Doaitse Swierstra Atze Dijkstra Doaitse Swierstra Atze Dijkstra Type Systems.

Doaitse SwierstraAtze Dijkstra

Doaitse SwierstraAtze Dijkstra

Type SystemsType Systems

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lectures from the book “Types and Programming Languages”, Benjamin Piercestudy the type inferencer/checker of the Utrecht Haskell Compiler (UHC)projectsmall exercises

Components

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Grammars and ParsingImplementation of Programming Languagesfluency in Haskell programming

Prerequisites

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should be relatively easy to follow, but it will take time (500 pages)well written, with extensive set of exercisesboth theory and practiceuses ML as a demonstration and implementation languagewritten at an easy pace

Book

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“extension” of IPT compilerdata typesmultiparameter type classesforall and existential quantifierspolymorphic kinds

UHC

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1.Modules

2.Extendible Records

3.Functional Dependencies

4.Typed AG

5.XMLambda

6.Unifying Kind and Type Inference

7.Kind Inferencing in Helium

8.Haskell Type System

9.Uniqueness Types

10.Implicit Arguments

Projects

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import and export typeskeep safetyclass issuesparallel importstyped annotationsML functors

1 Modules

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allow cartesian products with labelled fieldssubtype relationuse class sytem to express constraintsmany different proposals exits

2 Extendible Records

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allow parameters in instance definitions to have a functional relationshipvery useful in multi-parameter type classesimplemented in Hugs and GHC

3 Functional Dep’cies

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add elements of Haskell to AG systemdelicate choice of features has to be madeuseful work, as you will all recognise

Typed AG

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yet another approach to extendible recordsmoves further away from Haskellhas nice applications

XMLambda

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type inference and kind inference are very similarmake the code look just as similarcan this be extended to even more levels?

Unifying Type and Kind Inference

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Helium: light weight Haskell for first year studentsexcellent error messagestranport kind inference to heliumcontraint based type inferencer

Kind Inference in Helium

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study the full haskell type systemdocument missing features from UHCadd some of the still missing elements

Haskell Type System

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the type system keep tracks of the number of references to a value (1 or >1)replacement for IO monadinvented and implemented in Cleanmakes efficient implementation possibleavoids dynamic garbage collection

Uniqueness Types

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generalise the class parametersextend haskell so you can make classes into first class citizens

Implicit Arguments

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if you want a specific partner then choose a partnerdecide on your preferences ranking 1-5 for your top 5 preferred projectsmail to [email protected] before tomorrow 17.00

What to do now?

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motivationmathematical preliminariesuntyped arithmetic expressionsthe untyped lambda calculusnameless representation of terms

Chapter 1-10

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ML implementationTyped arithmetic expressionsSimply typed lanbda calculusML implementation

1-10 continued

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nothing in these chapters should come as a surpsise

and you should just browse/read

ML is used:

somewhat baroque notation!!

strict evaluation!!

the theory treated at some points relies on this!, so keep this in mind

Observations

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originally: data organistation, layout of data in memory: PIC (99)overloading, nice notation: 3+5.0programs should not go wrong

elementary operations are applied to bit patterns that represent the kind of data they expect

programs do terminate

Why types?

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assist in organisation of program code

dependent types => total correctness

a type system constrains the number of legal programs is some useful way

Why Types?

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dynamic typing (LISP, Java coercions)static typing (as you know it)Higher Order Logic, Automath

integration with theorem proversthe type system can express every property of a program

Kind of Typing Systems

operational semanticsbig step semanticssmall step semantics

denotational semanticsaxiomatic semantics

Proving program properties

Our preference here

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we can express every possible computation in the untyped lambda calculus

Lambda calculus

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Booleans t ::= true | false | if t then t else t

v ::= true | false

if true then t1 else t2 => t1if false then t1 else t2 => t2

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Booleans (strict evaluation)

t1 -> t1’ if t1 then t2 else t3 =>

if t1’ then t2 else t3

note that the condition is evaluated before the expressions themselves are

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evaluation is deterministic (i.e. always exactly one rule applies)evaluation always terminatesnormal forms are unique

Observe

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Arithmetic Expressions

t ::= 0 | succ t | pred t | iszero t

nv :: = 0 | succ nv

t1 => t2succ t1 => succ t2

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Booleans tru = \t.\f.tfls = \t.\f.fif = \c.\t.\e. c t e

if tru 2 3 =>(\c.\t.\e. c t e) tru 2 3 =>( \t.\e. tru t e) 2 3 =>( \e. tru 2 e) 3 => tru 2 3 => (\t.\f.t) 2 3 => ( \f.2) 3 => ( 2)

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Integersc0 = \s.\z. zc1 = \s.\z. s zc2 = \s.\z.s (s z)c3 = \s.\z.s (s (s z))

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Recursion!!omega = (\x. x x)(\x. x x) ??

fix = \f.(\x. f(x x)) (\x. f(x x))

fix g => (\x. g(x x))(\x. g(x x)) =>g((\x. g(x x))(\x. g(x x))) =>g (fix g)

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Recursion (cont.)

note that fix generates copies of g if necessaryand passes the generater on to this gso that g can generate new copies is necessary

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try to find a type for fixif you cannot find one, try to explain why you cannot

Exercise

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Untyped lambda calculus

t ::= x | \x.t | t t

v :: = \x.t

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De Bruijn Indices

represent lambda terms without using identifiersan identifier is replaced by a number indicating how far the argument is removed from the top of the stack

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Example

is represented by

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Terms

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Exercise

define a Haskell data type for the untyped lambda calculus with variablesdefine a Haskell data type for representing de Bruijn termsdefine a translation from the first to the secondwrite a small interpreter for the latter

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Chapter 6: de Bruijn numbersChapter 7: an interpreter in ML

Hint