CSE 6341 (755) Programming Languages

1
CSE 6341 (755)Programming Languages

• Neelam Soundarajan
• Computer Sc. Eng.
• Dreese Labs 579
• e-mail neelam_at_cse

2
Outline

• Main Topic
• Ways to formally define syntax and semantics of
  PLs
• Plus a bit about programming methodologies
• Tentative Schedule
• Attribute grammars 3 weeks
• Operational Semantics (including Lisp its
  interpreter) 3.5 weeks
• Axiomatic Semantics 3.5 weeks
• Denotational Semantics 1 week
• Other topics ??
• Exams etc. 0.5 week

3
References

• Unfortunately, no good books for the course.
• Mainly depend on slides, class discussions, your
  notes.
• DO NOT miss classes.
• Some useful references
• Formal specification of programming languages, F.
  Pagan
• Formal syntax and semantics of programming
  languages,Kurtz and Slonnegar
• Lisp 1.5 programmers manual, McCarthy and others
• Copies of all on reserve in the Sc./Eng. Library

4
Attribute Grammars

• (Ref. Pagan (Ch. 2.3) Kurtz (Ch. 3))
• Fact Context-free conditions specified using
  BNF
• Question How do we specify context-sensitive
  (CS) conds?
• Answer Using Attribute Grammars (AGs)
• An AG is
• a BNF grammar
• attributes
• rules for evaluating attributes
• conditions (to capture c.s. requirements)

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Example

• L an bn cn n gt 1
• ltasgt a altasgt1
• Na(ltasgt)? 1 Na(ltasgt)? Na(ltasgt1)1
• ltbsgt b bltbsgt1
• Nb(ltbsgt)? 1 Nb(ltbsgt)? Nb(ltbsgt1)1
• ltcsgt c cltcsgt1
• Nc(ltcsgt)? 1 Nc(ltcsgt)? Nc(ltcsgt1)1
• ltlsgt ltasgtltbsgtltcsgt
• Cond Na(ltas) Nb(ltbsgt) Nc(ltcsgt)
• Na Synthesized attribute of ltasgt
• Nb Synthesized attribute of ltbsgt
• Nc Synthesized attribute of ltcsgt
• No inherited attributes in this grammar.

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Some Comments

1. Consider how the grammar works with a parse
   tree,allowing, say, "aabbcc", and disallowing
   "aabcc"
2. Attributes are NOT program variables can't
   have Na(ltasgt) ? Na(ltasgt) 1
3. In rules/conditions, can only refer to attributes
   of non-terminal on the left and the non-terminals
   in the current alternative. Can't look at "grand
   children" etc.
4. Could have used N (instead of Na, Nb, Nc) as the
   name of all three attributes.

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Example (revisited)

• L an bn cn n gt 1
• ltlsgt ltasgtltbsgtltcsgt
• ExpNb(ltbsgt) ? Na(ltasgt) ExpNc(ltcsgt) ?
  Na(ltasgt)
• ltasgt a altasgt1
• Na(ltasgt)? 1 Na(ltasgt)? Na(ltasgt1)1
• ltbsgt b bltbsgt1
• Cond ExpNb(ltbsgt) 1 ExpNb(ltbsgt1) ?
  ExpNb(ltbsgt) -1
• ltcsgt c cltcsgt1
• Cond ExpNc(ltcsgt) 1 ExpNc(ltcsgt1) ?
  ExpNc(ltcsgt) -1
• Na Synthesized attribute of ltasgt
• ExpNb Inherited attribute of ltbsgt
• ExpNc Inherited attribute of ltcsgt
• Consider all strings over a,b,c (i.e., a's,
  b's, c's may be in any order) and require no. of
  a's no. of b's no. of c's (using synh.
  synthinh. attr)

8
Some Comments

1. An AG is a BNF grammar plus a set of attributes,
   some synth., others inh. Each attribute is
   associated with a specific non-terminal.
2. If S is a synth. attrib. of ltNgt, then for each
   alternative in ltNgt's production, must have an
   eval. rule that will be used to compute S(ltNgt)
   whenever that alternative is used.
3. If I is an inh. attrib. of ltNgt, then for each
   occurrence of ltNgt on the right side of any
   production, must have eval. rule that will be
   used to computer I(ltNgt) if that alternative is
   used.
4. Conditions are associated with individual
   alternatives of individual productions.
5. If any condition in a tree evaluates to false,
   the tree collapses

9
Context-free condns. using AGs

• CF conditions can also be expressed using AGs.
• L an bn n gt 1
• ltlsgt ltasgtltbsgt
• Cond Na(ltasgt) Nb(ltbsgt)
• AGs can be used to capture precedence, i.e.,
  specify how a string is to be parsed Consider
  all strings over a, b.Given "abab", want to
  ensure it is parsed as a(b(a(b))), notas
  (ab)(ab) etc.
• ltstrgt a b
• N(ltstrgt)? 1 N(ltstrgt)? 1
• ltstrgt1ltstrgt2 N(ltstrgt) ? N(ltstrgt1)
  N(ltstrgt2) Cond (N(ltstrgt1) 1)

10
Context-sens. condns. in PLs

• Main condition Id's that are used must have been
  declared
• ltproggt ltblockgt
• ltblockgt begin ltdecl seqgt ltstmt seqgt end
• How to ensure that in ltstmt seqgt we only use
  objects declared in ltdecl seqgt?
• Using synthesized attributes
• ltblockgt begin ltdecl seqgt ltstmt seqgt end
• Cond UsedIds(ltstmt seqgt) ? DeclIds(ltdecl
  seqgt)
• Using inherited attributes
• ltblockgt begin ltdecl seqgt ltstmt seqgt end
• AllowedIds(ltstmt seqgt) ? DeclIds(ltdecl
  seqgt)

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CS conditions in PLs (contd.)

• Problem Nested blocks (what are they?)
• Solution Use a sequence of sets, each containing
  Ids declared in a (surrounding) block
• ltblockgt begin ltdecl seqgt ltstmt seqgt end
• Nest(ltstmt seqgt) ? append(Nest(ltblockgt),
  Decs(ltdecl seqgt))
• ltstmt seqgt ltstmtgt Nest(ltstmtgt
  ) ? Nest(ltstmt seqgt) ltstmtgtltstmt
  seqgt1 Nest(ltstmtgt), Nest(ltstmt seqgt1) ?
  Nest(ltstmt seqgt)
• ltprogramgt ltblockgt Nest(ltblock
  gt) ? ltgt

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CS conditions in PLs (contd.)

• Problem Different types of Ids
• Solution An element of Decs() is of the
  form ("xy", int), or ("ab", bool), or ("PQ",
  proc)
• (For procedures, also need info about no./types
  of pars)
• Where do we check (that the CS conds. are
  satisfied)?
• ltassigngt ltidgt ltint expgt
• Cond lastType(Name(ltidgt, Nest(ltassigngt)
  int
• ltidgt ltbool expgt Cond
  lastType(Name(ltidgt, Nest(ltassigngt) bool

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CS conditions in PLs (contd.)

• ltproc callgt call ltidgt()
• Cond lastType(Name(ltidgt), Nest(ltproc
  callgt)) proc ? parTypes(Name(ltidgt),
  Nest(ltproc callgt)) ltgt
• call ltidgt(ltarg list)
• Cond lastType(Name(ltidgt), Nest(ltproc
  callgt)) proc ? parTypes(Name(ltidgt),
  Nest(ltproc callgt)) ...
• Question What about double declarations?
• ltdsgt ltdeclgt
• Decs(ltdsgt) ? Decs(ltdeclgt)
  Nest(ltdeclgt)?Nest(ltdsgt)
• ltdeclgt ltdsgt1
• Decs(ltdsgt) ? Decs(ltdeclgt) ? Decs(ltdsgt1)
• Nest(ltdeclgt), Nest (ltdsgt1) ? Nest(ltdsgt)
• Cond Decs(ltdeclgt) ? Decs(ltdsgt1) ? //
  Not quite?

14

• Note Maybe better to look at e.g. in pp. 15/16
  before grammar rules below
• Questions How do elements get into
  Decs()? How do procs access surr. block/call
  other procs? (Ans Need to pass Nest also
  to the ltdeclgts.)
• ltblockgt begin ltdecl seqgt ltstmt seqgt end
• Nest(ltstmt seqgt) ? append(Nest(ltblockgt),
  Decs(ltdecl seqgt)) Nest(ltdecl seqgt) ?
  append(Nest(ltblockgt), Decs(ltdecl seqgt))
• ltdeclgt int ltidgt
• Decs(ltdeclgt) ? (Name(ltidgt), int)
• bool ltidgt
• Decs(ltdeclgt) ? (Name(ltidgt), bool)
• proc ltidgt() ltblockgt
• Decs(ltdeclgt) ? (Name(ltidgt), proc, ltgt)
• Nest(ltblockgt) ? Nest(ltdeclgt) // need to
  change?
• proc ltidgt(ltpar listgt) ltblockgt
• Decs(ltdeclgt) ? (Name(ltidgt), proc,
  Partypes(ltpar listgt))
• Nest(ltblockgt) ? append(Nest(ltdeclgt),
  Decs(ltpar listgt))

15
ltproggt
ltblockgt
ltdsgt
ltssgt
ltdgt
ltdsgt
int X, Y
ltdsgt
ltdgt
ltblockgt
proc Q()
ltdsgt
ltstmtgt
ltblockgt
ltdsgt
ltssgt
16
ltproggt
ltblockgt
N1
D1
ltdsgt
ltssgt
D2
N2
N3
D3
ltdgt
ltdsgt
N5
N4
D4
D5
int X, Y
ltdsgt
D6
N6
N7
D7
ltdgt
D17
D18
ltblockgt
proc Q()
N8
D8
N18
D18
ltdsgt
N9
N10
D9
D10
ltstmtgt
N10
D10
ltblockgt
N12
D12
ltdsgt
N11
D11
ltssgt
D14
N14
N13
D13
N15
D15
N16
D16
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Translational Semantics

• AGs also used to specify translations (code
  generation) (e.g. YACC) (Ref Pagan (ch. 3.2)
  Kurtz (ch. 7))
• Basic idea
• ltstmtgt ltstmtgt1 ltstmtgt2 Code(ltstmtgt)?
  append(Code(ltstmtgt1), Code(ltstmtgt2))
• but the details are more complex ...
• E.g. How to ensure the same label is not used
  inCode(ltstmtgt1) and Code(ltstmtgt2) ?
• A simple imperative language (we will call it
  IMP taken from Pagan)
• ltproggt ltstmtgt
• ltstmtgt skip ltassigngt
  ltstmtgt1ltstmtgt2 if ltbegt thenltstmtgt1 else
  ltstmtgt2 while ltbegt do ltstmtgt1
• ltassigngt ltidgt ltaegt
• ltaegt ltidgt ltintgt ltaegt1ltaegt2 ltaegt1 ?
  ltaegt2 ltaegt1 ltaegt2
• ltbegt true false ltaegt1ltaegt2 ltaegt1 lt
  ltaegt2 ?ltbegt ltbegt1 ? ltbegt2 ltbegt1 ?
  ltbegt2

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Translational Semantics (contd.)

• Key attributes
• Code a synth. attribute of ltstmtgt, ltaegt, ltbegt
  The seq. of assembly instructions corresponding
  to a particular ltstmtgt, ltaegt, ltbegt
• Labin (inh.), Labout (synth.) keep track of next
  available label
• Temp (inh.) keeps track of next available memory
  loc. for temporary use
• ltproggt ltstmtgt
• Code(ltproggt) ? Code(ltstmtgt)
• Labin(ltstmtgt) ? 1
• ltstmtgt ltstmtgt1ltstmtgt2 Code(ltstmtgt) ?
  append(Code(ltstmtgt1), Code(ltstmtgt2))
• Labin(ltstmtgt1) ? Labin(ltstmtgt)
• Labin(ltstmtgt2) ? Labout(ltstmtgt1)
• Labout(ltstmtgt) ? Labout(ltstmtgt2)

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Translational Semantics (contd.)

• ltstmtgt ltassigngt
• Code(ltstmtgt) ? Code(ltassigngt)
• Labout(ltstmtgt) ? Labin(ltstmtgt) // why?
• if ltbegt then ltstmtgt1 else
  ltstmtgt2 Labin(ltstmtgt1) ? Labin(ltstmtgt) 2
  // why?
• Labin(ltstmtgt2) ? Labout(ltstmtgt1)
• Labout(ltstmtgt) ? Labout(ltstmtgt2)
• Code(ltstmtgt) ? append(
• Code(ltbegt), ("BZ", Labin(ltstmtgt)), //
  slight problem
• Code(ltstmtgt1), ("BR", label(Labin(ltstmtgt)
  1)),
• (label(Labin(ltstmtgt)) "No-Op"),
• Code(ltstmtgt2), (label(Labin(ltstmtgt)1)
  "No-Op") )
• while ltbegt do ltstmtgt1 Labin(ltstmtgt1
  ) ? ...
• Labout(ltstmtgt) ? ...
• Code(ltstmtgt) ? ...
• BZ "branch on zero" BR "unconditional
  branch" "No-Op" "continue".

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Translational Semantics (contd.)

• ltassigngt ltidgt ltaegt
• Code(ltassigngt) ? append(Code(ltaegt), ("STO",
  Name(ltidgt))
• Temp(ltaegt) ? 1
• ltaegt ltintgt
• Code(ltaegt) ? lt("LOAD" Value(ltintgt))gt
• ltidgt
• Code(ltaegt) ? lt("LOAD" Name(ltidgt))gt
• ltaegt1ltaegt2
• Code(ltaegt) ? append(
• Code(ltaegt1), ("STO" temp(Temp(ltaegt)),
• Code(ltaegt2), ("ADD" temp(Temp(ltaegt))) )
  Temp(ltaegt1) ?Temp(ltaegt)
• Temp(ltaegt2) ?Temp(ltaegt) 1 // why?

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Static Scope (Algol, Pascal, C, C,...)

• Entities accessible in a procedure Entities
  declared in that procedure Entities declared in
  the surrounding procedure (less those with
  name conflicts) Entities declared in procedure
  surrounding the surrounding procedure ...
• Visualize Each procedure is a box whose sides
  are one-way mirrors you can look out of the box,
  but you cant look into a box
• Some languages are not quite static scope but are
  close.

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Example

• program
• A, B, C integer
• Q procedure // no parameters begin B
  B2 C C2 print A, B, C end (Q)
• R procedure A integer begin A 3 C
  2 call Q B AC print A, B, C end (R)
• S procedure A, C integer
• Q procedure // nested in S C
  integer begin A A1 C B1
  print A, B, C end (S.Q)
• begin // body of S B 3 C 1
  A 4 print A, B, C call R print A, B, C
  end (S)
• begin // main body A 1 B 1 C
  1 call R print A, B, C call S print A,
  B, C end (main)