ocaml datatypes part ii: an exercise in type design · changing fonts in an element 14 • change...

OCamlDatatypesPartII:AnExerciseinTypeDesign

COS326DavidWalker

PrincetonUniversity

slidescopyright2017DavidWalkerpermissiongrantedtoreusetheseslidesfornon-commercialeducaJonalpurposes

1

ExampleTypeDesign 2

IBMdevelopedGML(GeneralizeMarkupLanguage)in1969•  hSp://en.wikipedia.org/wiki/IBM_Generalized_Markup_Language•  PrecursortoSGML,HTMLandXML

:h1.Chapter 1: Introduction :p.GML supported hierarchical containers, such as :ol :li.Ordered lists (like this one), :li.Unordered lists, and :li.Definition lists :eol. as well as simple structures. :p.Markup Minimization (later generalized and formalized in SGML), allowed the end-tags to be omitted for the “h1” and “p” elements.

SimplifiedGML 3

ToprocessaGMLdocument,anOCamlprogramwould:•  Readaseriesofcharactersfromatextfile&ParseGMLstructure•  RepresenttheinformaJoncontentasanOCamldatastructure•  Analyzeortransformthedatastructure•  Print/Store/CommunicateresultsWewillfocusonhowtorepresentandtransformtheinformaJoncontentofaGMLdocument.

ExampleTypeDesign 4

•  AGMLdocumentconsistsof:–  alistofelements

•  Anelementiseither:–  awordormarkupappliedtoanelement

•  Markupiseither:–  italicize,bold,orafontname

ExampleTypeDesign 5

type markup = Ital | Bold | Font of string type elt = Words of string list | Formatted of markup * elt type doc = elt list

•  AGMLdocumentconsistsof:–  alistofelements

•  Anelementiseither:–  awordormarkupappliedtoanelement

•  Markupiseither:–  italicize,bold,orafontname

ExampleData 6


let d = [ Formatted (Bold, Formatted (Font “Arial”,

Words [“Chapter”;“One”])); Words [“It”; ”was”; ”a”; ”dark”;

”&”; ”stormy; ”night.”; "A"];

Formatted (Ital, Words[“shot”]);

Words [“rang”; ”out.”] ];;

Challenge 7

•  Changeallofthe“Arial”fontsinadocumentto“Courier”.•  Ofcourse,whenweprogramfuncJonally,weimplement

changeviaafuncJonthat–  receivesonedatastructureasinput–  buildsanew(different)datastructureasanoutput

Challenge 8

•  Changeallofthe“Arial”fontsinadocumentto“Courier”.


Challenge 9


•  Technique:approachtheproblemtopdown,workondocfirst:

let rec chfonts (elts:doc) : doc =


Challenge 10



let rec chfonts (elts:doc) : doc = match elts with | [] -> | hd::tl ->


Challenge 11



let rec chfonts (elts:doc) : doc = match elts with | [] -> [] | hd::tl -> (chfont hd)::(chfonts tl)


Changingfontsinanelement 12


•  Nextworkonchangingthefontofanelement:

let rec chfont (e:elt) : elt =





let rec chfont (e:elt) : elt = match e with | Words ws -> | Formatted(m,e) ->





let rec chfont (e:elt) : elt = match e with | Words ws -> Words ws | Formatted(m,e) ->





let rec chfont (e:elt) : elt = match e with | Words ws -> Words ws | Formatted(m,e) -> Formatted(chmarkup m, chfont e)




•  Nextworkonchangingamarkup:

let chmarkup (m:markup) : markup =




•  Nextworkonchangingamarkup:

let chmarkup (m:markup) : markup = match m with | Font “Arial” -> Font “Courier” | _ -> m


Summary:Changingfontsinanelement 18

•  Changeallofthe“Arial”fontsinadocumentto“Courier”•  Lesson:funcJonstructurefollowstypestructure

let chmarkup (m:markup) : markup = match m with | Font “Arial” -> Font “Courier” | _ -> m let rec chfont (e:elt) : elt = match e with | Words ws -> Words ws | Formatted(m,e) -> Formatted(chmarkup m, chfont e) let rec chfonts (elts:doc) : doc = match elts with | [] -> [] | hd::tl -> (chfont hd)::(chfonts tl)

PoorStyle 19

•  ConsideragainourdefiniJonofmarkupandmarkupchange:

type markup = Ital | Bold | Font of string let chmarkup (m:markup) : markup = match m with | Font “Arial” -> Font “Courier” | _ -> m

PoorStyle 20

•  Whatifwemakeachange:

type markup = Ital | Bold | Font of string | TTFont of string let chmarkup (m:markup) : markup = match m with | Font “Arial” -> Font “Courier” | _ -> m

theunderscoresilentlycatchesallpossiblealternaJvesthismaynotbewhatwewant--perhapsthereisanArialTTfontitisbeSerifwearealertedofallfuncJonswhoseimplementaJonmayneedtochange

AcoupleofpracJceproblems 24

•  WriteafuncJonthatgetsridofimmediatelyredundantmarkupinadocument.–  FormaSed(Ital,FormaSed(Ital,e))canbesimplifiedtoFormaSed(Ital,e)

–  writemapsandfoldsovermarkups•  Designadatatypetodescribebibliographyentriesfor

publicaJons.SomepublicaJonsarejournalarJcles,othersarebooks,andothersareconferencepapers.Journalshaveaname,numberandissue;bookshaveanISBNnumber;AlloftheseentriesshouldhaveaJtleandauthor.–  designasorJngfuncJon–  designmapsandfoldsoveryourbibliographyentries

ToSummarize 25

•  DesignrecipeforwriJngOCamlcode:–  writedownEnglishspecificaJons

•  trytobreakproblemintoobvioussub-problems–  writedownsomesampletestcases–  writedownthesignature(types)forthecode–  usethesignaturetoguideconstrucJonofthecode:

•  tearapartinputsusingpaSernmatching– makesuretocoverallofthecases!(OCamlwilltellyou)

•  handleeachcase,buildingresultsusingdataconstructor–  thisiswherehumanintelligencecomesintoplay–  the“skeleton”givenbytypescanalmostbedoneautomaJcally!

•  cleanupyourcode–  useyoursampletests(andideallyothers)toensurecorrectness

ocaml datatypes part ii: an exercise in type design · changing fonts in an element 14 • change...

Documents