sequentially until a match is found.
 When a match is found all variables occurring in the head
become bound.
 Variables are local to each clause, and
are allocated and deallocated automatically.
 The body is evaluated sequentially.
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Functions (cont)
module(mathStuff).
export([factorial/1, area/1]).
factorial(0) > 1;
factorial(N) > N * factorial(N1).
area({square, Side}) >
Side * Side;
area({circle, Radius}) >
% almost :)
3 * Radius * Radius;
area({triangle, A, B, C}) >
S = (A + B + C)/2,
math:sqrt(S*(SA)*(SB)*(SC));
area(Other) >
{invalid_object, Other}.
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factorial(0) > 1;
factorial(N) >
N * factorial(N1)
> factorial(3)
matches N = 3 in clause 2
== 3 * factorial(3  1)
== 3 * factorial(2)
matches N =2 in clause 2
== 3 * 2 * factorial(2  1)
== 3 * 2 * factorial(1)
matches N = 1 in clause 2
== 3 * 2 * 1 * factorial(1  1)
== 3 * 2 * 1 * factorial(0)
== 3 * 2 * 1 * 1 (clause 1)
== 6
 Variables are local to each clause.
 Variables are allocated and deallocated
automatically.
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factorial(0) > 1;
factorial(N) when N > 0 >
N * factorial(N  1).
 The reserved word when introduces a
guard.
 Fully guarded clauses can be reordered.
factorial(N) when N > 0 >
N * factorial(N  1);
factorial(0) > 1.
factorial(N) >
N * factorial(N  1);
factorial(0) > 1.
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number(X)  X is a number
integer(X)  X is an integer
float(X)  X is a float
atom(X)  X is an atom
tuple(X)  X is a tuple
list(X)  X is a list
length(X) == 3  X is a list of length 3
size(X) == 2  X is a tuple of size 2.
X > Y + Z  X is > Y + Z
X == Y  X is equal to Y
X =:= Y  X is exactly equal to Y
(i.e. 1 == 1.0 succeeds but
1 =:= 1.0 fails)
 All variables in a guard must be bound.
 See the User Guide for a full list of
guards and allowed function calls.
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average(X) > sum(X) / len(X).
sum([HT]) > H + sum(T);
sum([]) > 0.
len([_T]) > 1 + len(T);
len([]) > 0.
 Note the pattern of recursion is the same
in both cases. This pattern is very common.
Two other common patterns:
double([HT]) > [2*Hdouble(T)];
double([]) > [].
member(H, [H_]) > true;
member(H, [_T]) > member(H, T);
member(_, []) > false.
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average(X) > average(X, 0, 0).
average([HT], Length, Sum) >
average(T, Length + 1, Sum + H);
average([], Length, Sum) >
Sum / Length.
 Only traverses the list ONCE
 Executes in constant space (tail recursive)
 The variables Length and Sum play the
role of accumulators
 N.B. average([]) is not defined  (you cannot
have the average of zero elements) 
evaluating average([]) would cause a runtime error
 we discuss what happens when run time
errors occur in the section on
error handling .
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h()  history . Print the last 20 commands.
b()  bindings. See all variable bindings.
f()  forget. Forget all variable bindings.
f(Var)  forget. Forget the binding of variable
X. This can ONLY be used as a command to
the shell  NOT in the body of a function!
e(n)  evaluate. Evaluate the n:th command
in history.
e(1)  Evaluate the previous command.
 Edit the command line as in Emacs
 See the User Guide for more details and
examples of use of the shell.
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apply(Mod, Func, Args)
 Apply the function Func in the module Mod
to the arguments in the list Args.

Mod
and Func
must be atoms (or
expressions which evaluate to atoms).
1> apply( lists1,min_max,[[4,1,7,3,9,10]]).
{1, 10}
 Any Erlang expression can be used in
the arguments to apply.
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case lists:member(a, X) of
true >
... ;
false >
...
end,
...
if
integer(X) > ... ;
tuple(X) > ...
end,
...
 Not really needed  but useful.
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