Data-Driven Programs
TDDC74 Programming:
Abstraction and Modelling
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A modular way of organizing programs
The idea will be illustrated by examples
The idea is to tag data and use these tags for
dispatching to the right procedure
Later on in the course we’ll compare various ways of
organizing programs
SICP 2 (part 3)
Lecture 6
Anders Haraldsson
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Data-Driven Programs
Tagging – tagged data
Laboratory work
media-dbs-examples.scm
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Adds information about the type to data
((respect) (aretha franklin) 1967 soul) ->
(music (respect) (aretha franklin) 1967 soul)
(person (name kalle) (adress storgatan 15) (age 20))
Primitives for tagging:
attach-tag
z get-type
z get-content
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Dispatching on type
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Discussions and exercises in SICP section 2.4
Dispatching on type
Generalized procedures that works for different types of data or
different representations of data
Discussions and exercises in SICP section 2.4.3
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We need recognizers for each type of representation
(define (person-list? p)
; Person -> Bool
(eq? (get-type p) ’person-list))
In earlier lecture we had two representations for person. We want to use
both representations and tag them.
(define (person-assoc-list? p)
; Person -> Bool
(eq? (get-type p) ’person-assoc-list))
(define (make-person-list name age father mother sex)
; Name x Age x Name x Name x Sex -> Person
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(attach-tag ’person-list (list name age father mother sex)))
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(define (make-person-assoc-list name age father mother sex)
; Name x Age x Name x Name x Sex -> Person
(attach-tag ‘person-assoc-list (list (cons ‘name name)
(cons ‘dob dob)
(cons ‘father father)
(cons ‘mother mother)
(cons ‘sex sex))))
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Now we want selectors for the person-dataobject knowing both
types of representation.
(define (get-name p)
; Person -> Name
(cond ((person-list? p) (car (get-contents p)))
((person-assoc-list? p) (cdr (assq (get-contents p))))
(else (error ”Bad type” p))))
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Tagging and Dispatching on types
Packages – data directed programming
Problems
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procedures
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Approach not scalable
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Lab 3 problem 8 - Table organization
Install-package
There are procedures in table-helper.scm, which help
us to organize procedures in packages in a 2
dimensional table.
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type1
proc11
proc21
type2
proc12
proc22
Define procedure install-type
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–
type3
proc13
proc23
Table primitives:
z put: row x column x item-to-store ->
z get: row x column -> item-to-store
z display-table ->
prints table
Define the procedures
Store them in table (put)
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Evaluate procedure install-type
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Lab 3 problem 8
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Generic procedures
Scheme interpreter procedures
From the data object find the apropriate procedure
The interpreter can be called by
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Discussions and exercises in SICP section 2.4.3
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operation1
operation2
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Simple procedures for each data-type grouped in
packages
Store packages independently of each other ->
easy to add or remove packages
Create generic operations that retrieves appropriate
procedures from storage
(eval Scheme-expression)
(apply Scheme-procedures argumentlist)
Get the type from the tag in the data object
Get the procedure for the type from the table
Apply the procedure to the data object
(eval (list ’+ 1 2 3)) -> (+ 1 2 3) = 6
(eval (list ’cons (list ’quote ’a) 1)) -> (cons (quote a) 1) =
(a . 1)
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(apply + (list 1 2 3)) -> (apply + ’(1 2 3)) = 6
(apply (lambda (x y) (list y x)) (list ’a ’b)) ->
-> (apply (lambda (x y) (list y x)) ’(a b)) = (b a)
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