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Chapter 21 Memory Model Fortress programs are highly multithreaded by design; the language makes it easy to expose parallelism. However, many Fortress objects are mutable; without judicious use of synchronization constructs—reductions and atomic expressions—data races will occur and programs will behave in an unpredictable way. The memory model has two important functions: 1. Define a programming discipline for the use of data and synchronization, and describe the behavior of programs that obey this discipline. This is the purpose of Section 21.2. 2. Define the behavior of programs that do not obey the programming discipline. This constrains the optimizations that can be performed on Fortress programs. The remaining sections of this chapter specify the detailed memory model that Fortress programs must obey. 21.1 Principles The Fortress memory model has been written with several important guiding principles in mind. Violations of these principles should be taken as a flaw in the memory model specification rather than an opportunity to be exploited by the programmer or implementor. The most important principle is this: violations of the Fortress memory model must still respect the underlying data abstractions of the Fortress programming language. All data structures must be properly initialized before they can be read by another thread, and a program must not read values that were never written. When a program fails, it must fail gracefully by throwing an exception. The second goal is nearly as important, and much more difficult: present a memory model which can be understood thoroughly by programmers and implementors. It should never be difficult to judge whether a particular program behavior is permitted by the model. Where possible, it should be possible to check that a program obeys the programming discipline. The final goal of the Fortress memory model is to permit aggressive optimization of Fortress programs. A multiprocessor memory model can rule out optimizations that might be performed by a compiler for a uniprocessor. The Fortress memory model attempts to rule out as few behaviors as possible, but more importantly attempts to make it easy to judge whether a particular optimization is permitted or not. The semantics of Fortress already allows permissive operation reordering in many programs, simply by virtue of the implicitly parallel semantics of tuple evaluation and looping. 156
21.2 Programming Discipline If Fortress programmers obey the following discipline, they can expect sequentially consistent behavior from their Fortress programs: • Updates to shared locations should always be performed using an atomic expression. A location is considered to be shared if and only if that location can be accessed by more than one thread at a time. For example, statically partitioning an array among many threads need not make the array elements shared; only elements actually accessed by more than one thread are considered to be shared. • Within a thread or group of implicit threads objects should not be accessed through aliased references; this can yield unexpected results. Section 21.2.3 defines the notion of apparently disjoint references. An object should not be written through one reference when it is accessed through another apparently disjoint reference. The following stylistic guidelines reduce the possibility of pathological behavior when a program strays from the above discipline: • Where feasible, reduction should be used in favor of updating a single shared object. • Immutable fields and variables should be used wherever practical. We discuss this further in Section 21.2.1. • A getter or a setter should behave as though it is performing a simple field access, even if it internally accesses many locations. The simplest (but not necessarily most efficient) way to obtain the appropriate behavior is to make hand-coded accessors atomic . Section 21.2.2 expands on this. 21.2.1 Immutability Recall from Section 4.3 that we can distinguish mutable and immutable memory locations. Any thread that reads an immutable field will obtain the initial value written when the object was constructed. In this regard it is worth re-emphasizing the distinction between an object reference and the object it refers to. A location that does not contain a value object contains an object reference. If the field is immutable, that means the reference is not subject to change; however, the object referred to may still be modified in accordance with the memory model. By contrast, recall that all the fields of a value object are immutable; however, a mutable location may have a value type. Such a location can be written, completely replacing the value object it contains. Similarly, reading the value contained in such a location conceptually causes the entire value object to be read; if this isn’t followed by an immediate field reference, the read must be performed atomically. This may potentially be expensive (see Section 21.3). 21.2.2 Providing the Appearance of a Single Object In practice, most accesses to fields occur through a mediating getter or setter method. It should not be possible for the programmer to tell whether a given getter or setter directly accesses a field or if it performs additional computations. Similarly, any subscripting operation must be indistinguishable from accessing a single field. Thus, accessor methods and subscripting methods should provide the appearance of atomicity, as described in Section 21.3. The Fortress standard libraries are written to preserve this abstraction. For example, the Array type in Fortress makes use of one or more underlying HeapSequences, and array subscripting preserves the atomicity guarantees of this underlying object. 21.2.3 Modifying Aliased Objects In common with Fortran, and unlike most other popular programming languages, Fortress gives special treatment to accesses to a location through different aliases. For the purposes of variable aliasing, it is important to define the notion 157
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The Fortress Language Specification
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4 Evaluation 36 4.1 Values . . . .
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12 Functions 85 12.1 Function Decla
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17.7 Coercions for Tuple and Arrow
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IV Fortress for Library Writers 214
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40.10The Trait Fortress.Core.Binary
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Chapter 1 Introduction The Fortress
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A note on the presented libraries i
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component HelloWorld export Executa
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foo:String → () baz: String → S
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fortress upgrade Gary with Ralph No
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halve(x : R64) : R64 = x/2 Predicta
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while x < 10 do print x x += 1 end
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It is also possible to convert a me
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factorial(n) = ∏ i←1:n i This f
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In both methods cons and append , t
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default distributions will provide
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Chapter 3 Programs A program consis
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Chapter 10), a literal (see Section
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(discussed in Section 13.13) to a s
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Chapter 5 Lexical Structure A Fortr
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U+0021 EXCLAMATION MARK ! U+0023 NU
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If a character (or any other syntac
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BIG SI unit absorbs abstract also a
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escape sequences are useful for ASC
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5.14.1 Multicharacter Enclosing Ope
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Note: the elegant way to write Avog
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• top-level variable declarations
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declares id to be a mutable variabl
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several new variables. This syntax
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Chapter 7 Names Names are used to r
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7.3 Qualified Names Fortress provid
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Fortress also allows coercion betwe
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• element types of a tuple type c
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TypeAlias ::= type Id [StaticParams
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‘}’. If such a clause contains
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Note that a trait declaration inher
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the parametric trait WrappedDiction
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Chapter 10 Objects An object is a v
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FldDef ::= FldMod ∗ Id [IsType] (
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Chapter 11 Static Parameters Trait,
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11.5 Operator and Identifier Parame
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Chapter 12 Functions Functions are
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swap(x :Object, y : Object) = (y, x
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A function declaration may be separ
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Chapter 13 Expressions Fortress is
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Negating the literal 0 produces a s
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the function is evaluated with the
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13.10 Assignments Syntax: Expr ::=
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treeSum(t : TreeLeaf) = 0 treeSum(t
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Several common mathematical operato
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The body of each iteration is run i
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Page 119 and 120: trait CheckedException extends { Ex
Page 121 and 122: Chapter 15 Overloading and Multiple
Page 123 and 124: e called with. In other words, we e
Page 125 and 126: Chapter 16 Operators Operators are
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Page 129 and 130: left context whitespace operator fi
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Page 137 and 138: Chapter 17 Conversions and Coercion
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Page 143 and 144: trait B extends A end trait C exten
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Page 149 and 150: Chapter 19 Tests and Properties For
Page 151 and 152: 19.5 Test Suites In order to allow
Page 153 and 154: Chapter 20 Type Inference Type infe
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Page 179 and 180: Part III Fortress APIs and Document
Page 181 and 182: 23.1.3 hash(maxval: N64): N64 23.1.
Page 183 and 184: opr =(self,other: Boolean):Boolean
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Page 189 and 190: 24.2.11 opr ∧(self,other: Boolean
Page 191 and 192: 24.2.19 possibly(self): Boolean 24.
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Chapter 29 Dimensions and Units 29.
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unit secondOfAngle secondsOfAngle:
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Chapter 30 Tests 30.1 The Object Fo
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31.2 Convenience Types An optional
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Chapter 32 Parallelism and Locality
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y evaluating the two elements of th
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There is a DefaultDistribution whic
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v = spawn at a.region(i) do a i end
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expr ∑ type wrapper ∑ body R,
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32.8.3 Using Overloading to Adapt G
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Chapter 33 Overloaded Functional De
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coercions: T U ⇐⇒ T ♦ U ∧
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The More Specific Rule for Function
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An infix/multifix operator declarat
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may be so defined except ordinary p
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and here some of these computed dim
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unit name 1 name 2 name 3 : . . . u
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Chapter 36 Support for Domain-Speci
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Because syntax expanders are define
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Part V Fortress APIs and Documentat
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property ∀(a:T) ¬(a ∼ a) end A
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trait PartialOrderOperatorsT extend
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The HasMinimalElement trait specifi
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The trait Commutative requires the
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A value e is a left identity for a
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trait ApproximatelyHasInverses T ex
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end extends { ApproximateCommutativ
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A default definition is provided fo
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trait RationalQuantityunit U absorb
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(self,other:RationalQuantityU,ninf
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38.2.4 getter hashCode(): Z64 38.2.
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Chapter 39 Components and APIs We d
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Chapter 40 Memory Sequences and Bin
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updatenat k(j: IntegerStatick, v: T
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40.1.12 update(j:IndexInt, v: T): L
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40.3.2 opr nat m[r:RangeOfStaticSiz
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40.5 The Trait Fortress.Core.Binary
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40.5.5 bit(m:IndexInt): Bit The res
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40.6 The Trait Fortress.Core.Binary
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40.6.4 opr nat m[r:RangeOfStaticSiz
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40.6.16 opr ‖ nat m,bool bigEndia
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countLeadingZeroBits():IndexInt cou
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40.7.41 signedShift(j:IndexInt):T 4
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40.8.6 signedDivide(other: T,overfl
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littleEndian():BinaryEndianLinearEn
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40.9.16 opr ‖ nat m,nat radix,nat
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v: BinaryEndianWordb,bigEndianBytes
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40.10.9 opr nat m[r:RangeOfStaticSi
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40.10.24 splitnat b ′ () : Binary
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itAnd(selfStart: IndexInt,source:T,
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40.13.2 wrappingAdd(selfStart: Inde
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40.13.30 unsignedMax(selfStart: Ind
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40.13.55 gatherBits(selfStart:Index
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Appendix A Fortress Calculi A.1 Bas
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Values, evaluation contexts, redexe
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Expression typing: p; ∆; Γ ⊢ e
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α, β type variables f method name
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Function/method name lookup: Fname(
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One owner for all the visible metho
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Traits defining a method: defining
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Values, evaluation contexts, and re
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Valid function declarations: p ⊢
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Method definition lookup: visible p
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Values, intermediate expressions, e
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Field typing: p; ∆; Γ ⊢ vd ok
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Appendix B Overloaded Functional De
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Proof. We prove this for δ, but th
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Upgrade A predicate upg? takes two
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a is rendered as a foobar is render
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• If a portion is sub and another
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supercalifragilisticexpialidocious
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any alternative names, with undersc
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Appendix F Operator Precedence, Cha
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U+007C VERTICAL LINE | | U+2016 DOU
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The following two operators have th
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U+003D EQUALS SIGN = = EQ U+2243 AS
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U+22DD EQUAL TO OR GREATER-THAN U+2
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U+2289 NEITHER A SUPERSET OF NOR EQ
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F.4.8 Miscellaneous Relational Oper
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U+21A5 UPWARDS ARROW FROM BAR U+21A
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U+2224 DOES NOT DIVIDE ∤ U+2225 P
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U+27F8 LONG LEFTWARDS DOUBLE ARROW
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U+2960 UPWARDS HARPOON WITH BARB LE
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U+29ED BLACK CIRCLE WITH DOWN ARROW
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Appendix G Concrete Syntax In this
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Extends ::= extends TraitTypes Excl
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StaticParam ::= Id [Extends] [absor
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Value ::= Literal | fn ValParam [Is
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Appendix H Generated Concrete Synta
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-> fn_decl -> object_decl -> var_de
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-> id extends_opt absorbs_opt -> "n
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-> w "excludes" w type_ref_list com
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-> obj_decl_body_elem br obj_decl_b
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-> op wr op_follows_op_w -> op wr e
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-> no_space_op_expr_result no_space
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id_opt -> -> w id with_opt -> -> w
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-> unpasting_elem rect_separator un
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-> "}" comprehension -> set_compreh
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-> do_expr -> for_expr -> spawn_exp
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-> type_ref -> "(" w type_ref w ","
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enc -> enc_literal op_or_enc -> op
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Bibliography [1] O. Agesen, L. Bak,
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