COP 4020 Lecture -*- Outline -*-

* Records in Erlang

** motivation

------------------------------------------
         WHY RECORDS?

Tuples can store multiple, heterogenous
data

Compare:

 {"Northern Cardinal",
  "Cardinalis cardinalis"
  {8.3, 9.3, inch}
 }           

to:

 {species,
     common_name,
      "Northern Cardinal",
     scientific_name,
      "Cardinalis cardinalis"
     length,
      {min, 8.3, max, 9.3, unit, inch}
 }           

------------------------------------------
    See birdtuples.erl

    Q: Which is better?
       the second one is easier to follow
    Q: Why?
       the information
    Q: Why have the atoms in positions 1, 2, 4, 6, and 8?
        to help us remember what those positions mean

    To deal with tuples we might do the following...
------------------------------------------
        FUNCTIONS ON TUPLES

species_common_name({Name,_,_,_}) ->
   Name.
species_scientific_name({_,SN,_,_}) ->
   SN.

measure_avg({Min,Max,Unit}) ->
   {Min+Max/2,Unit}.

species_length({_,_,Len}) ->
   measure_avg(Len).
   
------------------------------------------
        See birdtuples.erl      

    Q: What happens if we change the order of some components in these tuples?
       (some) functions break!
    Q: What happens if we add new components to these tuples?
       All the functions break!

    Try adding wingspan to the records in birdtuples.erl

------------------------------------------
           GOALS FOR RECORDS

We would like to:
   - access data without knowing


   - pattern match against data without
      knowing



------------------------------------------
        ... the order of the components
        ... all the components (or how many there are).
        
** syntax

*** definition

------------------------------------------
         RECORD DEFINITIONS

<Record Def> ::=
   -record(<Name>, { <Fields> }).

<Fields> ::= <Field>
           | <Field> , <Fields>

<Field> ::= <Name>
          | <Name> = <DefaultValue>

<DefaultValue> ::= <Expression>

Examples:

-record(measure, {min = 0.0, max, unit}).

-record(bird,
        {common_name, scientific_name,
         length, wingspan}).

------------------------------------------


*** creation

------------------------------------------
    CREATION OF RECORD VALUES IN ERLANG

% in measure.hrl
-record(measure, {min = 0.0, max, unit}).

  #measure{min = 8.3, max = 9.3,
           unit = inch}

In the Erlang shell need to use rr

1> rr("measure.hrl").
[measure]
2> CM = #measure{min = 8.3, max = 9.3,
                 unit = inch}.
#measure{min = 8.3,max = 9.3,unit = inch}


Or if the records are in a module:

3> rr(birds).
[bird,measure]
------------------------------------------
        See also birds.erl

*** access

------------------------------------------
          FIELD ACCESS TO RECORDS

Normally, use pattern matching:

measure_avg(#measure{min = Min,
		     max = Max}) ->
    (Min+Max)/2.0.

Can also use access syntax (dot):

    CM#measure.min
    CM#measure.max
    
------------------------------------------
        See also birds.erl