GeneralizedNewtypeDeriving ! is!now!type+safe! - - PowerPoint PPT Presentation

GeneralizedNewtypeDeriving! is!now!type+safe!

How!roles!save!the!day

Richard!Eisenberg University!of!Pennsylvania eir@cis.upenn.edu Haskell!Implementors’!Workshop Sunday,!September!22,!2013 Boston,!MA,!USA

GHC!7.6.3!!segfault

newtype7Age7=7MkAge7Int 77deriving7Frob type7family7Discern7a type7instance7Discern7Int7=7Bool type7instance7Discern7Age7=7[Char] class7Frob7a7where 77baz7::7a77Discern7a instance7Frob7Int7where 77baz7=7(>70) segfault7=7head7(baz7(MkAge75))

GHC!7.6.3!!segfault

newtype7Age7=7MkAge7Int 77deriving7Frob type7family7Discern7a type7instance7Discern7Int7=7Bool type7instance7Discern7Age7=7[Char] class7Frob7a7where 77baz7::7a77Discern7a instance7Frob7Int7where 77baz7=7(>70) segfault7=7head7(baz7(MkAge75))

Two!equalities

Nominal!(N)!equality:!two!types!are!the!same.

• reflexivity
• type!synonyms
• type!families
• GADT!pattern+matching
• any!use!of!(~)

Representational!(R)!equality:!two!types!have! the!same!runtime*representation.

• newtypes
• any!nominal!equality

“Haskell!equality” “compile+time!equality” “runtime!equality” R+equality!is!coarser!than!N+equality

Safety!of!GND

• GeneralizedNewtypeDeriving!(GND)!

requires!representational!equality: class7C7a7where7... deriving7instance7C7Age !soundness!requires!(C7Age!~R!!C7Int)

• How!do!we!know!if!this!holds?

!depends!on!the!definition!of!C

• Use!a!role!for!the!parameter!a

Parameter!roles

• All!type!parameters!have!a!role:

data7Foo7b7=7... If!b!is!nominal: If!b!is!representational:

• b’s!role!says!what!notion!of!equality!between!

Baz!and!Boz!is!necessary!to!prove!that!Foo7 Baz!is!representationally!equal!to!Foo7Boz.

• A!parameter!at!representational!role!is!more!

flexible!because!R+equality!is!coarser!than!N+ equality Foo7Age7~R7Foo7Int Foo7Age7~R7Foo7Int

Type+safe!GND

Last!parameter!of!a!class!has!representational!role

• GND!is!type+safe.

Role!inference

• Roles!are!inferred!from!a!type’s!definition
• A!role!is!representational!by!default,!or!nominal!

if!a!parameter!is!used!in!a!nominal!context

• Nominal!contexts:

✦Type!families ✦GADT+like!parameters ✦Use!with!(~) ✦Other!nominal!contexts ✦plus!one!more...

Roles!examples

class7C17a7where7m17::7a77[a] class7C27a7where7m27::7a77Discern7a data7T17a7=7MkT17a data7T27a7where7MkT27::7T27Bool data7T37a7=7MkT37(T27a) !a!is!representational !a!is!nominal !a!is!representational !a!is!nominal !a!is!nominal

Tricky!role!inference

data7Tricky7a7b7=7MkTricky7(a7b) !a!is!representational,!b!is!nominal

Role!inference

• Roles!are!inferred!from!a!type’s!definition
• A!role!is!representational!by!default,!or!nominal!

if!a!parameter!is!used!in!a!nominal!context

• Nominal!contexts:

✦Type!families ✦GADT+like!parameters ✦Use!with!(~) ✦Other!nominal!contexts ✦Argument!to!another!type!variable

Role!annotations

data7Set7a7=7... instance7Ord7Age7where7... 77SS7inverse7of7Int’s7Ord7instance class7HasSet7a7where7mkSet7::7Set7a instance7HasSet7Int7where7mkSet7=7... deriving7instance7HasSet7Age type7role7Set7nominal

Role!annotations

data7Set7a7=7... instance7Ord7Age7where7... 77SS7inverse7of7Int’s7Ord7instance class7HasSet7a7where7mkSet7::7Set7a instance7HasSet7Int7where7mkSet7=7... deriving7instance7HasSet7Age type7role7Set7nominal

Can't7make7a7derived7instance7of7‛HasSet7Age’ 77(even7with7cunning7newtype7deriving): 77it7is7not7typeSsafe7to7use7GeneralizedNewtypeDeriving7on7this7class; 77the7last7parameter7of7‛HasSet’7is7at7role7nominal

Roles!break!code!(1)

• Increase!in!type!safety!!less!code!compiles
• Case!study:!Only!2!changes!required!in!GHC
• In!cmm/SMRep.lhs:

newtype7StgWord7=7StgWord7Word64 77deriving7(IArray7UArray,7...)

• In!Data.Array.Base:

class7IArray7a7e7where 77bounds7::7Ix7i77a7i7e77(i,7i) 77...

• Had!to!manually!write!wrapper!functions

Roles!break!code!(2)

• Increase!in!type!safety!!less!code!compiles
• Case!study:!Only!2!changes!required!in!GHC
• In!utils/UniqFM.lhs:

newtype7UniqFM7ele7=7UFM7(IntMap7ele) 77deriving7(Traversable,7...)

• In!Data.Traversable:

class7(...)77Traversable7t7where 77traverse7::7...77f7(t7b) 77...

• Just!add!SXDeriveTraversable

Roles!in!GHC

• Most!functions!that!produce!a!Coercion!now!

take!a!Role:

mkTyConAppCo7::7Role7S>7TyCon 7777777777777S>7[Coercion]7S>7Coercion dsTcCoercion7::7Role7S>7TcCoercion 7777777777777S>7(Coercion7S>7CoreExpr) 7777777777777S>7DsM7CoreExpr

• Role!conversion!is!available:

maybeSubCo2_maybe7::7Role77SS7desired 777777777777777777S>7Role77SS7current 777777777777777777S>7Coercion7S>7Maybe7Coercion

• How!to!know!which!role!to!use?!See!

ghc/docs/core*spec/core*spec.pdf

Roles!in!libraries

• Step!1:!Make!sure!your!code!compiles
• diagrams!didn’t!due!to!potential!bug
• ...!but!lens!did,!as!did!every!other!library!tested
• Step!2:!Think!about!adding!role!annotations
• Set,!Map
• Other!abstract!types!with!class+based!

invariants

• Use!CPP!to!keep!compatibility!with!older!

versions!of!GHC

Further!reading

• “Generative!Type!Abstraction!and!Type+Level!

Computation”!by!Weirich,!Vytiniotis,!Peyton! Jones,!and!Zdancewic!(POPL!’11)

• Roles!wiki!page:

http://ghc.haskell.org/trac/ghc/wiki/Roles

• Roles!implementation!wiki!page:

http://ghc.haskell.org/trac/ghc/wiki/ RolesImplementation

• Blog!post:

http://typesandkinds.wordpress.com/ 2013/08/15/roles+a+new+feature+of+ghc/