SPARK 0.8 David Morley, Nov 2005

Core Data Types Includes values most often used by SPARK Primitive –Integer, e.g. 1 –Float, e.g., 1.0 –String, e.g., “one” –Symbol, e.g., one –Variable designator, e.g., $var (new in 0.8) Compound –Structure, e.g., (foo 1 2 3) - has functor symbol foo –List, e.g., [1 2 3]

SPARK-L Source Represented by core data values (cf. Lisp, Prolog) SPARKL source to generate a value usually different from the value: –(+ 1 2) ⇒ 3, (- 4 1) ⇒ 3, 3 ⇒ 3 –[(+ 1 2) 3] ⇒ [3 3] “one”) ⇒ one “foo” 1 2 3) ⇒ (foo 1 2 3) “bar” (+ 1 1) (+ 2 2)) ⇒ (bar 2 4)

Backquote and comma Used for easy construction of basic values (cf. Lisp) Backquote ` prevents evaluation –`one ⇒ one –`(foo 1 2 3) ⇒ (foo 1 2 3) Comma, performs evaluation within backquote –`(bar,(+ 1 1),(+ 2 2)) ⇒ (bar 2 4)

File Contains –Package statement –Identifier declarations –Import statements –Export statements –Require statements –Facts to be loaded Every identifier (unquoted symbol or functor) used in the file must have a visible declaration. –Declarations in the file are visible –Imported declarations are visible –Any declaration visible to the package is visible

Package A Package is –A “bucket” that accumulates declarations, imports, exports from files –A means of communicating declarations between files Files can use identifiers from their package Files import identifiers from other packages into their package Files export identifiers from their package Unlike old concept of module: –Not a fixed set of files –New files can add identifiers to a package

Specifying the Package of a File “package: P” For backward compatibility: –“module: P” = “package: P” –If no package statement, package = filename

File-Local Names Not all declarations are shared with the package Identifiers starting with underscore are only visible in the file –_foo, _LocalPredicate, _Procedure1 It is conceivable (but no syntax exists) to import an identifier foo as a different identifer _foo. That import would not be visible to the package.

Identifiers and Symbols Identifers are local names –interpreted with respect to a file To distinguish identifiers in different files we associate a symbol with every identifier: For identifiers declared in file F in package P: –Id maps to symbol P.Id –_Id maps to symbol F._Id When an identifier declared as a functor appears instead as a symbol, it evaluates to the corresponding symbol: –Given {defpredicate (Id $x)} {defpredicate (_Id $y)} –Id ⇒ P.Id, _Id ⇒ F._Id (cf. `Id ⇒ Id and `_Id ⇒ _Id) –Useful for passing as an argument to applyfun, etc. –Less need for “(qent Id)”

Importing from a package Q. When you say “importfrom: P” which files do you get declarations from? Ans. All files in the transitive closure of the “requires” relation that are in package P. –F requires F’ if F contains requires: F’ –F requires P if F contains importfrom: P … –F requires P if F is in package P –P requires F if F has the same name as P

Usage SPARKL source expressions can be used in different ways Each way designated by a usage character: + evaluate term [set: $x (+ 1 2)] - match term (= [$x $y] [1 2]) s solve predicate [context: (P $x) ] u update predicate [conclude: (P 7) ] r retract predicate [retract: (P 4) ] … SPARKL source processing assigns a usage to each expression in source

Mode declaration for a functor Specifies argument usages Used for static mode checking of action/predicate usages before file is loaded Prefix inputs with “+”, outputs with “-” –{defaction (act +$x $y -$z)} In any use of this action: $x must be evaluable (I.e., is an input) $y need only be matchable (I.e., can be an output) $z must not be evaluable (I.e, must be output)

Formal parameters Richer formal parameters for: –defprocedure cues –task closure parameters –pred closure parameters Parameter may be one of: –+X - at start, match X to the actual parameter value –-X - at end, match actual parameter to the value of X –$x - at start, constrain $x to match actual parameter Examples: –+[$x $y] - match input parameter to [$x $y] Less need for Ground

Accessing Core Values from Python See Python module spark.internal.parse.basicvalues Core data types are called: String, Float, Integer, Symbol, Variable, Structure, List –Symbol(“foo”), List((1,2,3)), isinstance(x,List) Structure args/List elements access: –myStruct[0], len(myStrict), myList[1:2] Structure functor access: –myStruct.functor Symbol name access: –mySym.name or str(mySym) Construct Structure: –Structure(functorSym, argsSequence) or functorSym.structure(arg0, arg1, …)

Random Bits “precondition: X” is now optional No distinction between task modifiers and basic tasks –[context: X do: Y] is a special case of using […] for sequential composition (technically contiguous: rather than seq: )