Alloy: Difference between revisions

As of version 1.8 ProB provides support to load Alloy models. The Alloy models are translated to B machines by a Java frontend.

This work and web page is still experimental.

The work is based on a translation of the specification language Alloy to classical B. The translation allows us to load Alloy models into ProB in order to find solutions to the model's constraints. The translation is syntax-directed and closely follows the Alloy grammar. Each Alloy construct is translated into a semantically equivalent component of the B language. In addition to basic Alloy constructs, our approach supports integers and orderings.

Installation

Alloy2B is included as of version 1.8.2 of ProB.

You can build Alloy2B yourself:

• Clone or download Alloy2B project on Github.
• Make jar file (gradle build) and
• put resulting alloy2b-*.jar file into ProB's lib folder.

Examples

N-Queens

module queens
open util/integer
sig queen { x:Int, x':Int, y:Int } {
    x >= 1
    y >= 1
    x <= #queen
    y <= #queen
    x' >=1
    x' <= #queen
    x' = minus[plus[#queen,1],x]
}
fact { all q:queen, q':(queen-q) {
    ! q.x = q'.x
    ! q.y = q'.y
    ! plus[q.x,q.y] = plus[q'.x,q'.y]
    ! plus[q.x',q.y] = plus[q'.x',q'.y]
}}
pred show {}
run show for exactly 4 queen, 5 int

This can be loaded in ProB, as shown in the following screenshot. To run the "show" command you have to use "Find Sequence..." command for "run_show" in the "Constraint-Based Checking" submenu of the "Verify" menu.

Internally the Alloy model is translated to the following B model:

MACHINE alloytranslation
SETS /* deferred */
  queen
CONCRETE_CONSTANTS
  x,
  x_,
  y
/* PROMOTED OPERATIONS
  run0 */
PROPERTIES
    x : queen --> INTEGER
  & x_ : queen --> INTEGER
  & y : queen --> INTEGER
  & !this.(this : queen => x(this) >= 1 & y(this) >= 1 & x(this) <= card(queen) & y(this) <= 
  card(queen) & x_(this) >= 1 & x_(this) <= card(queen) & x_(this) = (card(queen) + 1) - x(this)
  )
  & card(queen) = 4
  & !(q,q_).(q_ : queen - {q} => not(x(q) = x(q_)) & not(y(q) = y(q_)) & not(x(q) + y(q) = x(
  q_) + y(q_)) & not(x_(q) + y(q) = x_(q_) + y(q_)))
INITIALISATION
    skip
OPERATIONS
  run0 = 
    PRE 
        card(queen) = 4
      & !(q,q_).(q_ : queen - {q} => not(x(q) = x(q_)) & not(y(q) = y(q_)) & not(x(q) + y(q)
   = x(q_) + y(q_)) & not(x_(q) + y(q) = x_(q_) + y(q_)))
    THEN
      skip
    END
/* DEFINITIONS
  PREDICATE show; */
END

River Crossing Puzzle

module river_crossing
open util/ordering[State]
abstract sig Object { eats: set Object }
one sig Farmer, Fox, Chicken, Grain extends Object {}
fact { eats = Fox->Chicken + Chicken->Grain}
sig State { near, far: set Object }
fact { first.near = Object && no first.far }
pred crossRiver [from, from', to, to': set Object] {
  one x: from | {
    from' = from - x - Farmer - from'.eats
    to' = to + x + Farmer
  }
}
fact {
  all s: State, s': s.next {
    Farmer in s.near =>
      crossRiver [s.near, s'.near, s.far, s'.far]
    else
      crossRiver [s.far, s'.far, s.near, s'.near]
  }
}
run { last.far=Object } for exactly 8 State

This can be loaded in ProB, as shown in the following screenshot. To run the "show" command you have to use "Find Sequence..." command for "run_show" in the "Constraint-Based Checking" submenu of the "Verify" menu (after enabling Kodkod in the Preferences menu).

Internally the Alloy model is translated to the following B model:

/*@ generated */
MACHINE river_crossing
SETS
    Object_
CONSTANTS
    Farmer_, Fox_, Chicken_, Grain_, eats_Object, near_State, far_State
DEFINITIONS
    crossRiver_(from_,from__,to_,to__) == from_ <: Object_ 
    									  & from__ <: Object_ & to_ <: Object_ 
    									  & to__ <: Object_  &  (card({x_ | {x_} <: from_
    									  & (((from__ = (((from_ - {x_}) - {Farmer_}) - eats_Object[from__])))
    									  & ((to__ = ((to_ \/ {x_}) \/ {Farmer_}))))}) = 1) ;
    next_State_(s) == {x|x=s+1 & x:State_} ;
    nexts_State_(s) == {x|x>s & x:State_} ;
    prev_State_(s) == {x|x=s-1 & x:State_} ;
    prevs_State_(s) == {x|x<s & x:State_} ;
    State_ == 0..7
PROPERTIES
    {Farmer_} <: Object_ &
    {Fox_} <: Object_ &
    {Chicken_} <: Object_ &
    {Grain_} <: Object_ &
    ((eats_Object = (({Fox_} * {Chicken_}) \/ ({Chicken_} * {Grain_})))) &
    (((near_State[{min(State_)}] = Object_) & far_State[{min(State_)}] = {})) &
    (!(s_, s__).({s_} <: State_ & {s__} <: next_State_(s_) => 
    			((({Farmer_} <: near_State[{s_}]) => 
    					crossRiver_(near_State[{s_}], near_State[{s__}],
    					 far_State[{s_}], far_State[{s__}])) 
    					& (not(({Farmer_} <: near_State[{s_}])) =>
    							 crossRiver_(far_State[{s_}], far_State[{s__}],
    							  near_State[{s_}], near_State[{s__}]))))) &
    Farmer_ /= Fox_ &
    Farmer_ /= Chicken_ &
    Farmer_ /= Grain_ &
    Fox_ /= Chicken_ &
    Fox_ /= Grain_ &
    Chicken_ /= Grain_ &
    {Farmer_} \/ {Fox_} \/ {Chicken_} \/ {Grain_} = Object_ &
    eats_Object : Object_ <-> Object_ &
    near_State : State_ <-> Object_ &
    far_State : State_ <-> Object_
OPERATIONS
    run_2 = PRE (far_State[{max(State_)}] = Object_) THEN skip END
END

Proof with Atelier-B Example

sig Object {}
sig Vars {
    src,dst : Object
}
pred move (v, v': Vars, n: Object) {
    v.src+v.dst = Object
    n in v.src
    v'.src = v.src - n
    v'.dst = v.dst + n
	}
assert add_preserves_inv {
    all v, v': Vars, n: Object |
         move [v,v',n] implies  v'.src+v'.dst = Object
}
check add_preserves_inv for 3 

Note that our translation does not (yet) generate an idiomatic B encoding, with move as B operation

and src+dst=Object as invariant: it generates a check operation encoding the predicate
 add_preserves_inv
 with universal quantification.

Below we shoe the B machine we have input into AtelierB. It was obtained by pretty-printing from \prob, and putting the negated guard

of theadd_preserves_inv into an assertion (so that AtelierB generates the desired proof obligation).
 

MACHINE alloytranslation
SETS /* deferred */
  Object_; Vars_
CONCRETE_CONSTANTS
  src_Vars, dst_Vars
PROPERTIES
    src_Vars : Vars_ --> Object_
  & dst_Vars : Vars_ --> Object_
ASSERTIONS
  !(v_,v__,n_).(v_ : Vars_ & v__ : Vars_ & n_ : Object_
   => 
   (src_Vars[{v_}] \/ dst_Vars[{v_}] = Object_ & 
    v_ |-> n_ : src_Vars &
    src_Vars[{v__}] = src_Vars[{v_}] - {n_} &
    dst_Vars[{v__}] = dst_Vars[{v_}] \/ {n_} 
    =>
    src_Vars[{v__}] \/ dst_Vars[{v__}] = Object_)
   )
END

The following shows AtelierB proving the above assertion:

Alloy Syntax

Logical predicates:
-------------------
 P and Q       conjunction
 P or Q        disjunction
 P implies Q   implication
 P iff Q       equivalence
 not P         negation

Alternative syntax:
 P && Q        conjunction
 P || Q        disjunction
 P => Q        implication
 P <=> Q       equivalence
 ! P           negation

Quantifiers:
-------------
 all DECL | P   universal quantification
 some DECL | P  existential quantification
 one DECL | P   existential quantification with exactly one solution
 lone DECL | P  quantification with one or zero solutions
 
where the DECL follow the following form: 
 x : S          choose a singleton subset of S (like x : one S)
 x : one S      choose a singleton subset of S
 x : S          choose x to be any subset of S
 x : some S     choose x to be any non-empty subset of S
 x : lone S     choose x to be empty or a singleton subset of S
 x : Rel        where Rel is a cartesian product / relation: see multiplicity declarations x in Rel
 x,y... : S, v,w,... : T  means x:S and y : S and ... v:T and w:T and ...
 disjoint x,y,... : S     means x : S and y : S and ... and x,y,... are all pairwise distinct

Set Expressions:
----------------
 univ           all objects
 none           empty set
 S + T          set union
 S & T          set intersection
 S - T          set difference
 # S			cardinality of set

Set Predicates:
---------------
 no S           set S is empty
 S in T         R is subset of S
 S = T          set equality
 S != T         set inequality
 some S         set S is not empty
 one S          S is singleton set
 lone S         S is empty or a singleton
 {x:S | P}      set comprehension
 {DECL | P}     set comprehension, DECL has same format as for quantifiers
 let s : S | P 	identifier definition

Relation Expressions:
----------------------
 R -> S         Cartesian product
 R . S          relational join
 S <: R         domain restriction of relation R for unary set S
 R :> S         range restriction of relation R for unary set S
 R ++ Q         override of relation R by relation Q
 ~R             relational inverse
 ^R             transitive closure of binary relation
 *R             reflexive and transitive closure
 
Multiplicity Declarations:
---------------------------
The following multiplicity annotations are supported for binary (sub)-relations:

f in S -> T            f is any relation from S to T (subset of cartesian product)
f in S -> lone T       f is a partial function from S to T
f in S -> one T        f is a total function from S to T
f in S -> some T       f is a total relation from S to T
f in S one -> one T    f is a total bijection from S to T
f in S lone -> lone T  f is a partial injection from S to T
f in S lone -> one T   f is a total injection from S to T
f in S some -> lone T  f is a partial surjection from S to T
f in S some -> one T   f is a total surjection from S to T
f in S some -> T       f is a surjective relation from S to T
f in S some -> some T  f is a total surjective relation from S to T

Ordered Signatures:
-------------------
A signature S can be defined to be ordered:
 open util/ordering [S] as s

 s/first	first element
 s/last		last element
 s/next[s2]	element after s2
 s/nexts[s2]	all elements after s2
 s/prev[s2]	element before s2
 s/prevs[s2]	all elements before s2

Sequences:
----------

s : seq S 	ordered and indexed sequence
s.first		head element
s.rest 		tail of the sequence
s.idxOf [x] 	returns the first index of the occurence of x in s, returns the empty set if x does not occur in s
s.insert[i,x]	returns a new sequence where x is inserted at index position i

Arithmetic Expressions and Predicates:
--------------------------------------
You need to open util/integer:

 plus[X,Y]       addition
 minus[X,Y]      subtraction
 mul[X,Y]        multiplication
 div[X,Y]        division
 rem[X,Y]        remainder
 sum[S]          sum of integers of set S

 X < Y           less
 X = Y           integer equality
 X != Y          integer inequality
 X > Y           greater
 X =< Y          less or equal
 X >= Y          greater or equal

Structuring:
------------
fact NAME { PRED }
fact NAME (x1,...,xk : Set) { PRED }

pred NAME { PRED }
pred NAME (x1,...,xk : Set) { PRED }

assert NAME { PRED }

fun NAME : Type { EXPR }

Commands:
---------

run NAME
check NAME

run NAME for x 				global scope of less or equal x
run NAME for exactly x1 but x2 S 	global scope of x1 but less or equal x2 S
run NAME for x1 S1,...,xk Sk 		individual scopes for signatures S1,..,Sk
run NAME for x Int 			specify the integer bitwidth (integer overflows might occur)