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Generics Everywhere CONTACT@ADAMFURMANEK.PL HTTP://BLOG.ADAMFURMANEK.PL FURMANEKADAM 1 25.10.2020 GENERICS EVERYWHERE - ADAM FURMANEK About me Experienced with backend, frontend, mobile, desktop, ML, databases. Blogger, public speaker.

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Generics Everywhere

CONTACT@ADAMFURMANEK.PL HTTP://BLOG.ADAMFURMANEK.PL FURMANEKADAM

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About me

Experienced with backend, frontend, mobile, desktop, ML, databases. Blogger, public speaker. Author of .NET Internals Cookbook. http://blog.adamfurmanek.pl contact@adamfurmanek.pl furmanekadam

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Agenda

Why do we need reusability. Templates in C++. Erasure in Java. Reification in .NET.

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Reusability

Use of existing assets in some form within the software product development process.

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Why do we need reusability

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Why do we need reusability

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Generic programming

Style of computer programming in which algorithms are written in terms of types to- be-specified-later that are then instantiated when needed for specific types provided as parameters.

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Generic programming

Not new – started in ML in 1973. Most popular on function and class level. Multiple flavours – templates, generics, type classes, polytypic functions. Used for code reuse, static-time reflection, metaprogramming.

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Templates in C++

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C++ templates C++ „copies” each template and replaces type placeholder with actual type. Then it tries to compile it. If it works – great!

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How to see the code?

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What is T?

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template<typename T> template<typename T = void> template<typename... Ts> template<int I> template<typename K, typename V, template<typename> typename C = my_array>

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Raytracing

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https://github.com/phresnel/metatrace

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C++ templates

PROS

Relatively easy to understand. Doesn’t require sophisticated runtime support. Works without any notion of „supertype” or „base type” for all types. Doesn’t require extensive constraints for passed types. Can be easily specialized for specific types. Works nice with const expressions. Turing complete – perfect for compile time calculations.

CONS „Something” needs to instantiate the type for given parameter. A lot of machine code duplication. We cannot instantiate the code in runtime. Compiler messages are often less than ideal. We cannot constrain input type declaratively – we can only use metaprogramming tricks.

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JVM Erasure

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Java type system

OBJECTS java.lang.Object is the root of the class

  • hierarchy. Every class has Object as a
  • superclass. All objects, including arrays,

implement the methods of this class. Provides method for equality checking (equals), calculating hashcode (hashCode), checking runtime type (getClass), serializing to string literal (toString). String is an Object. PRIMITIVE VALUES Primitive types correspond to primitive values. These include int, short, long, byte, char, float, double, boolean. We can implicitly convert between them (most

  • f the times).

There is a reference type for each primitive type respectively (Integer, Short, Long, Byte, Character, Float, Double, Boolean).

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JVM erasure

Java replaces generic placeholder with java.lang.Object. In other words, it gets rid of the generic type. There is one version of the code – no copying of generic for different types. It then compiles the code with the java.lang.Object as a generic type.

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JVM erasure

PROS Maintains backwards comaptibility. Stores one code for all types. Works with higher kinded types (partially). We can constrain generic parameter (bounds). Relatively easy to implement. Works with covariance and contravariance better. CONS

We lose the type information. We have a performance hit for primitive types. We don’t have full compiler support and type safety. It is harder to perform optimizations for particular types. We need to encode „trivial” things with type system. We cannot provide specialization for given type.

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Reification in .NET

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.NET Type system

Mostly based on Java type system. System.Object is the root of the hierarchy. Primitive types are called value types. Users can create their

  • wn value types (structures).

Few more things: delegates, events, expression trees, dynamic.

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Reification in .NET

.NET replaces type placeholder with System.__Canon for reference types and actual type for value types. It has one code for all reference types and different code for each value type. It doesn’t lose the generic type. Generics are reified and remember their type.

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Reification in .NET

PROS Maintains generic type – no need for passing type information in runtime. Guarantees type safety. Easier optimizations for value types. No overhead for value type (no boxing). CONS No way to do higher kinded types without runtime support. Harder to implement. Duplicates code for value types. Need to encode „trivial” things with type system. No easy way to provide specialization for given type.

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Summary

3 different approaches for reusability:

  • C++ copying code, changing the type and compiling it.
  • JVM removing generic type and pretending it is still there.
  • .NET storing generic type and generating different code for value types.

Each approach has pros and cons

  • C++ approach is Turing complete but requires a lot of code duplication.
  • JVM approach is easy to implement but doesn’t guarantee type safety.
  • .NET approach is much stronger and guarantees type safety but doesn’t

support higher kinded types = less powerful type system.

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Q&A

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References

Jeffrey Richter — „CLR via C#” Mario Hewardt — „Advanced .NET Debugging” Adam Furmanek – „.NET Internals Cookbook” Andrei Alexandrescu — „Modern C++ Design: Generic Programming and Design Patterns Applied: Applied Generic and Design Patterns (C++ in Depth)”

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References

https://blog.adamfurmanek.pl/2019/08/31/net-inside-out-part-10-using-type-markers-for-low- level-optimizations/ — type markers in .NET http://dreixel.net/research/pdf/gdmh.pdf — generics in Haskell https://alexandrnikitin.github.io/blog/dotnet-generics-under-the-hood/ — .NET generics under the hood http://www.angelikalanger.com/GenericsFAQ/FAQSections/TechnicalDetails.html — Java generics under the hood

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Thanks!

CONTACT@ADAMFURMANEK.PL HTTP://BLOG.ADAMFURMANEK.PL FURMANEKADAM

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