1. Basic Syntax and Semantics
Computer Science 325
Overview of Erlang I
Basic Syntax and Semantics 1

2. Erlang Pragmatics
Expressions can be evaluated interactively in an interpreter
Programs consist of modules of function definitions, which must be saved in files, compiled, and imported for use

3. Firing up the Erlang Shell

4. Numbers, Arithmetic, Comparisons
1> % Syntax: <expression>. 53
2> 3.14.
3.14
3> * 2. 11
4> 3 == 4.
false
5> 3 =< 4.
true
Syntax is derived from Prolog, in which a set of clauses ends with a ‘.’

5. Numbers and Function Calls
6> abs(-2). 2
7> sqrt(2).
** exception error: undefined shell command sqrt/1
8> math:sqrt(2).
9> % Syntax: <module name>:<function name>
No need to import, if it’s a standard module
Other modules must be in the current working directory

6. Booleans and Logic Note the =< for less than 10> 3 != 2.
* 1: syntax error before: '='
11> not (3 == 2).
true
12> (7 >= 2) and (7 =< 6).
false
13> false or true.
Note the =< for less than

7. Atoms and Strings
14> false.
false
15> apple.
apple
16> "Erlang rules!".
"Erlang rules!”
17> [1,2,3].
Atoms are like enumerated values in C, Java, or Haskell
Must begin with a lowercase letter
Lists are not atomic, so they don’t evaluate to themselves

8. Variables and Assignment
18> Area = 3.14 * 6 * 6.
19> Area.
20> Apple.
* 1: variable 'Apple' is unbound
Single assignment only, for binding.
Variables must be capitalized.

9. Assignment Is Really Pattern Matching
21> Area.
22> Area = 3.14 * 5 * 5.
** exception error: no match of right hand side value 78.5
When Erlang sees the = operator it tries to match the expressions on the left and right, to answer the question, what can I do to make this statement true?
A match won’t succeed if a variable already has a binding

10. Tuples and Pattern Matching
21> Ken = {person, {name, ken}, {eyeColor, blue}}.
{person, {name, ken}, {eyeColor, blue}}
22> Ken.
{person, {name, ken}, {eyeColor, blue}}.
23> {person, {name, Name}, {eyeColor, EyeColor}} = Ken.
{person,{name,ken},{eyeColor,blue}}
24> Name.
ken
25> EyeColor.
blue
Tuples use {} as delimiters

11. Lists
26> Numbers = [100, 66, 33 22, 15].
[100, 66, 33 22, 15]
27> Numbers.
If T is a list, then [H|T] is also a list, with head H and tail T.
[] is the empty list.
| is like : in Haskell.

12. Lists
26> Numbers = [100, 66, 33 22, 15].
[100, 66, 33 22, 15]
27> Numbers.
If T is a list, then [H|T] is also a list, with head H and tail T.
[] is the empty list.
28> MoreNumbers = [99 | Numbers]. % Like : in Haskell
[99, 100, 66, 33 22, 15]
29> MoreNumbers.

13. Lists 32> MoreNumbers. [99, 100, 66, 33, 22, 15]
33> [One, Two, Three|Tail] = MoreNumbers.
34> One. 99
35> Two.
100
36> Three. 66

14. Getting Help 37> help(). ** shell internal commands **
b() display all variable bindings
e(N) repeat the expression in query <N>
f() forget all variable bindings
f(X) forget the binding of variable X
h() history
history(N) -- set how many previous commands to keep
results(N) -- set how many previous command results to keep
catch_exception(B) -- how exceptions are handled
v(N) use the value of query <N>
rd(R,D) define a record
rf() remove all record information
rf(R) remove record information about R
rl() display all record information
rl(R) display record information about R
rp(Term) -- display Term using the shell's record information
. . . etc.

15. Modules
As in Python, Erlang programs consist of modules, which map directly to source (.erl) files
A module must be saved and compiled before its code can be run
Modules typically consist of function definitions and other executable code, such as startup code

16. An Example Module
% geometry.erl
-module(geometry).
-export([area/1]). % Function name and arity
area({rectangle, Width, Height}) -> Width * Height; % First clause
area({circle, Radius}) -> * Radius * Radius. % Second clause
When the function is called, the actual argument is matched to the function’s parameter.
Each clause is tried until a match occurs or an exception is raised.
Think of the ; as an OR, without the Boolean result

17. Usage of the Module A module must be compiled before it can be used.
% geometry.erl
-module(geometry).
-export([area/1]). % Function name and arity
area({rectangle, Width, Height}) -> Width * Height; % First clause
area({circle, Radius}) -> * Radius * Radius. % Second clause
1> c(geometry).
{ok, geometry}
2> geometry:area({rectangle, 10, 5}). 50
3> geometry:area({circle, 1.4}).
A module must be compiled before it can be used.
Use c(<module name>) in the Erlang shell.

18. Another Example Compile and run from the terminal: % hello.erl
-module(hello).
-export([start/1]).
start() -> io:format("Hello world!~n").
Compile and run from the terminal:
$ erlc hello.erl
$ erl –noshell –s hello start –s init stop
Hello world! $

19. Recursive List Processing
% mylist.erl
-module(mylist).
-export([sum/1]). % Function name and arity
sum(L) -> sum(L, 0).
sum([], N) -> N;
sum([H|T]) -> sum(T, H + N).
Same function name, different arities.
Tail recursive?

20. Recursive List Processing
% mylist.erl
-module(mylist).
-export([sum/1]). % Function name and arity
sum(L) -> sum(L, 0).
sum([], N) -> N;
sum([H|T], N) -> sum(T, H + N).
1> c(mylist).
{ok, mylist}
2> mylist:sum([10, 20, 30]). 60

21. Anonymous Functions (funs)
1> fun(X, Y) -> X * Y end.
#Fun<erl_eval >
2> Multiply = fun(X, Y) -> X * Y end.
3> Multiply(2, 5). 10
funs can be passed as arguments to other functions and returned as their values.
But other functions are not first-class data.

22. Map and Filter 1> lists:map(abs, [-1, 2, -3, -6]).
** exception error: bad function abs
in function lists:map/2 (lists.erl, line 1224)
2> lists:map(fun(X) -> abs(X) end, [-1, 2, -3, -6]).
[1,2,3,6]
3> lists:filter(fun(X) -> X > 0 end, [-1, 2, -3, -6]).
[2]

23. Quitting the Shell When a process hangs, ctrl-c can exit the shell.
38> q(). % or halt().
When a process hangs, ctrl-c can exit the shell.