I discuss Haste, which compiles Haskell code to Javascript to be run on the browser. I then cover Tagless Final Style, which is a technique for creating flexible and extensible DSLs.

1. Haste
Same Language, Multiple latforms
Tagless Final Style
Same Syntax, Multiple nterpretations
Nathan Sorenson
@takeoutweight
i
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2. haste-lang.org
A Haskell-to-Javascript Compiler
created by Anton Ekblad

3. + Full Haskell 2010 Support
(Proper Numbers, Lazy, Pure, Type Classes, …)
+ Nearly all GHC extensions
+ Supports large amount of Hackage
+ Cabal-style build
+ Compact output (~2k hello world)
+ Fast
+ Javascript FFI
+ Browser API

4. - No Template Haskell
- No GHCi
- No forkIO
- No Weak Pointers

5. Elm
Haskell-inspired. Strict. Structural typing and FRP.
Purescript
Haskell-inspired. Strict. Structural typing and Effect typing.
Fay
Haskell subset. Lazy. Small & Fast code. No type classes.
GHCJS
Full GHC. Big runtime with GC, Thread scheduler, etc

6. Browser-Friendly GHC-Compatible
Elm
PureScript
Fay GHCJSHaste

7. ghc-7.4.2.9: The GHC API
parseModule :: GhcMonad m => ModSummary → m ParsedModule
typeCheckModule :: GhcMonad m => ParsedModule → m TypecheckedModu
desugarModule :: GhcMonad m => TypecheckedModule → m DesugaredModule
coreToStg :: DynFlags → CoreProgram → IO [ StgBinding ]

8. +---------+
LLVM backend /--->| LLVM IR |--
| +---------+ | LLVM
| v
+------------+ Desugar +------+ STGify +-----+ CodeGen +-----+ | NCG +----------+
| Parse tree |--------->| Core |-------->| STG |--------->| C-- |----+-------->| Assembly |
+------------+ +------+ +-----+ +-----+ | +----------+
| ^
| +---+ | GCC
C backend ---->| C |--/
+---+
https://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/GeneratedCode
ghc-7.4.2.9: The GHC API

9. module StgSyn where
data GenStgExpr bndr occ Source
= StgApp occ [GenStgArg occ]
| StgLit Literal
| StgConApp DataCon [GenStgArg occ]
| StgOpApp StgOp [GenStgArg occ] Type
| StgLam Type [bndr] StgExpr
| StgCase (GenStgExpr bndr occ) (GenStgLiveVars occ) …
| StgLet (GenStgBinding bndr occ) (GenStgExpr bndr occ)
| StgLetNoEscape (GenStgLiveVars occ) (GenStgLiveVars …
| StgSCC CostCentre !Bool !Bool (GenStgExpr bndr occ)
| StgTick Module Int (GenStgExpr bndr occ)

10. module Data.JSTarget.AST where
-- | Expressions. Completely predictable.
data Exp where
Var :: Var → Exp
Lit :: Lit → Exp
Not :: Exp → Exp
BinOp :: BinOp → Exp → Exp → Exp
Fun :: Maybe Name → [Var] → Stm → Exp
Call :: Arity → Call → Exp → [Exp] → Exp
Index :: Exp → Exp → Exp
Arr :: [Exp] → Exp
AssignEx :: Exp → Exp → Exp
IfEx :: Exp → Exp → Exp → Exp
deriving (Eq, Show)

11. https://ghc.haskell.org/trac/ghc/ticket/3693

13. Installing
$ cabal install haste-compiler
$ haste-boot
# Or from source
$ git clone https://github.com/valderman/haste-compiler.git
$ cd haste-compiler
$ cabal sandbox init
$ cabal install
$ haste-boot --local

14. Building a Haste Project
$ hastec Main.hs # → Main.js
$ hastec --start=asap Main.hs # node Main.js
# Or with via cabal-install
$ haste-inst configure
$ haste-inst build
# installing dependencies (if lucky)
$ haste-inst install contravariant mtl semigroups

15. Remove Haste Unfriendly Things
use Cabal build-type: Simple, not Custom
remove use of Template Haskell
remove use of ‘vector’ package
# installing dependencies (if unlucky)
$ cabal unpack time
# … remove Haste Unfriendly Things …
$ haste-inst configure
$ haste-inst build --install-jsmods --ghc-options=-UHLINT
$ haste-install-his time-1.4.2 dist/build
$ haste-copy-pkg time-1.4.2 --package-
db=dist/package.conf.inplace

16. ./libraries/haste-lib
module Haste.DOM
addChild :: MonadIO m => Elem → Elem → m ()
elemById :: MonadIO m => ElemID → m (Maybe Elem)
module Haste.JSON
encodeJSON :: JSON → JSString
decodeJSON :: JSString → Either String JSON
module Haste.Graphics.Canvas
setFillColor :: Color → Picture ()
line :: Point → Point → Shape ()
module Haste.Concurrent.Monad
forkIO :: CIO () → CIO ()
putMVar :: MVar a → a → CIO ()

17. FFI
// javascript.js
function jsGetAttr(elem, prop) {
return elem.getAttribute(prop).toString();
}
-- haskell.hs (compile-time ffi)
foreign import ccall jsGetAttr :: Elem → JSString → IO JSString
-- (in-line javascript, run-time ffi)
f :: String → String → IO Int
f a b = ffi “(function (a,b) {window.tst = a; return 3;})” a b
-- expose to JS, via Haste[“myInc”] or Haste.myInc
export :: FFI a => JSString → a → IO ()
export “myInc” ((x -> return (x + 1)) :: Int → IO Int)
// javascript.js
Haste.myInc(3) // 4

18. facebook.github.io/react

19. <div id=“mydiv”>
<button>clickme</button>
</div>
React.DOM.div({idName:“mydiv”},
[React.DOM.button({},
[“clickme”])])

20. React.DOM.div({idName:“mydiv”},
[React.DOM.button({},
[“clickme”])])
<div id=“mydiv”>
<button>click{{me}}</button>
</div>

21. function(me) {
return React.DOM.div({idName:“mydiv”},
[React.DOM.button({},
[“click”+me])]);
}
<div id=“mydiv”>
<button>click{{me}}</button>
</div>

22. div :: [Attr] → [JSPtr] → JSPtr
button :: [Attr] → [JSPtr] → JSPtr
text :: String → JSPtr
EDSL

23. div :: [Attr] → [JSPtr] → JSPtr
button :: [Attr] → [JSPtr] → JSPtr
text :: String → JSPtr
EDSS
Embedded Domain Specific Syntax

24. Tagless Final Style

25. Tagless Final Style
Discovered by Oleg Kiselyov

26. Tagless Final Style
Discovered by Oleg Kiselyov
But don’t be scared.

27. Initial Style
data Html = Div [Attr] [Html]
| Button [Attr] [Html]
| Text String
client :: Html → JSPtr
client (Div attrs children) = …
client (Button attrs children) = …
client (Text str) = …
server :: Html → String
server (Div attrs children) = …
server (Button attrs children) = …
server (Text str) = …
i
i

28. server :: Html → String
server (Div attrs children) =
“<div” ++ show attrs ++ “>”
++ concatMap server children
++ “</div>”
server (Button attrs children) =
“<button” ++ show attrs ++ “>”
++ concatMap server children
++ “</button>”
server (Text str) = str

29. Initial Style
data Html =
Div [Attr] [Html]
| Button [Attr] [Html]
| Text String

30. class Html i where
div :: [Attr] → [ i ] → i
button :: [Attr] → [ i ] → i
text :: String → i
Final Style

31. Final Style
class Html i where
div :: [Attr] → [i] → i
button :: [Attr] → [i] → i
text :: String → i
-- Initial Style
data Html =
Div [Attr] [Html]
| Button [Attr] [Html]
| Text String

32. Final Style
class Html i where
div :: [Attr] → [i] → i
button :: [Attr] → [i] → i
text :: String → i
-- Initial Style (GADT)
data Html where
Div :: [Attr] → [Html] → Html
Button :: [Attr] → [Html] → Html
Text :: String → Html

33. Final Style
class Html i where
div :: [Attr] → [i] → i
button :: [Attr] → [i] → i
text :: String → i
instance Html String where
div attrs children = …
button attrs children = …
text str = …
instance Html JSPtr where
div attrs children = …
button attrs children = …
text str = …
i
i

34. srv :: Html→String
srv (Div attrs children) =
“<div” ++ show attrs ++ “>”
++ concatMap srv children
++ “</div>”
srv (Button attrs children) =
“<button” ++ show attrs ++ “>”
++ concatMap srv children
++ “</button>”
srv (Text str) = str

35. instance Html String where
div attrs children =
“<div” ++ show attrs ++ “>”
++ concatMap srv children
++ “</div>”
button attrs children =
“<button” ++ show attrs ++ “>”
++ concatMap srv children
++ “</button>”
text str = str

36. instance Html String where
-- div :: [Attr] → [i] → i
div attrs children =
“<div” ++ show attrs ++ “>”
++ concatMap ??? children
++ “</div>”
-- button :: [Attr] → [i] → i
button attrs children =
“<button” ++ show attrs ++ “>”
++ concatMap ??? children
++ “</button>”
-- text:: String → i
text str = str

37. instance Html String where
-- div :: [Attr] → [i] → i
div attrs children =
“<div” ++ show attrs ++ “>”
++ concatMap id children
++ “</div>”
-- button :: [Attr] → [i] → i
button attrs children =
“<button” ++ show attrs ++ “>”
++ concatMap id children
++ “</button>”
-- text:: String → i
text str = str

38. instance Html String where
-- div :: [Attr] → [i] → i
div attrs children =
“<div” ++ show attrs ++ “>”
++ concat children
++ “</div>”
-- button :: [Attr] → [i] → i
button attrs children =
“<button” ++ show attrs ++ “>”
++ concat children
++ “</button>”
-- text:: String → i
text str = str

39. -- Initial Style
i :: Html
i = Div [] [(Button [] [Text “clickMe”])]
iOut :: String
iOut = server i
-- Final Style
f :: (Html i) => i
f = div [] [(button [] [text “clickMe”])]
fOut = f :: String

40. class Math (i :: * ) where
lit :: Int → i
(+) :: i → i → i
(>) :: i → i → i
instance Math Int where …
instance Math String where …
i
i

41. class Math (i :: *→*) where
lit :: Int → i Int
(+) :: i Int → i Int → i Int
(>) :: i Int → i Int → i Bool

42. class Math (i :: *→*) where
lit :: Int → i Int
(+) :: i Int → i Int → i Int
(>) :: i Int → i Int → i Bool
newtype Eval a = Eval {eval :: a}
instance Math Eval where …
newtype Pretty a = Pretty {pp :: String}
instance Math Pretty where …
a = (lit 1) > ((lit 2) + (lit 3))
e = eval a -- False
p = pp a -- “(1 > (2 + 3))”
i
i

43. class Html i where
div :: [Attr] → [i] → i
button :: [Attr] → [i] → i
text :: String → i
instance SafariHtml String where
webkitElt attrs children) = …
instance SafariHtml JSPtr where
webkitElt attrs children) = …
class SafariHtml i where
webkitElt :: [Attr] → [i] → i
Language Extensibility
i
i

44. f :: (Html i, SafariHtml i) => i
f = div [] [(webkitElt [] [text “clickMe”])]
fOut = f :: String
Language Extensibility

45. div :: (Attr a, Html i) => [a] → [i] → i
-- div [idName “mydiv”] []
button :: (Attr a, Html i) => [a] → [i] → i
-- button [idName “mybtn”, disabled True] []
class Attr a where
idName :: String → a
disabled :: Bool → a
instance Attr DivAttr where
idName s = …
disabled b = … instance Attr ButtonAttr where
idName s = …
disabled b = …
newtype ButtonAttr
newtype DivAttr
i
i

46. div :: (Html i) => [DivAttr] → [i] → i
-- div [idName “mydiv”] []
button :: (Html i) => [ButtonAttr] → [i] → i
-- button [idName “mybtn”, disabled True] []
class IdA a where
idName :: String → a
class DisabledA a where
disabled :: Bool → a
instance IdA ButtonAttr where
idName s = …
instance DisabledA ButtonAttr where
disabled b = …
newtype ButtonAttrinstance IdA DivAttr where
idName s = …
newtype DivAttr
i
i
i

47. type src form
<img>
instance
SrcA ImgAttr
<input>
instance
TypeA InputAttr
instance
SrcA InputAttr
instance
FormA InputAttr
<button
>
instance
TypeA ButtonAttr
instance
FormA ButtonAttr
<label>
instance
FormA LabelAttr
i
i
i
i i
i
i

48. Typed Tagless Final Course Notes
okmij.org/ftp/tagless-final/course/lecture.pdf
haste-lang.org
“Haskell in the Browser With Haste” Lars Kuhtz
alephcloud.github.io/bayhac2014/slides
facebook.github.io/react
github.com/takeoutweight @takeoutweight