Declarative Programming & Algebraic Data Types from Django's perspective
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This document discusses using declarative programming and algebraic data types (ADTs) to concentrate on important aspects of code and check as much as possible early. It provides examples of using ADTs in Python to define types for multilingual content and ensure consistency. ADTs allow defining types as variants like an enumeration that can be matched on to concentrate on "what" rather than "how". Consistency is ensured by checking that all variants are handled. This improves coherence of components.
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Declarative Programming & Algebraic Data Types from Django's perspective
- 1. Declarative Programming & Algebraic Data Types * Maxim Avanov maximavanov.com * from Django's perspective 19thMoscow Django Meetup
- 2. Our Goal 1. 束Outsource損 boilerplate (i.e. concentrate on important). 2. Check as much as possible, and as soon as possible. 3. Component coherence.
- 3. 1. A story of How & What Concentrate on important
- 6. A few things to worry about defhandle_article_form(request): ifrequest.method=='POST': form=ArticleForm(request.POST) ifform.is_valid(): save_article(form.cleaned_data) returnHttpResponseRedirect('/success/') else: form=ArticleForm() returnrender(request,'article_form.html',{'form':form}) 束What損 is obscured by 束How損 redundant details Single Responsibility principle is violated
- 7. If we could get rid of How's once and for all, would we miss them?
- 10. What it actually means #WehaveanArticlesHandler:ArticlesHandler #Itrendersatemplate:article_form.html #Ausershallbeabletoaddnewentries:article_form() #IfwesubmitvalidArticleForm:save_article() #IfwesubmitinvalidArticleForm:invalid_article_form() classArticlesHandler(object): def__init__(self,request): self.request=request defarticle_form(self): form=ArticleForm() return{'form':form} defsave_article(self): save_article(self.request.form.cleaned_data) returnHttpResponseRedirect('/success/') definvalid_article_form(self): return{'form':self.request.form}
- 11. ...or in other words --- view:ArticlesHandler GET:article_form POST: -validate:articles.ArticleForm view:save_article -view:invalid_article_form classArticlesHandler(object): def__init__(self,request): self.request=request defarticle_form(self): form=ArticleForm() return{'form':form} defsave_article(self): save_article(self.request.form.cleaned_data) returnHttpResponseRedirect('/success/') definvalid_article_form(self): return{'form':self.request.form}
- 12. A couple of other examples
- 13. Different ways to do the same thing @view_defaults(route_name='authentication',renderer='auth_form.html') classAuthenticationHandler(object): @view_config(request_method='POST',validate_form=EmailAuthForm) defauth_with_email(self): #... @view_config(request_method='POST',validate_form=SMSAuthForm) defauth_with_sms(self): #... @view_config(request_method='POST',validate_form=LoginAuthForm) defauth_with_login(self): #... @view_config(request_method='POST') defon_invalid_form(self): #...
- 16. OCaml ADT example Watch a 束Caml Trading損 talk by Yaron Minsky at http://youtu.be/hKcOkWzj0_s?t=31m6s
- 19. 2. Trying to reproduce Check as much as possible, and as soon as possible
- 20. Python ADT * * kind of
- 21. Python ADT example Models (product types) fromdjango.dbimportmodels classOrder(models.Model): tid=models.IntegerField() price=models.DecimalField(max_digits=16,decimal_places=4) size=models.IntegerField() classCancel(models.Model): xtid=models.IntegerField() classCancelReplace(models.Model): xr_tid=models.IntegerField() new_price=models.DecimalField(max_digits=16,decimal_places=4) new_size=models.IntegerField()
- 22. Python ADT example Smart Enums (union types) fromrhetoric.adtimportadt from.modelsimportOrder,Cancel,CancelReplace classInstruction(adt): ORDER=Order CANCEL=Cancel CANCEL_REPLACE=CancelReplace
- 23. Python ADT example Cases (match statement) from.typesimportInstruction @Instruction.ORDER('filter_by_oid') deffilter_order_by_oid(order,oid): returnorder.tid==oid @Instruction.CANCEL('filter_by_oid') deffilter_cancel_by_oid(cancel,oid): returncancel.xtid==oid @Instruction.CANCEL_REPLACE('filter_by_oid') deffilter_cancel_replace_by_oid(cr,oid): returncr.xr_tid==oid deffilter_by_oid(instructions,oid): returnlist(filter( lambdai:Instruction.match(i)['filter_by_oid'](i,oid), instructions))
- 24. Python ADT example Cases (2) from.typesimportInstruction inline_matcher=Instruction.inline_match( ORDER=(lambdao,oid:o.tid==oid), CANCEL=(lambdac,oid:c.xtid==oid), CANCEL_REPLACE=(lambdacr,oid:cr.xr_tid==oid)) deffilter_by_oid_alt(instructions,oid): returnlist(filter(lambdai:inline_matcher(i)(i,oid), instructions))
- 25. 3. Use case: multilingual content Component coherence
- 29. Consistency check Adding a new language classLanguage(adt): ENGLISH='en' GERMAN='de' SPANISH='es' ConfigurationError: Casedb:articlesof<class'project.articles.types.Language'> isnotexhaustive.Hereisthevariantthatisnotmatched:SPANISH #Wehavetoregisterthiscase @Language.SPANISH('db:articles') classSpanishArticle(IRegionalArticle): pass
- 30. Consistency check Using undefined variant fromrhetoric.adtimportadt classLanguage(adt): ENGLISH='en' GERMAN='de' inline_matcher=Language.inline_match( ENGLISH=lambda:EnglishArticle.objects.all(), GERMAN=lambda:GermanArticle.objects.all(), SPANISH=lambda:SpanishArticle.objects.all() ) PatternError:VariantSPANISHdoesnotbelong tothetype<class'project.articles.types.Language'>
- 31. Consistency check Guard boundaries from.typesimportLanguage defincludeme(config): RULES={'language':Language} config.add_route('articles.regional.index','/articles/{language}',RULES ) /articles/{language} => ^articles/(?P<language>(?:de|en))$ classLanguage(adt): ENGLISH='en' GERMAN='de' SPANISH='es' /articles/{language} => ^articles/(?P<language>(?:de|en|es))$
- 33. Beyond today's topic Things worth mentioning github.com/lihaoyi/macropy github.com/benanhalt/PyAlgebraicDataTypes (cons ADT) 束Say What You Mean損 talk by Ryan Kelly
- 34. Thank you Q & A
- 35. Credits talk by Yaron Minsky束Caml Trading損 Rhetoric Project Venusian Project