Common Lisp. In Practical Usage

Transcription

1 Common Lisp In Practical Usage

2 Contents A walk through Lisp history What the man in the street seems to think Some highlights of Common Lisp Lisp in production

3 History

4

5 The Beginnings Lisp grew from a research paper by John McCarthy He showed how you could build a language from basic operators And a simple data structure - the list He called it List Processor - Lisp Importantly, this data structure was used for data and code

6 Academia Lisp grew as a research language Used as a language with which to research languages Its academic and theoretical roots made it ideal for fundamental research And it grew into a practical language with multiple university-supported implementations But having a such a high-level language with First-class functions Garbage-collection Many other abstractions (bignums for example) Was a major resource hog on early computers - leading to:

7 Lisp Machines - Custom hardware designed to run Lisp - Solved many problems such as slow garbage collection - And lead to operating systems written entirely in Lisp - Lisp machines were commercialized (Symbolics etc) - Which lead to many Lisp users outside of Academia

8 Lisp Machines (2)

9 Lisp Machines (3)

10 Lisp Machines (4) An O/S written almost to the metal in Lisp Everything was editable Hyperlinked documentation A version of Lisp now featuring A full object system Threading A large collection of data structures Graphics and windowing system All of this led to a demand for a standard, Common Lisp was born

11 Nothing good ever lasts Commodity hardware became much cheaper US government funding cuts removed a large part of the LM market In the case of Symbolics, bad real-estate investments The result being that cheap and good enough won But (Common) Lisp didn t die as various free and commercial implementations grew from all of this work and running on all common platforms

12 Today There are multiple Common Lisp implementations running on commodity hardware and under development Commercial Lispworks Franz Allegro Clozure (Former MCL) - free but backed by a consulting company Open Source SBCL Clisp ABCL - runs on the JVM And various other projects

13 Libraries Common Lisp has plenty of libraries But until recently was very badly organized Now we have Perennial BETA

14 Resources a good gateway to resources The various provider websites Books Practical Common Lisp, free or in book form On Lisp Assorted other books #lisp on freenode

15 Conferences The next European Lisp Symposium will be April 16th, 17th 2018 In Marbella

16 What the man on the street thinks

17 General criticisms that we hear Lisp is slow Original implementations, particularly of scheme (used in teaching) were entirely interpreted All modern implementations are compiled GC technology has advanced hugely Too many parenthesis It doesn t look like C or Java, this is not a problem There s a reason why Lisp code looks like an abstract syntax tree Because it was, originally. Alternative syntax projects all stalled And now it s recognised as an important strength Different!= bad

18 Fun features

19 Basic syntax Lisp s syntax is about as simple as can be The magic is that code and basic data structures share notation Which makes playing games in code very easy EXAMPLE

20 Basic data structures Lists, for code and data Some formalised List structures with API PLISTS (:a 1 :b 2) with getf ALISTS ((:a. 1) (:b. 2)) with assoc Arrays Hash tables Structures Classes (CLOS)

21 About the language Let s look at a few strengths of Common Lisp Lambda Macros CLOS Conditions We ll see what still sets the language apart And why it has been so influential And why we might still want to use it

22 Lambda Lambda is possibly Lisp s most famous export to the programming world Know to some as the anonymous function Lisp Lambdas can be passed around and called just as in any other typical functional programming language Nothing here will be new to functional programmers EXAMPLE

23 Macros Macros take advantage of Lisp s regular syntax to allow the writing and rewriting of code We can use this to do things like Create DSLs New control structures Manage resources Just make some code prettier They have their downsides Hygiene Compilation issues Complexity

24 CLOS The Common Lisp Object System works from the point of view of Generic Functions and not classes We have multimethods and can specialise on mutiple arguments Around, Before and After methods are part of the language We can customize method how methods are applied CLOS is implemented in CLOS so we can rewrite the object system if we wanted to. EXAMPLE

25 Conditions The Lisp conditions system is a superset of the traditional try/catch construct Conditions are objects and can be subclassed We can unwind the stack if we want, or not We can restart computations in arbitrary places, changing values as we go For development we can catch and retry areas of code that we re working on EXAMPLE

26 Real life usage

27 RavenPack We process and extract structured information from unstructured text We operate mostly on news sources from various providers Speed is important as well as maintaining a large archive Customers are mostly large institutions and universities

28 RavenPack infrastructure Running 24/7 Data is streamed through the system When reclassifying historical data we can be running ~1000 machines Each machine needs immediate access to Gigs of metadata All of this makes certain demands that Lisp helps with Not the only solution of course

29 Development Keep your new app running all day Working on individual functions Start and stop threads as necessary Protect parts of the program as you develop Doing all of this well requires a functional mindset Restarting function calls requires no global state and immutable arguments Closures can also be problematic Functions must be insulated from their exterior loops

30 Deployment We can start a Lisp image from zero But that can take a few minutes to compile, load and load data And requires repositories on the server And access to the data in a DB or AWS Or we can ship a prebuilt Lisp image With all application data already loaded (2GB+) Starts in 5 seconds Either way we have a running REPL So we can connect and debug the image while it runs Load updates Fix problems at the REPL

31 When things go wrong So connecting to a production server for debugging allows We can stop individual threads Examine the stack, inspect the objects Change their values Restart the threads Usually this is just a precursor to developing the fix in development Then shipping it out And loading it into the running Lisp image Or a restart if necessary But it means that errors don t result in a corpse, it s still alive

32 Downsides Limited ecosystem if you re doing something typical Encourages is some a cowboy mindset And individualism Too much fun? :)

33 Questions? Ask me now I m not here tomorrow Or me alawson@ravenpack.com And we are hiring, attitude is more important than experience