Erlang variables, functions and modules - University of Kentsjt/co545/Lecture3.pdf · Erlang variables, functions and modules Understanding variables in Erlang. Simple function deﬁnitions

Erlang variables, functions and modules

Simon [email protected]

CO545 Lecture 3

Values in Erlang

Numbers 71820809342981189182192

Atoms atom 'i am an atom' true

Booleans false true

Tuples and lists {[1,2,3],{1,2},true}

Strings "atom" "i am an atom"

Functions fun (X,Y) -> X*Y end


Understanding variables in Erlang.

Simple function definitions in Erlang.

Erlang modules, compilation and use.

Erlang so far

Erlang works like a calculator …

… computation is expression evaluation.

Values come from these built-in types …

… numbers, atoms, lists, tuples and functions.

Erlang has variables

Erlang has variables …

1> A=2+3. 5 2> B=A-1. 4

Erlang has variables

Erlang has variables …

1> A=2+3. 5 2> B=A-1. 4

… but not as we know it, Jim.

3> A=B+1. 5 4> A=B. ** exception error: no match of right hand side value 4

What's going on?

Erlang has single assignment variables

If a variable doesn't have a value already – it isn't bound … … then ‘=’ gives it a value:

1> A=2+3. 5 2> B=A-1. 4

Erlang has single assignment variables

If a variable doesn't have a value already – it isn't bound … … then ‘=’ gives it a value:

1> A=2+3. 5 2> B=A-1. 4

If it is bound, then ‘=’ is a check on its value: does the value of the RHS match the value of the variable on the LHS?

3> A=B+1. 5 4> A=B. ** exception error: no match of right hand side value 4

So how can we use variables in Erlang?

We can use variables to name values …

1> A=2+3. 5 2> B=A-1. 4

… and then use those variables in defining other values.

5> C={A,B,A}. {5,4,5}

No updateable variables

In Java, we might write something like this …

for(int i = 1; i <= 200; i++) { if(i%2 == 0) { sum = sum + i; } }

We can't do that in Erlang.

... Sum = Sum + i ...

We can build loops … but in a functional way … see below!

Variables in the shell

Erlang has variables.

1> A=2+3. 5 2> B=A-1. 4 3> A=B+1. 5 4> A=B. ** exception error: no match of right hand side value 4

We can – only in the shell – forget bindings …

5> f(A). ok 6> A=B. 4 7> f(). --- forgets all bindings.

Defining functions for ourselves.

The simplest function definitions look like this

double(X) -> times(X,2).

times(X,Y) -> X*Y.

Defining functions for ourselves.

The body can contain multiple expressions: the return value of the function is the value of the last expression in the body:

triArea(A,B,C) -> S = (A+B+C)/2, math:sqrt(S*(S-A)*(S-B)*(S-C)).

Modules

These definitions need to go into a module, in the file first.erl

-module(first). -export([double/1,triArea/3]).


times(X,Y) -> X*Y. triArea(A,B,C) -> S = (A+B+C)/2, math:sqrt(S*(S-A)*(S-B)*(S-C)).




Modules

-module declaration: the modulefoo lives in the file foo.erl




Modules

-export declaration: need to list all the functions that are exported





Modules



Identify a function by its name and itsarity: how many arguments it takes.




Modules

When you call a function from another module, you need to use its fully-qualified name: module:function as in math:sqrt



Identify a function by its name and itsarity: how many arguments it takes.

Erlang infrastructure – the basics

Erlang runs on a virtual machine called the BEAM.

Just like the JVM for Java, this makes it Erlang portable, potentially more agile, …

An Erlang module needs to be compiled into BEAM code, before it can be used, e.g. in the Erlang shell, or called from another module.

Once compiled, a file foo.beam is produced for each foo.erl.

Using our functions

The command c(foo). compiles the file foo.erl.

Remember that to use a function … … you need to use fully-qualified names, and … the function must be exported:

1> c(first). {ok,first} 2> double(2). ** exception error: undefined shell command double/1 3> first:double(2). 4 4> first:times(2,3). ** exception error: undefined function first:times/2 5> first:triArea(3,4,5). 6.0

Making a choice: pattern matching

Suppose that we want to make a choice in defining a function

is_zero(0) -> true; is_zero(X) -> false.

Instead of writing (just) a variable as an argument, we can write a literal (or more complicated things … see below).


Boolean or usually allows both possibilities to be true.

“You can choose pizza or pasta …”

In a café, you can have pizza or pasta, but not both: this is called or is exclusive or. How can we define exclusive or on booleans.?

xOr(true,false) -> true; xOr(false,true) -> true; xOr(X,Y) -> false.

A common idiom: “don’t care”

In the final case here, X and Y aren't used in the definition …

… we don’t case about their values, as this is the catch all case, and we can replace them with _ …

xOr(true,false) -> true; xOr(false,true) -> true; xOr(_,_) -> false.


You can also use pattern matching to check equality between (parts of) patterns. In this case between two arguments:

xOr(X,X) -> false; xOr(_,_) -> true.


Understanding variables in Erlang.

Simple function definitions in Erlang.

Erlang modules, compilation and use.

Simple pattern matching.

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Erlang variables, functions and modules - University of Kentsjt/co545/Lecture3.pdf · Erlang variables, functions and modules Understanding variables in Erlang. Simple function deﬁnitions