DEFUN 2008 - Real World Haskell

Real World Haskell Bryan O’Sullivan bos@serpentine.com 2008-09-27

Welcome! A few things to expect about this tutorial: The pace will be rapid Stop me and ask questions—early and often I assume no prior Haskell exposure

A little bit about Haskell Haskell is a multi-paradigm language. It chooses some unusual, but principled, defaults: Pure functions Non-strict evaluation Immutable data Static, strong typing Why default to these behaviours? We want our code to be safe, modular, and tractable.

Pure functions Definition The result of a pure function depends only on its visible inputs: Given identical inputs, it always computes the same result. It has no other observable effects. What are some consequences of this? Modularity leads to simplified reasoning about behaviour. Straightforward testing: no need for elaborate frameworks.

Immutable data Definition Data is immutable (or purely functional) if it is never modified after construction. To “modify” a value, we create a new value. Both new and old versions can coexist afterwards, so we get persistent, versioned data for free. Modification is often easier than with mutable data. In multithreaded code, we do away with much elaborate locking.

Static, strong typing Deﬁnition A program is statically typed if we know the type of every expression before the program is run. Deﬁnition Code is strongly typed if the absence of certain classes of error can be proven statically.

Safety, modularity, and tractability Safety: As few nasty surprises at runtime as possible. Static typing and eased testing give us conﬁdence. Modularity: We can build big pieces of code from smaller components. No need to focus on the details of the smaller parts. Tractability: All of this ﬁts in our brain comfortably... ...leaving plenty of room for the application we care about.

GHC, the Glorious Glasgow Haskell Compiler Have you got GHC yet? Download installer for Windows, OS X, or Linux here: http://www.haskell.org/ghc/download_ghc_683.html

What’s special about GHC? Mature, portable, optimising compiler Great tools: interactive shell and debugger time and space proﬁlers code coverage analyser BSD-licensed, hence suitable for OSS and commercial use

Counting lines The classic Unix wc command counts the lines in some ﬁles: $ time wc -l *.fasta 9975 1000-Rn_EST.fasta 14032 chr18.fasta 14005 chr19.fasta 13980 chr20.fasta 42017 chr_all.fasta 94009 total real 0m0.017s

Breaking the problem down Subproblems to consider: Get our command line arguments Read a ﬁle Split it into lines Count the lines Let’s work through these in reverse order.

Type signatures Deﬁnition A type signature describes the type of a Haskell expression: e : : Double We read :: as “left has the type right”. So “e has the type Double”. Here’s the accompanying deﬁnition: e = 2.7182818

Type signatures are optional In Haskell, most type signatures are optional. The compiler can automatically infer types based on our usage. Why write type signatures at all, then? Mostly as useful documentation to ourselves.

GHC’s interactive interpreter GHC includes an interactive expression evaluator, ghci. Run it from a terminal window or command prompt: $ ghci GHCi, version 6.8.3: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> The Prelude> text is ghci’s prompt. Type :? at the prompt to get (terse) help.

Basic interaction Let’s enter some expressions: Prelude> 2 + 2 4 Prelude> True && False False We can ﬁnd out about types: Prelude> :type True True :: Bool

Writing a list Here’s an empty list: Prelude> [] [] What do we need to create a longer list? A value An existing list Some glue—the : operator Prelude> 1:[] [1] Prelude> 1:2:[] [1,2]

Syntactic sugar for lists What’s the diﬀerence between these? 1:2:[] [1,2] Nothing—the latter is purely a notational convenience.

Characters and strings One character: Prelude> :type ’a’ ’a’ :: Char A string is a list of characters: Prelude> ’a’ : ’b’ : [] quot;abquot; Notation: Single quotes for one Char Double quotes for a string (written [Char])

Function application We apply a function to its arguments by juxtaposition: Prelude> length [2,4,6] 3 Prelude> take 2 [3,6,9,12] [3,6] Why refer to this as application, instead of the more familiar calling? Haskell is a non-strict language The result may not be computed immediately

Lists are inductive Haskell lists are deﬁned inductively. A list can be one of two things: An empty list A value in front of an existing list We call our friends [] and : value constructors: They construct values that have the type “list of something.”

Counting lines Haskell programmers love abstraction. We won’t worry about counting lines. Instead, we’ll count the elements in any kind of list.

The type signature of a function How do we describe a function that computes the length of a list? l e n : : [ a ] −> I n t e g e r The −> notation denotes a function. The function accepts an [a], and returns an Integer. What’s an [a]? A list, whose elements must all be of some type a.

Counting by induction: the base case An empty list has the length zero. len [ ] = 0 This is our ﬁrst example of pattern matching. Our function accepts one argument. If the argument is an empty list, we return zero. We call this the base case.

Counting by induction: the inductive case Let’s see if a list value was created using the : constructor. len ( x : xs ) = 1 + len xs If the pattern match succeeds: The name x is bound to the head of the list. The name xs is bound to the tail of the list. The body of the deﬁnition is used as the result.

The complete function Save this in a ﬁle named Length.hs: l e n : : [ a ] −> I n t e g e r len [ ] = 0 len ( x : xs ) = 1 + len xs

Load the ﬁle into ghci In the same directory, run ghci: Prelude> :load Length [1 of 1] Compiling Main ( Length.hs, interprete Ok, modules loaded: Main. *Main> The ghci prompt changes when we load ﬁles. Let’s try out our function: *Main> len [] 0 *Main> len (1:[]) 1 *Main> len [4,5,6] 3

Generating a list from a list How might we double every other element of a list? double ( a : b : cs ) = a : b ∗ 2 : double cs double cs = cs Save this in a ﬁle named Double.hs. Load the ﬁle into ghci. Try the following expressions: [1..10] double [1..10]

Your turn: axpy The classic Linpack function axpy computes a × xi + yi over a scalar a and each element i of two vectors x and y . Deﬁne it over two lists of numbers in Haskell. How do we handle lists of diﬀerent lengths?

Splitting text on line boundaries Haskell provides a large library of built-in functions, the Prelude. Here’s the Prelude’s function for splitting text by lines: l i n e s : : S t r i n g −> [ S t r i n g ] The type String is a synonym for [Char]. A ghci experiment: *Main> lines quot;foonbarnquot; [quot;fooquot;,quot;barquot;] *Main> len (lines quot;foonbarnquot;) 2

Reading a file To read a file, we use the Prelude’s readFile function: *Main> :type readFile readFile :: FilePath -> IO String What’s this signature mean? The FilePath type is just a synonym for String. The type IO String means here be dragons! A signature that ends in IO something can have externally visible side effects. Here, the side effect is “read the contents of a file”.

Side effects That innocuous IO in the type is a big deal. We can tell by its type signature whether a value might have externally visible effects. If a type does not include IO, it cannot: Read files Make network connections Launch torpedoes The ideal is for most code to not have an IO type.

Counting lines in a file If we invoke code that has side effects, our code must by implication have side effects too. c o u n t L i n e s : : F i l e P a t h −> IO I n t e g e r c o u n t L i n e s p a t h = do c o n t e n t s <− r e a d F i l e p a t h return ( len ( l i n e s contents )) We had to add IO to our type here because we use readFile, which has side effects. Add this code to Length.hs.

A few explanations The <− notation means “perform the action on the right, and assign the result to the name on the left.” name <− a c t i o n The return function takes a pure value, and (here) adds IO to its type.

Command line arguments We use getArgs to obtain command line arguments. import System . E n v i r o n m e n t ( getArgs ) main = do a r g s <− getArgs putStrLn ( ” h e l l o , a r g s a r e ” ++ show a r g s ) What’s new here? The import directive imports the name getArgs from the System.Environment module. The ++ operator concatenates two lists.

Pattern matching in an expression We use case to pattern match inside an expression. −− Does l i s t c o n t a i n two o r more e l e m e n t s ? atLeastTwo m y L i s t = case m y L i s t o f ( a : b : c s ) −> True −> F a l s e The expression between case and of is matched in turn against each pattern, until one matches.

Irrefutable and wild card patterns A pattern usually matches against a value’s constructors. In other words, it inspects the structure of the value. A simple pattern, e.g. a plain name like a, contains no constructors. It thus matches any value. Deﬁnition A pattern that always matches any value is called irrefutable. The special wild card pattern is irrefutable, but does not bind a value to a name.

Tuples A tuple is a ﬁxed-size collection of values. Items in a tuple can have diﬀerent types. Example: (True,”foo”) This has the type (Bool,String) Contrast tuples with lists, to see why we’d want both: A list is a variable-sized collection of values. Each value in a list must have the same type. Example: [True, False]

The zip function What does the zip function do? Adventures in function discovery, courtesy of ghci: Start by inspecting its type, using :type. Try it with one set of inputs. Then try with another.

Making our program runnable Add the following code to Length.hs: main = do −− E x e r c i s e : g e t t h e command l i n e a r g u m e n t s l e n g t h s <− mapM c o u n t L i n e s a r g s mapM p r i n t L e n g t h ( z i p a r g s l e n g t h s ) case a r g s o f ( : : ) −> p r i n t L e n g t h ( ” t o t a l ” , sum l e n g t h s ) −> r e t u r n ( ) Don’t forget to add an import directive at the beginning!

The mapM function This function applies an action to a list of arguments in turn, and returns the list of results. The mapM function is similar, but returns the value (), aka unit (“nothing”). The mapM function is useful for the eﬀects it causes, e.g. printing every element of a list.

Write your own printLength function Hint: we’ve seen a similar example already, with our getArgs example.

Compiling your program It’s easy to compile a program with GHC: $ ghc --make Length What does the compiler do? Looks for a source ﬁle named Length.hs. Compiles it to native code. Generates an executable named Length.

Running our program Here’s an example from my laptop: $ time ./Length *.fasta 1000-Rn_EST.fasta 9975 chr18.fasta 14032 chr19.fasta 14005 chr20.fasta 13980 chr_all.fasta 42017 total 94009 real 0m1.533s Oh, no! Look at that performance! 90 times slower than wc

Faster ﬁle processing Lists are wonderful to work with But they exact a huge performance toll The current best-of-breed alternative for ﬁle data: ByteString

What is a ByteString? They come in two ﬂavours: Strict: a single packed array of bytes Lazy: a list of 64KB strict chunks Each ﬂavour provides a list-like API.

Retooling our word count program All we do is add an import and change one function: import q u a l i f i e d Data . B y t e S t r i n g . Lazy . Char8 a s B c o u n t L i n e s p a t h = do c o n t e n t s <− B . r e a d F i l e p a t h r e t u r n ( l e n g t h (B . l i n e s c o n t e n t s ) ) The “B.” preﬁxes make us pick up the readFile and lines functions from the bytestring package.

What happens to performance? Haskell lists: 1.533 seconds Lazy ByteString: 0.022 seconds wc command: 0.015 seconds Given the tiny data set size, C and Haskell are in a dead heat.

When to use ByteStrings? Any time you deal with binary data For text, only if you’re sure it’s 8-bit clean For i18n needs, fast packed Unicode is under development. Great open source libraries that use ByteStrings: binary—parsing/generation of binary data zlib and bzlib—support for popular compression/decompression formats attoparsec—parse text-based ﬁles and network protocols

Part 2

A little bit about JSON A popular interchange format for structured data: simpler than XML, and widely supported. Basic types: Number String Boolean Null Derived types: Object: unordered name/value map Array: ordered collection of values

JSON at work: Twitter’s search API From http://search.twitter.com/search.json?q=haskell: {quot;textquot;: quot;Why Haskell? Easiest way to be productivequot;, quot;to_user_idquot;: null, quot;from_userquot;: quot;galoisincquot;, quot;idquot;: 936114469, quot;from_user_idquot;: 1633746, quot;iso_language_codequot;: quot;enquot;, quot;created_atquot;:quot;Fri, 26 Sep 2008 19:15:35 +0000quot;}

What is a JSString? We hide the underlying use of a String: newtype J S S t r i n g = JSONString { f r o m J S S t r i n g : : S t o J S S t r i n g : : S t r i n g −> J S S t r i n g t o J S S t r i n g = JSONString We do the same with JSON objects: newtype J S O b j e c t a = JSONObject { f r o m J S O b j e c t : : [ ( t o J S O b j e c t : : [ ( S t r i n g , a ) ] −> J S O b j e c t a t o J S O b j e c t = JSONObject

JSON conversion In Haskell, we capture type-dependent patterns using typeclasses: The class of types whose values can be converted to and from JSON data R e s u l t a = Ok a | E r r o r S t r i n g c l a s s JSON a where readJSON : : J S V a l u e −> R e s u l t a showJSON : : a −> J S V a l u e

Why JSString, JSObject, and JSArray? Haskell typeclasses give us an open world: We can declare a type to be an instance of a class at any time In fact, we cannot declare the number of instances to be ﬁxed If we left the String type “naked”, what could happen? Someone might declare Char to be an instance of JSON What if someone declared a JSON a =>JSON [a] instance? This is the overlapping instances problem.

Relaxing the overlapping instances restriction By default, GHC is conservative: It rejects overlapping instances outright We can get it to loosen up a bit via a pragma: {−# LANGUAGE O v e r l a p p i n g I n s t a n c e s #−} If it ﬁnds one most speciﬁc instance, it will use it, otherwise bail as before.

Bool as JSON Here’s a simple way to declare the Bool type as an instance of the JSON class: i n s t a n c e JSON Bool where showJSON = JSBool readJSON ( JSBool b ) = Ok b readJSON = E r r o r ” Bool p a r s e f a i l e d ” This has a design problem: We’ve plumbed our Result type straight in If we want to change its implementation, it will be painful

Hiding the plumbing A simple (but good enough!) approach to abstraction: s u c c e s s : : a −> R e s u l t a s u c c e s s k = Ok k f a i l u r e : : S t r i n g −> R e s u l t a f a i l u r e errMsg = E r r o r errMsg Functions like these are sometimes called “smart constructors”.

Does this aﬀect our code much? We simply replace the explicit constructors with the functions we just deﬁned: i n s t a n c e JSON Bool where showJSON = JSBool readJSON ( JSBool b ) = success b readJSON = f a i l u r e ” Bool p a r s e f a i l e d ”

JSON input and output We can now convert between normal Haskell values and our JSON representation. But... ...we still need to be able to transmit this stuﬀ over the wire. Which is more fun to mull over? Parsing!

A functional view of parsing Here’s a super-simple perspective: Take a piece of data (usually a sequence) Try to apply an interpretation to it How might we represent this?

A basic type signature for parsing Take two type variables, i.e. placeholders for types that we’ll substitute later: s—the state (data) we want to parse a—the type of its interpretation We get this generic type signature: s −> a Let’s make the task more concrete: Parse a String as an Int S t r i n g −> I n t What’s missing?

Parsing as state transformation After we’ve parsed one Int, we might have more data in our String that we want to parse. How to represent this? Return the transformed state and the result in a tuple. s −> ( a , s ) We accept an input state of type s, and return a transformed state, also of type s.

Parsing is composable Let’s give integer parsing a name: p a r s e D i g i t : : S t r i n g −> ( I n t , S t r i n g ) How might we want to parse two digits? p a r s e T w o D i g i t s : : S t r i n g −> ( ( I n t , I n t ) , S t r i n g ) parseTwoDigits s = let ( i , t ) = parseDigit s ( j , u) = parseDigit t in (( i , j ) , u)

Chaining parses more tidily It’s not good to represent the guts of our state explicitly using pairs: Tying ourselves to an implementation eliminates wiggle room. Here’s an alternative approach. newtype S t a t e s a = S t a t e { r u n S t a t e : : s −> ( a , s ) } A newline declaration hides our implementation. It has no runtime cost. The runState function is a deconstructor: it exposes the underlying value.

Chaining parses Given a function that produces a result and a new state, we can “chain up” another function that accepts its result. c h a i n S t a t e s : : S t a t e s a −> ( a −> S t a t e s b ) −> S t a c h a i n S t a t e s m k = State chainFunc where c h a i n F u n c s = let (a , t ) = runState m s in runState (k a) t Notice that the result type is compatible with the input: We can chain uses of chainStates!

Injecting a pure value We’ll often want to leave the current state untouched, but inject a normal value that we can use when chaining. p u r e S t a t e : : a −> S t a t e s a p u r e S t a t e a = S t a t e $ s −> ( a , s )

What about computations that might fail? Try these in in ghci: Prelude> head [1,2,3] 1 Prelude> head [] What gets printed in the second case?

One approach to potential failure The Prelude deﬁnes this handy standard type: data Maybe a = Just a | Nothing We can use it as follows: s a f e H e a d ( x : ) = Just x safeHead [ ] = Nothing Save this in a source ﬁle, load it into ghci, and try it out.

Some familiar operations We can chain Maybe values: c h a i n M a y b e s : : Maybe a −> ( a −> Maybe b ) −> Maybe b c h a i n M a y b e s Nothing k = Nothing c h a i n M a y b e s ( Just x ) k = k x This gives us short circuiting if any computation in a chain fails: Maybe is the Ur-exception. We can also inject a pure value into a Maybe-typed computation: pureMaybe : : a −> Maybe a pureMaybe x = Just x

What do these types have in common? Chaining: chainMaybes : : Maybe a −> ( a −> Maybe b ) −> Maybe b chainStates : : State s a −> ( a −> S t a t e s b ) −> State s b Injection of a pure value: p u r e S t a t e : : a −> S t a t e s a pureMaybe : : a −> Maybe a Abstract away the type constructors, and these have identical types!

Monads More type-related pattern capture, courtesy of typeclasses: c l a s s Monad m where −− c h a i n (>>=) : : m a −> ( a −> m b ) −> m b −− i n j e c t a p u r e v a l u e r e t u r n : : a −> m a

Instances When a type is an instance of a typeclass, it supplies particular implementations of the typeclass’s functions: i n s t a n c e Monad Maybe where (>>=) = c h a i n M a y b e s r e t u r n = pureMaybe i n s t a n c e Monad ( S t a t e s ) where (>>=) = c h a i n S t a t e s return = pureState

Chaining with monads Using the methods of the Monad typeclass: parseThreeDigits = p a r s e D i g i t >>= a −> p a r s e D i g i t >>= b −> p a r s e D i g i t >>= c −> return (a , b , c ) Syntactically sugared with do-notation: p a r s e T h r e e D i g i t s = do a <− p a r s e D i g i t b <− p a r s e D i g i t c <− p a r s e D i g i t return (a , b , c ) This now looks suspiciously like imperative code.

Haven’t we forgotten something? What happens if we want to parse a digit out of a string that doesn’t contain any? We’d like to “break the chain” if a parse fails. We have this nice Maybe type for representing failure. Alas, we can’t combine the Maybe monad with the State monad. Diﬀerent monads do not combine.

But this is awful! Don’t we need lots of boilerplate? Are we condemned to a world of numerous slightly tweaked custom monads? We can adapt the behaviour of an underlying monad. newtype MaybeT m a = MaybeT { runMaybeT : : m (Maybe a ) }

Can we inject a pure value? pureMaybeT : : (Monad m) = a −> MaybeT m a > pureMaybeT a = MaybeT ( r e t u r n ( Just a ) )

Can we write a chaining function? chainMaybeTs : : (Monad m) = MaybeT m a −> ( a −> Ma > −> MaybeT m b x ‘ chainMaybeTs ‘ f = MaybeT $ do unwrapped <− runMaybeT x case unwrapped o f Nothing −> r e t u r n Nothing Just y −> runMaybeT ( f y )

Making a Monad instance Given an underlying monad, we can stack a MaybeT on top of it and get a new monad. i n s t a n c e (Monad m) = Monad ( MaybeT m) where > (>>=) = chainMaybeTs r e t u r n = pureMaybeT

A custom monad in 2 lines of code A parsing type that can short-circuit: {−# LANGUAGE G e n e r a l i z e d N e w t y p e D e r i v i n g #−} newtype MyParser a = MyP ( MaybeT ( S t a t e S t r i n g ) a ) d e r i v i n g (Monad , MonadState S t r i n g ) We use a GHC extension to automatically generate instances of non-H98 typeclasses: Monad MonadState String

What is MonadState? The State monad is parameterised over its underlying state, as State s: It knows nothing about the state, and cannot manipulate it. Instead, it implements an interface that lets us query and modify the state ourselves: c l a s s (Monad m) = MonadState s m > −− q u e r y t h e c u r r e n t s t a t e get : : m s −− r e p l a c e t h e s t a t e w i t h a new one p u t : : s −> m ( )

Parsing text In essence: Get the current state, modify it, put the new state back. What do we do on failure? s t r i n g : : S t r i n g −> MyParser ( ) s t r i n g s t r = do s <− g e t l e t ( hd , t l ) = s p l i t A t ( l e n g t h s t r ) s i f s t r == hd then p u t t l e l s e f a i l $ ” f a i l e d t o match ” ++ show s t r

Shipment of fail We’ve carefully hidden fail so far. Why? Many monads have a very bad deﬁnition: error. What’s the problem with error? It throws an exception that we can’t catch in pure code. It’s only safe to use in catastrophic cases.

Non-catastrophic failure A bread-and-butter activity in parsing is lookahead: Inspect the input stream and see what to do next JSON example: An object begins with “{” An array begins with “[” We look at the next input token to ﬁgure out what to do. If we fail to match “{”, it’s not an error. We just try “[” instead.

Giving ourselves alternatives We have two conﬂicting goals: We like to keep our implementation options open. Whether fail crashes depends on the underlying monad. We need a safer, abstract way to fail.

MonadPlus A typeclass with two methods: c l a s s Monad m = MonadPlus m where > −− non−f a t a l f a i l u r e mzero : : m a −− i f t h e f i r s t a c t i o n f a i l s , −− p e r f o r m t h e s e c o n d i n s t e a d mplus : : m a −> m a −> m a To upgrade our code, we replace our use of fail with mzero.

Writing a MonadZero instance We can easily make any stack of MaybeT atop another monad a MonadPlus: i n s t a n c e Monad m = MonadPlus ( MaybeT m) where > mzero = MaybeT $ r e t u r n Nothing a ‘ mplus ‘ b = MaybeT $ do r e s u l t <− runMaybeT a case r e s u l t o f Just k −> r e t u r n ( Just k ) Nothing −> runMaybeT b We simply add MonadPlus to the list of typeclasses we ask GHC to automatically derive for us.

Using MonadPlus Given functions that know how to parse bits of JSON: p a r s e O b j e c t : : MyParser [ ( S t r i n g , J S V a l u e ) ] p a r s e A r r a y : : MyParser [ J S V a l u e ] We can turn them into a coherent whole: parseJSON : : MyParser J S V a l u e parseJSON = ( p a r s e O b j e c t >>= o −> r e t u r n ( J S O b j e c t o ) ) ‘ mplus ‘ ( p a r s e A r r a y >>= a −> r e t u r n ( J S A r r a y a ) ) ‘ mplus ‘ ...

The problem of boilerplate Here’s a repeated pattern from our parser: f o o >>= x −> r e t u r n ( b a r x ) These brief uses of variables, >>=, and return are redundant and burdensome. In fact, this pattern of applying a pure function to a monadic result is ubiquitous.

Boilerplate removal via lifting We replace this boilerplate with liftM: l i f t M : : Monad m = ( a −> b ) −> m a −> m b > We refer to this as lifting a pure function into the monad. parseJSON = ( JSObject ‘ liftM ‘ parseObject ) ‘ mplus ‘ ( JSArray ‘ liftM ‘ parseArray ) This style of programming looks less imperative, and more applicative.

The Parsec library Our motivation so far: Show you that it’s really easy to build a monadic parsing library But we must concede: Maybe you simply want to parse stuﬀ Instead of rolling your own, use Daan Leijen’s Parsec library.

What to expect from Parsec It has some great advantages: A complete, concise EDSL for building parsers Easy to learn Produces useful error messages But it’s not perfect: Strict, so cannot parsing huge streams incrementally Based on String, hence slow Accepts, and chokes on, left-recursive grammars

Parsing a JSON string An example of Parsec’s concision: j s o n S t r i n g = between ( c h a r ’ ” ’ ) ( c h a r ’ ” ’ ) ( many j s o n C h a r ) Some parsing combinators explained: between matches its 1st argument, then its 3rd, then its 2nd many runs a parser until it fails It returns a list of parse results

Parsing a character within a string j s o n C h a r = c h a r ’ ’ >> ( p e s c <|> p u n i ) <|> s a t i s f y ( ‘ notElem ‘ ” ” ” ) Between quotes, jsonChar matches a string’s body: A backslash must be followed by an escape (“n”) or Unicode (“u2fbe” ) Any other character except “” or “”” is okay More combinator notes: The >> combinator is like >>=, but provides only sequencing, not binding The satisfy combinator uses a pure predicate.

Your turn! Write a parser for numbers. Here are some pieces you’ll need: import Numeric ( readFloat , readSigned ) import Text . P a r s e r C o m b i n a t o r s . P a r s e c import C o n t r o l . Monad ( mzero ) Other functions you’ll need: getInput setInput The type of your parser should look like this: parseNumber : : C h a r P a r s e r ( ) R a t i o n a l

Experimenting with your parser Simply load your code into ghci, and start playing: Prelude> :load MyParser *Main> parseTest parseNumber quot;3.14159quot;

My number parser parseNumber = do s <− g e t I n p u t case readSigned r e a d F l o a t s o f [ ( n , s ’ ) ] −> s e t I n p u t s ’ >> r e t u r n n −> mzero <?> ” number ”

Using JSON in Haskell A good JSON package is already available from Hackage: http://tinyurl.com/hs-json The module is named Text.JSON Doesn’t use overlapping instances

Part 3 This was going to be a concurrent web application, but I ran out of time. It’s still going to be informative and fun!

Concurrent programming The dominant programming model: Shared-state threads Locks for synchronization Condition variables for notiﬁcation

The prehistory of threads Invented independently at least 3 times, circa 1965: Dijkstra Berkeley Timesharing System PL/I’s CALL XXX (A, B) TASK; Alas, the model has barely changed in almost half a century.

What does threading involve? Threads are a simple extension to sequential programming. All that we lose are the following: Understandability, Predictability, and Correctness

Concurrent Haskell Introduced in 1996, inspired by Id. Provides a forkIO action to create threads. The MVar type is the communication primitive: Atomically modiﬁable single-slot container Provides get and put operations An empty MVar blocks on get A full MVar blocks on put We can use MVars to build locks, semaphores, etc.

What’s wrong with MVars? MVars are no safer than the concurrency primitives of other languages. Deadlocks Data corruption Race conditions Higher order programming and phantom typing can help, but only a little.

The fundamental problem Given two correct concurrent program fragments: We cannot compose another correct concurrent fragment from them without great care.

Message passing is no panacea It brings its own diﬃculties: The programming model is demanding. Deadlock avoidance is hard. Debugging is really tough. Don’t forget coherence, scaling, atomicity, ...

Lock-free data structures A focus of much research in the 1990s. Modus operandi: ﬁnd a new lock-free algorithm, earn a PhD. Tremendously diﬃcult to get the code right. Neither a scalable or sustainable approach! This inspired research into hardware support, followed by: Software transactional memory

Software transactional memory The model is loosely similar to database programming: Start a transaction. Do lots of work. Either all changes succeed atomically... ...Or they all abort, again atomically. An aborted transaction is usually restarted.

The perils of STM STM code needs to be careful: Transactional code must not perform non-transactional actions. On abort-and-restart, there’s no way to roll back dropNukes()! In traditional languages, this is unenforceable. Programmers can innocently cause serious, hard-to-ﬁnd bugs. Some hacks exist to help, e.g. tm callable annotations.

STM in Haskell In Haskell, the type system solves this problem for us. Recall that I/O actions have IO in their type signatures. STM actions have STM in their type signatures, but not IO. The type system statically prevents STM code from performing non-transactional actions!

Firing up a transaction As usual, we can explore APIs in ghci. The atomically action launches a transaction: Prelude> :m +Control.Concurrent.STM Prelude Control.Concurrent.STM> :type atomically atomically :: STM a -> IO a

Let’s build a game—World of Haskellcraft Our players love to have possessions. data I t e m = S c r o l l | Wand | Banjo d e r i v i n g ( Eq , Ord , Show) −− i n v e n t o r y data I n v = I n v { i n v I t e m s : : [ Item ] , invCapacity : : Int } d e r i v i n g ( Eq , Ord , Show)

Inventory manipulation Here’s how we set up mutable player inventory: import C o n t r o l . C o n c u r r e n t .STM type I n v e n t o r y = TVar I n v n e w I n v e n t o r y : : I n t −> IO I n v e n t o r y n e w I n v e n t o r y cap = newTVarIO I n v { i n v I t e m s = [ ] , i n v C a p a c i t y = cap } The use of curly braces is called record syntax.

Inventory manipulation Here’s how we can add an item to a player’s inventory: a d d I t e m : : I t e m −> I n v e n t o r y −> STM ( ) a d d I t e m i t e m i n v = do i <− readTVar i n v writeTVar inv i { i n v I t e m s = item : i n v I t e m s i } But wait a second: What about an inventory’s capacity? We don’t want our players to have inﬁnitely deep pockets!

Checking capacity GHC deﬁnes a retry action that will abort and restart a transaction if it cannot succeed: i s F u l l : : I n v −> Bool i s F u l l ( I n v i t e m s cap ) = l e n g t h i t e m s == cap a d d I t e m i t e m i n v = do i <− readTVar i n v when ( i s F u l l i ) retry writeTVar inv i { i n v I t e m s = item : i n v I t e m s i }

Let’s try it out Save the code in a ﬁle, and ﬁre up ghci: *Main> i <- newInventory 3 *Main> atomically (addItem Wand i) *Main> atomically (readTVar i) Inv {invItems = [Wand], invCapacity = 3} What happens if you repeat the addItem a few more times?

How does retry work? In principle, all the runtime has to do is retry the transaction immediately, and spin tightly until it succeeds. This might be correct, but it’s wasteful. What happens instead? The RTS tracks each mutable variable touched during a transaction. On retry, it blocks the transaction until at least one of those variables is modiﬁed. We haven’t told GHC what variables to wait on: it does this automatically!

Your turn! Write a function that removes an item from a player’s inventory: r e m o v e I t e m : : I t e m −> I n v e n t o r y −> STM ( )

My item removal action r e m o v e I t e m i t e m i n v = do i <− readTVar i n v case break (==i t e m ) ( i n v I t e m s i ) o f ( ,[]) −> r e t r y ( h , ( : t ) ) −> w r i t e T V a r i n v i { i n v I t e m s = h ++ t }

Your turn again! Write an action that lets us give an item from one player to another: g i v e I t e m : : I t e m −> I n v e n t o r y −> I n v e n t o r y −> STM ( )

My solution g i v e I t e m i t e m a b = do removeItem item a addItem item b

What about that blocking? If we’re writing a game, we don’t want to block forever if a player’s inventory is full or empty. We’d like to say “you can’t do that right now”.

One approach to immediate failure Let’s call this the C programmer’s approach: a d d I t e m 1 : : I t e m −> TVar I n v −> STM Bool a d d I t e m 1 i t e m i n v = do i <− readTVar i n v if isFull i then r e t u r n F a l s e e l s e do writeTVar inv i { i n v I t e m s = item : i n v I t e m s i } r e t u r n True

What is the cost of this approach? If we have to check our results everywhere: The need for checking will spread Sadness will ensue

The Haskeller’s ﬁrst loves We have some fondly held principles: Abstraction Composability Higher-order programming How can we apply these here?

A more abstract approach It turns out that the STM monad is a MonadPlus instance: i m m e d i a t e l y : : STM a −> STM (Maybe a ) immediately act = ( Just ‘ l i f t M ‘ a c t ) ‘ mplus ‘ r e t u r n Nothing

What does mplus do in STM? This combinator is deﬁned as orElse : o r E l s e : : STM a −> STM a −> STM a Given two transactions j and k: If transaction j must abort, perform transaction k instead.

A complicated speciﬁcation We now have all the pieces we need to: Atomically give an item from one player to another. Fail immediately if the giver does not have it, or the recipient cannot accept it. Convert the result to a Bool.

Compositionality for the win Here’s how we glue the whole lot together: import Data . Maybe ( i s J u s t ) giveItemNow : : I t e m −> I n v e n t o r y −> I n v e n t o r y −> IO Bool giveItemNow i t e m a b = liftM isJust . atomically . immediately $ r e m o v e I t e m i t e m a >> a d d I t e m i t e m b Even better, we can do all of this as nearly a one-liner!

Thank you! I hope you found this tutorial useful! Slide source available: http://tinyurl.com/defun08

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DEFUN 2008 - Real World Haskell

by Bryan O'Sullivan

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