Logic programming Logic program A set of logical formulas Logic - - PowerPoint PPT Presentation

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Principles of programming languages Maarit Harsu / Matti Rintala / Henri Hansen

TUT Software Systems

Logic programming

• Logic program

– A set of logical formulas

• Logic programming

– Writing formulas, "describing rules of the game"

• Execution of a program

– Searching for a result that fulfills the rules

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Principles of programming languages Maarit Harsu / Matti Rintala / Henri Hansen

TUT Software Systems

Features of Logic programming

• Declarative semantics

– Simpler than for imperative languages – Semantics of a proposition can be deduced from the expression

• Programmer describes the result, not how it is

achieved

– e.g., sorting – c.f. Definition of a system

sort ( old_list, new_list ) ← permutation ( old_list, new_list ) ∧ sorted ( new_list ) sorted ( list ) ← ∀j: 1 ≤ j < n, list ( j ) ≤ list ( j + 1 ) ”The challenge of Imperative programming is that all details

• f computation

must be expressed.”

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Principles of programming languages Maarit Harsu / Matti Rintala / Henri Hansen

TUT Software Systems

Curry - functional logic language

• "Modern" research language
• Combines functional and logic paradigms
• Several implementations exist
• Syntax (almost) from Haskell
• Adds features for logic programming:

– Free variables – Non-deterministic functions – Logical constraints, built-in search (with several search strategies)

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Principles of programming languages Maarit Harsu / Matti Rintala / Henri Hansen

TUT Software Systems

New in Curry (vs Haskell)

Non-deterministic functions

• f x = x

f x = x+1 f 3 – Haskell: first match is chosen, 3 returned – Curry: both matches chosen, two execution branches, both 3 and 4 returned

• Builtin choice-operator ?:

0 ? 1 returns both 0 and 1

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Principles of programming languages Maarit Harsu / Matti Rintala / Henri Hansen

TUT Software Systems

New in Curry (vs Haskell)

Partial functions

• empty [] = []

empty [3]

– Haskell: Run-time error – Curry: "No solution" (continue search for other solutions)

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Principles of programming languages Maarit Harsu / Matti Rintala / Henri Hansen

TUT Software Systems

New in Curry (vs Haskell)

Constraints

• equality =:=, constraint combinator & (and &>),

anything returning success

• Unlike booleans (==), requires constraint to hold
• Can be used to limit function definitions (other

major uses too) f x y | x =:= reverse y = x++y function defined only for some lists

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Principles of programming languages Maarit Harsu / Matti Rintala / Henri Hansen

TUT Software Systems

New in Curry (vs Haskell)

Free variables

• Value not known beforehand
• Curry tries to deduce (search for) the value

based on constraints (unification)

• [1]++x =:= [1,2,3] where x free

returns binding x=[2,3]

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Principles of programming languages Maarit Harsu / Matti Rintala / Henri Hansen

TUT Software Systems

Curry examples

• take 3 x =:= [1,2,3] & reverse x =:= x

where x free

• insert x ys = x:ys

insert x (y:ys) = y : insert x ys permutate [] = [] permutate (x:xs) = insert x (permutate xs)

• permutate [1,2,3,1] =:= a++[1,2]++b

where a,b free

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Principles of programming languages Maarit Harsu / Matti Rintala / Henri Hansen

TUT Software Systems

Shortcomings of logic paradigm

• Controlling the order of resolution

– efficiency – Infinite loops possible

• Closed-world assumption

– A goal can be proven but not disproven

• Natural limits

– No need to describe computation – However, different solution strategies differ in efficiency

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Principles of programming languages Maarit Harsu / Matti Rintala / Henri Hansen

TUT Software Systems

Applications for logic programming

• Relational databases

– Input (facts) – Database relations (rules) – Queries

• AI
• Expert systems

– Fact-based deduction

• Language processing

– top-down –resolution resembles natural language

• Teaching