Foreign Inline Code in Haskell - YOW! Conferences and...

Manuel M T ChakravartyUniversity of New South Wales

Foreign Inline Code in Haskell

mchakravarty

α TacticalGraceTacticalGracejusttesting.org

1

30 minute time slot: 25min talking + 5min[15min The Problem; 5min TH+Quasiquoting; 5min Inline Objective-C]

http://justtesting.org

http://justtesting.org

λShiny new functional language

2

» Imagine, you have got a shiny, new functional language...

3

» ...and you want to use it to write a great new app...

λ4

» ...then you will need to use many existing frameworks and libraries.» Luckily, any serious language will have a foreign function interface!» Haskell standard includes a simple, but versatile FFI

“Problem solved?”

5

» Does that solve the problem of language interoperability?» Let me explain that at an example...

“Problem solved?”No!

5

» Does that solve the problem of language interoperability?» Let me explain that at an example...

dumpURL :: String -> IO () dumpURL urlString = do urlData <- stringWithContentsOfUrl urlString putStr urlData

What we want to write

6

dumpURL :: String -> IO () dumpURL urlString = do urlData <- stringWithContentsOfUrl urlString putStr urlData

What we want to write

char *stringWithContentsOfUrlCstub(char *urlString) { NSURL *url = [NSURL URLWithString:urlString]; [NSString stringWithContentsOfURL:url encoding:NSUTF8StringEncoding error:NULL]; }

What we want to call (& need to put into an extra file)

6

What we have to write as well…

foreign import stringWithContentsOfURLCstub :: CString -> IO CString

stringWithContentsOfURL :: String -> IO String stringWithContentsOfURL url = withCString url $ \urlC -> do resultC <- stringWithContentsOfURLCstub urlC result <- peek resultC free resultC return result

7

» FFI code: If you find it confusing, that’s fine, as I want to argue that you shouldn’t write it in the first place.* Anything extensively relying on foreign frameworks will be a pain[Hybrid languages (eg, F# & Scala) have a different set of trade offs, but don’t solve it either.]

Bridging Libraries 1-to-1 transliteration of types & functions

8

* Bridging or binding libraries transliterate types & functions* Need to be maintained and documented; track multiple versions of base library* Lack of type safety* Tools and dynamic transliteration (by reflection) help

C➙Haskellc2hs

A case study

GTK+

9

* c2hs: automation for bridging C libraries to Haskell <https://hackage.haskell.org/package/c2hs>

* Implements large parts of a C compiler front-end* Bridge for the cross-platform Gnome GUI library GTK+ <http://projects.haskell.org/

gtk2hs/>

10

* AFAIK, currently the only fully featured and properly maintained Haskell GUI library* Haskell GTK+ library is used for realistic applications

“Does this approach scale?”

11

» Read question* It requires significant resources & constant effort to track the original library* Application frameworks: enormous and growing footprint

February 2011Introduction of GTK+ 3

12

* Base GTK+ development isn’t even particularly fast* Bridging libraries lag behind their base libraries* Another example: Haskell OpenGL library

February 2011Introduction of GTK+ 3

December 2013GTK+ 3 in Haskell Bridge

12

* Base GTK+ development isn’t even particularly fast* Bridging libraries lag behind their base libraries* Another example: Haskell OpenGL library

13

* Compared to modern application frameworks, GTK+ is small!

Bridging libraries don’t scale Too much weight!

14

“Interoperability is an old problem — maybe an old

solution can help?”

15

16

* Modula-2 with inline 68000 assembly* Assembly code can symbolically refer to Modula-2 entities (e.g., variables)

“Inline C, C++, Objective-C, … in Haskell?”

17

* We don’t want to build front ends for a few more languages into GHC* How do we share entities between the languages?

Meta-programmingTemplate Haskell

18

» Generic infrastructure for program manipulationNOTE: I'll run through this quickly; I'll explain the details at the workshop.

HASKELL WORKSHOP 2002

19

* Template Haskell: Haskell extension implemented by GHC* Useful for: defining macros, code generators, code transformations…* Other languages have their own variants; eg., MetaOCaml» Let’s look at an example…


#define

macros

19



#define

macros

[| … |]code

generators19



#define

macros

[| … |]code

generators

trafo (ConE name) = …code

transformations19


$(sel 1 3) (a, b, c) = a

20

* Meta function executed at splice point, generating spliced code

$(sel 1 3) (a, b, c) = a

meta-programming function

20


$(sel 1 3) (a, b, c) = a


splice

20


$(sel 1 3) (a, b, c) = a


splice \tup -> case tup of {(x, y, z) -> x} :: (a, b, c) -> a

20


$(sel 1 3) (a, b, c) = a



$(sel 5 5) (a, b, c, d, e) = e

20


$(sel 1 3) (a, b, c) = a



$(sel 5 5) (a, b, c, d, e) = e

\tup -> case tup of {(x, y, z, v, w) -> w} :: (a, b, c, d, e) -> e

20


sel :: Int -> Int -> ExpQ sel i n = [| \tup -> case tup of {$pat -> $res} |] where pat = tupP (map varP names) res = varE (names !! (i - 1)) names = [mkName $ "v" ++ show i | i <- [1..n]]

21

* Quasiquotations in [|..|] brackets* Explain TH in more detail in the workshop


type of Haskell expressions

21



type of Haskell expressions quasi-quotation

21



type of Haskell expressions quasi-quotation

spliced expression(anti quote)

21


Processing other languagesGeneric quasi-quotation

22

* Quoting arbitrary languages


Quasiquoting for any language you can provide a parser for

23

Language.C.Quote

add n = [cfun| int addConstant(int x) { return x + $int:n; } |]

24

* QQ for C including some GNU extensions, parts of CUDA & OpenCL, and all of Objective-C

Language.C.Quote


quasi-quotation identifier

24


Language.C.Quote


quasi-quotation identifier

splice identifier

24


mkMap dev aenv fun arr = return $ CUTranslSkel "map" [cunit|

$esc:("#include <accelerate_cuda.h>") extern "C" __global__ void map ($params:argIn, $params:argOut) { const int shapeSize = size(shOut); const int gridSize = $exp:(gridSize dev); int ix;

for ( ix = $exp:(threadIdx dev) ; ix < shapeSize ; ix += gridSize ) { $items:(dce x .=. get ix) $items:(setOut "ix" .=. f x) } } |] where ...

25

* Accelerate: embedded high-performance array language for GPUs* Combinators as code skeletons (code templates with holes)* Yellow splices/anti-quotes are the holes (parameters) of the template* Doing this with strings or explicit AST construction would be much worse

Inlining foreign languagesInline C Code

26

Language.C.Inline

dumpURL :: String -> IO () dumpURL urlString = do urlData <-

putStr urlData

stringWithContentsOfUrl urlString

27

* Inline Objective-C* All the FFI code is generated automatically* Again, details in the workshop

Language.C.Inline

dumpURL :: String -> IO () dumpURL urlString = do urlData <-

putStr urlData

$(objc ['urlString] ''String [cexp| [NSString stringWithContentsOfURL: [NSURL URLWithString:urlString] encoding:NSUTF8StringEncoding error:NULL] |])

inline Objective-C splice

27

* Inline Objective-C* All the FFI code is generated automatically* Again, details in the workshop

Inline C…is simple

28

* Lack of bridge saves a lot of work; instant access to new versions* Bridges require familiarity with native libraries already; inline code leverages that directly* Haskell types are automatically mapped to types of the inline language


No bridging library needs to be maintained

28



No extra documentation


28



No extra documentation

Some type safety


28


Language integration bylanguage inlining

types >< state languages

29

Outline workshop* Get familiar with TH and Language.C.Quote* Run through three exercises using inline Objective-C including a graphical REPL

Workshop Part 1Template Haskell

30

* http://www.haskell.org/haskellwiki/Template_Haskell* http://hackage.haskell.org/package/template-haskell

Clone https://github.com/mchakravarty/ylj14-workshop

31

printf :: String -> ExpQ printf fmt = gen (parse fmt)

33


data Format = D | S | L String parse :: String -> [Format]

33


data Format = D | S | L String parse :: String -> [Format]

gen :: [Format] -> ExpQ gen [D] = [| \n -> show n |] gen [S] = [| \s -> s |] gen [L s] = [| s |]

33

Task ❶

A recursive generator

34

printf/

35

printf :: String -> ExpQ printf fmt = gen (parse fmt) [| "" |]

printf/

35


gen :: [Format] -> ExpQ -> ExpQ

printf/

35


gen :: [Format] -> ExpQ -> ExpQ gen [] prefix = prefix

printf/

35


gen :: [Format] -> ExpQ -> ExpQ gen [] prefix = prefix gen (D : fmt) prefix

printf/

35


gen :: [Format] -> ExpQ -> ExpQ gen [] prefix = prefix gen (D : fmt) prefix = [| \n -> $(gen fmt [| $prefix ++ show n |]) |]

printf/

35


gen :: [Format] -> ExpQ -> ExpQ gen [] prefix = prefix gen (D : fmt) prefix = [| \n -> $(gen fmt [| $prefix ++ show n |]) |] gen (S : fmt) prefix

printf/

35

Workshop Part 2Quasi-quotation of C variants

36

* http://hackage.haskell.org/package/language-c-quote

gensum/

37

genSum :: Name -> Int -> BlockItem genSum arr n = [citem| { int sum = 0; for (int i = 0; i++; i < $n) sum += $id:(show arr)[i]; } |]

gensum/

37


custom parser for C block items

gensum/

37



splicing of a name as an identifier

gensum/

37



splicing of a name as an identifier

default is to spliceas an expression

gensum/

37

Task ❷

Loop unrolling

38

Replace the for loop by a statement sequence

[cstms| … |] quote a statement sequence$stms:… splice a statement sequence

39

genSum arr n = [citem| { int sum = 0; $stms:(additions 0) } |] where additions i | i == n = [] | otherwise = [cstms| sum += $id:(show arr)[ $int:i ]; $stms:(additions (i + 1)) |]

Replace the for loop by a statement sequence

[cstms| … |] quote a statement sequence$stms:… splice a statement sequence

39

Workshop Part 3Inline Objective-C Code

40

* http://hackage.haskell.org/package/language-c-inline

{-# LANGUAGE TemplateHaskell, QuasiQuotes #-}

import Language.C.Quote.ObjC import Language.C.Inline.ObjC

objc_import ["<Foundation/Foundation.h>"]

nslog :: String -> IO () nslog msg = $(objc ['msg] ''() [cexp| NSLog(@"Here is a message from Haskell: %@”,! ! ! ! ! ! msg) |])

objc_emit

minimal/

41

* Complete the skeleton in the exercise pack





objc_emit

imports for the generatedObjective-C code

minimal/

41






objc_emit

free variables


minimal/

41






objc_emit

free variables


finalise inline code

minimal/

41


Task ❸

Bridging classes

42

particle/

Particle.hs Represent Haskell record as ObjC classMain.hs Allocate instance and use it

43

particle/

Particle.hs Represent Haskell record as ObjC classMain.hs Allocate instance and use it

Complete the ObjC class implementation

43

Task ❹

A simple application

44

app/

Interpreter.hs Haskell interface to package ghcAppDelegate.hs Haskell-Objective-C bridge

45

app/

Interpreter.hs Haskell interface to package ghcAppDelegate.hs Haskell-Objective-C bridge

Complete AppDelegate.hs

Run with open -a HSApp.app

45

Thank you!

46

Images from

http://wikimedia.org

http://openclipart.org

GitHub repo

https://github.com/mchakravarty/language-c-inline

Megamax Modula-2 Screenshot by Dirk Steins

Foreign Inline Code in Haskell - YOW! Conferences and...

Documents

Transcript of Foreign Inline Code in Haskell - YOW! Conferences and...