Compiler Construction | Lecture 2 | Declarative Syntax Definition

Lecture 2: Declarative Syntax Deﬁnition
CS4200 Compiler Construction
Eelco Visser
TU Delft
September 2018

This Lecture
!2
Java Type Check
JVM
bytecode
Parse CodeGenOptimize
Speciﬁcation of syntax deﬁnition from which we can derive parsers

The perspective of this lecture on declarative
syntax definition is explained more elaborately in
this Onward! 2010 essay. It uses an on older version
of SDF than used in these slides. Production rules
have the form
X1 … Xn -> N {cons(“C”)}
instead of
N.C = X1 … Xn
http://swerl.tudelft.nl/twiki/pub/Main/TechnicalReports/TUD-SERG-2010-019.pdf
https://doi.org/10.1145/1932682.1869535

The SPoofax Testing (SPT) language used in
the section on testing syntax definitions was
introduced in this OOPSLA 2011 paper.
https://doi.org/10.1145/2076021.2048080
http://swerl.tudelft.nl/twiki/pub/Main/TechnicalReports/TUD-SERG-2011-011.pdf

The SDF3 syntax definition
formalism is documented at the
metaborg.org website.
http://www.metaborg.org/en/latest/source/langdev/meta/lang/sdf3/index.html

What is Syntax?
!8
https://en.wikipedia.org/wiki/Syntax
In mathematics, syntax refers to the rules governing the behavior of
mathematical systems, such as formal languages used in logic. (See
logical syntax.)
In linguistics, syntax (/ˈsɪntæks/[1][2]) is the set of rules, principles, and
processes that govern the structure of sentences in a given language,
speciﬁcally word order and punctuation.

The term syntax is also used to refer to the study of such principles
and processes.[3]

The goal of many syntacticians is to discover the syntactic rules
common to all languages.
The word syntax comes from Ancient Greek: σύνταξις "coordination",
which consists of σύν syn, "together," and τάξις táxis, "an ordering".

Syntax (Programming Languages)
!9
In computer science, the syntax of a computer language is the set of
rules that deﬁnes the combinations of symbols that are considered to be
a correctly structured document or fragment in that language.

This applies both to programming languages, where the document
represents source code, and markup languages, where the document
represents data.

The syntax of a language deﬁnes its surface form.[1]

Text-based computer languages are based on sequences of characters,
while visual programming languages are based on the spatial layout and
connections between symbols (which may be textual or graphical).

Documents that are syntactically invalid are said to have a syntax error.
https://en.wikipedia.org/wiki/Syntax_(programming_languages)

Syntax
- The set of rules, principles, and processes that govern the structure
of sentences in a given language

- The set of rules that deﬁnes the combinations of symbols that are
considered to be a correctly structured document or fragment in that
language

How to describe such a set of rules?
!10
That Govern the Structure

What do we call the elements of programs?
!12
#include <stdio.h>
int power(int m, int n);
/* test power function */
main() {
int i;
for (i = 0; i < 10; ++i)
printf("%d %d %dn", i, power(2, i), power(-3, i));
return 0;
}
/* power: raise base to n-th power; n >= 0 */
int power(int base, int n) {
int i, p;
p = 1;
for (i = 1; i <= n; ++i)
p = p * base;
return p;
}

What kind of program element is this?
!13
#include <stdio.h>
main() {
int i;
for (i = 0; i < 10; ++i)
return 0;
}
int i, p;
p = 1;
for (i = 1; i <= n; ++i)
p = p * base;
return p;
}
Program
Compilation Unit

!14
#include <stdio.h>
main() {
int i;
for (i = 0; i < 10; ++i)
return 0;
}
int i, p;
p = 1;
for (i = 1; i <= n; ++i)
p = p * base;
return p;
}
Preprocessor Directive

!15
#include <stdio.h>
main() {
int i;
for (i = 0; i < 10; ++i)
return 0;
}
int i, p;
p = 1;
for (i = 1; i <= n; ++i)
p = p * base;
return p;
}
Function Declaration
Function Prototype

!16
#include <stdio.h>
main() {
int i;
for (i = 0; i < 10; ++i)
return 0;
}
int i, p;
p = 1;
for (i = 1; i <= n; ++i)
p = p * base;
return p;
}
Comment

!17
#include <stdio.h>
main() {
int i;
for (i = 0; i < 10; ++i)
return 0;
}
int i, p;
p = 1;
for (i = 1; i <= n; ++i)
p = p * base;
return p;
}
Function Deﬁnition

!18
#include <stdio.h>
main() {
int i;
for (i = 0; i < 10; ++i)
return 0;
}
int i, p;
p = 1;
for (i = 1; i <= n; ++i)
p = p * base;
return p;
}
Variable Declaration

!19
#include <stdio.h>
main() {
int i;
for (i = 0; i < 10; ++i)
return 0;
}
int i, p;
p = 1;
for (i = 1; i <= n; ++i)
p = p * base;
return p;
}
Statement
For Loop

!20
#include <stdio.h>
main() {
int i;
for (i = 0; i < 10; ++i)
return 0;
}
int i, p;
p = 1;
for (i = 1; i <= n; ++i)
p = p * base;
return p;
}
Statement
Function Call

!21
#include <stdio.h>
main() {
int i;
for (i = 0; i < 10; ++i)
return 0;
}
int i, p;
p = 1;
for (i = 1; i <= n; ++i)
p = p * base;
return p;
}
Expression

!22
#include <stdio.h>
main() {
int i;
for (i = 0; i < 10; ++i)
return 0;
}
int i, p;
p = 1;
for (i = 1; i <= n; ++i)
p = p * base;
return p;
}
Formal Function Parameter

!23
#include <stdio.h>
main() {
int i;
for (i = 0; i < 10; ++i)
return 0;
}
int i, p;
p = 1;
for (i = 1; i <= n; ++i)
p = p * base;
return p;
}
Type

Syntactic Categories
!24
Program
Compilation Unit
Preprocessor Directive
Function Prototype
Statement
For Loop
Expression
Formal Function

Parameter
Type
Function Call
Programs consist of diﬀerent kinds of elements

Hierarchy of Syntactic Categories
!25
Program
Compilation Unit
Preprocessor

Directive
Function Prototype
Function

Deﬁnition
Variable

Declaration
Statement
For LoopExpression
Formal Function

Parameter
Type
Function Call
Some kinds of constructs are contained in others

The Tiger Language
!26
Example language used in lectures
https://www.lrde.epita.fr/~tiger/tiger.html
Documentation
https://github.com/MetaBorgCube/metaborg-tiger
Spoofax project

!27
let
var N := 8
type intArray = array of int
var row := intArray[N] of 0
var col := intArray[N] of 0
var diag1 := intArray[N + N - 1] of 0
function printboard() = (
for i := 0 to N - 1 do (
for j := 0 to N - 1 do
print(if col[i] = j then
" O"
else
" .");
print("n")
);
print("n"))
function try(c : int) = (
if c = N then
printboard()
else
for r := 0 to N - 1 do
if row[r] = 0 & diag1[r + c] = 0 & diag2[r + 7 - c] = 0 then (
row[r] := 1;
diag1[r + c] := 1;
diag2[r + 7 - c] := 1;
col[c] := r;
try(c + 1);
row[r] := 0;
diag1[r + c] := 0;
diag2[r + 7 - c] := 0))
in
try(0)
end
A Tiger program that solves
the n-queens problem

!28
let
var N := 8
for i := 0 to N - 1 do (
" O"
else
" .");
print("n")))
if c = N then
printboard()
else
if row[r] = 0 & diag1[r + c] = 0 then (
diag2[r + 7 - c] := 1;
try(c + 1);))
in
try(0)
end
A mutilated n-queens Tiger
program with ‘redundant’
elements removed
What are the syntactic
categories of Tiger?
What are the language
constructs of Tiger called?

!29
let
var N := 8
for i := 0 to N - 1 do (
" O"
else
" .");
print("n")))
if c = N then
printboard()
else
diag2[r + 7 - c] := 1;
try(c + 1);))
in
try(0)
end
Program
Expression
Let Binding

!30
let
var N := 8
for i := 0 to N - 1 do (
" O"
else
" .");
print("n")))
if c = N then
printboard()
else
diag2[r + 7 - c] := 1;
try(c + 1);))
in
try(0)
end

!31
let
var N := 8
for i := 0 to N - 1 do (
" O"
else
" .");
print("n")))
if c = N then
printboard()
else
diag2[r + 7 - c] := 1;
try(c + 1);))
in
try(0)
end
Type Declaration

!32
let
var N := 8
for i := 0 to N - 1 do (
" O"
else
" .");
print("n")))
if c = N then
printboard()
else
diag2[r + 7 - c] := 1;
try(c + 1);))
in
try(0)
end
Type Expression

!33
let
var N := 8
for i := 0 to N - 1 do (
" O"
else
" .");
print("n")))
if c = N then
printboard()
else
diag2[r + 7 - c] := 1;
try(c + 1);))
in
try(0)
end
Expression
Array Initialization

!34
let
var N := 8
for i := 0 to N - 1 do (
" O"
else
" .");
print("n")))
if c = N then
printboard()
else
diag2[r + 7 - c] := 1;
try(c + 1);))
in
try(0)
end

!35
let
var N := 8
for i := 0 to N - 1 do (
" O"
else
" .");
print("n")))
if c = N then
printboard()
else
diag2[r + 7 - c] := 1;
try(c + 1);))
in
try(0)
end
Function Name

!36
let
var N := 8
for i := 0 to N - 1 do (
" O"
else
" .");
print("n")))
if c = N then
printboard()
else
diag2[r + 7 - c] := 1;
try(c + 1);))
in
try(0)
end
Function Body
Expression
For Loop

!37
let
var N := 8
for i := 0 to N - 1 do (
" O"
else
" .");
print("n")))
if c = N then
printboard()
else
diag2[r + 7 - c] := 1;
try(c + 1);))
in
try(0)
end
Expression
Sequential Composition

!38
let
var N := 8
for i := 0 to N - 1 do (
" O"
else
" .");
print("n")))
if c = N then
printboard()
else
diag2[r + 7 - c] := 1;
try(c + 1);))
in
try(0)
end
Formal Parameter

!39
let
var N := 8
for i := 0 to N - 1 do (
" O"
else
" .");
print("n")))
if c = N then
printboard()
else
diag2[r + 7 - c] := 1;
try(c + 1);))
in
try(0)
end
Expression
Assignment

!40
let
…
function prettyprint(tree : tree) : string =
let
var output := ""
function write(s : string) =
output := concat(output, s)
function show(n : int, t : tree) =
let
function indent(s : string) = (
write("n");
for i := 1 to n do
write(" ") ;
output := concat(output, s))
in
if t = nil then
indent(".")
else (
indent(t.key);
show(n + 1, t.left);
show(n + 1, t.right))
end
in
show(0, tree);
output
end
…
in
…
end
Functions can be nested

Structure
- Programs have structure

Categories
- Program elements come in multiple categories

- Elements cannot be arbitrarily interchanged

Constructs
- Some categories have multiple elements

Hierarchy
- Categories are organized in a hierarchy
!41
Elements of Programs

Decomposing a Program into Elements
!43
let function fact(n : int) : int =
if n < 1 then
1
else
n * fact(n - 1)
in
fact(10)
end

!44
function fact(n : int) : int =
if n < 1 then
1
else
n * fact(n - 1)
fact(10)
Exp.Let

!45
fact(10)
Exp.Let
n : int if n < 1 then
1
else
n * fact(n - 1)
FunDec
fact int

!46
fact(10)
Exp.Let
int
if n < 1 then
1
else
n * fact(n - 1)
FunDec
fact
n
FArg
Tid
int
Tid

!47
fact(10)
Exp.Let
int
n < 1
FunDec
fact
n
FArg
Tid
Exp.If
1 n * fact(n - 1)
int
Tid

!48
fact(10)
Exp.Let
int
n
FunDec
fact
n
FArg
Tid
Exp.If
1
n * fact(n - 1)
int
Tid
Exp.Lt
Exp.Int
1
Exp.Var
Exp.Int

!49
fact(10)
Exp.Let
int
n
FunDec
fact
n
FArg
Tid
Exp.If
1
fact
int
Tid
Exp.Lt
Exp.Int
1
Exp.Var
Exp.Int Exp.Times
n
Exp.Var Exp.Call
n - 1
Etc.
if n < 1 then
1
else
n * fact(n - 1)
in
fact(10)
end

Tree Structure Represented as (First-Order) Term
!50
Mod(
Let(
[ FunDecs(
[ FunDec(
"fact"
, [FArg("n", Tid("int"))]
, Tid("int")
, If(
Lt(Var("n"), Int("1"))
, Int("1")
, Times(
Var("n")
, Call("fact", [Minus(Var("n"), Int("1"))])
)
)
)
]
)
]
, [Call("fact", [Int("10")])]
)
)
if n < 1 then
1
else
n * fact(n - 1)
in
fact(10)
end

Textual representation
- Convenient to read and write (human processing)

- Concrete syntax / notation

Structural tree/term representation
- Represents the decomposition of a program into elements

- Convenient for machine processing

- Abstract syntax
!51
Decomposing Programs

What are well-formed textual programs?
What are well-formed terms/trees?
How to decompose programs automatically?
!52
Formalizing Program Decomposition

Abstract Syntax:
Formalizing Program
Structure
53

Algebraic Signatures
!54
signature
sorts S0 S1 S2 …
constructors
C : S1 * S2 * … -> S0
…
Sorts: syntactic categories
Constructors: language constructs

Well-Formed Terms
!55
The family of well-formed terms T(Sig) deﬁned by signature
Sig is inductively deﬁned as follows:

If C : S1 * S2 * … -> S0 is a constructor in Sig and  
if t1, t2, … are terms in T(Sig)(S1), T(Sig)(S2), …,  
then C(t1, t2, …) is a term in T(Sig)(S0)

Well-Formed Terms: Example
!56
signature
sorts Exp
constructors
Int : IntConst -> Exp
Var : ID -> Exp
Times : Exp * Exp -> Exp
Minus : Exp * Exp -> Exp
Lt : Exp * Exp -> Exp
If : Exp * Exp * Exp -> Exp
Call : ID * List(Exp) -> Exp
if n < 1 then
1
else
n * fact(n - 1)
If(
, Int("1")
, Times(
Var("n")
)
)
decompose
well-formed wrt

Lists of Terms
!57
signature
sorts Exp Var
constructors
…
[Minus(Var("n"), Int(“1”))]
[Minus(Var("n"), Int(“1”)), Lt(Var("n"), Int("1"))]
[]

Well-Formed Terms with Lists
!58
The family of well-formed terms T(Sig) deﬁned by signature
Sig is inductively deﬁned as follows:

If C : S1 * S2 * … -> S0 is a constructor in Sig and  
if t1, t2, … are terms in T(Sig)(S1), T(Sig)(S2), …,  
then C(t1, t2, …) is a term in T(Sig)(S0)

If t1, t2, … are terms in T(Sig)(S),

Then [t1, t2, …] is a term in T(Sig)(List(S))

Abstract syntax of a language
- Deﬁned by algebraic signature

- Sorts: syntactic categories

- Constructors: language constructs

Program structure
- Represented by (ﬁrst-order) term

- Well-formed with respect to abstract syntax

- (Isomorphic to tree structure)
!59
Abstract Syntax

From Abstract Syntax to
Concrete Syntax
60

What does Abstract Syntax Abstract from?
!61
signature
sorts Exp Var
constructors
Var : ID -> Exp
Signature does not deﬁne ‘notation’

What is Notation?
!62
signature
sorts Exp Var
constructors
Var : ID -> Exp
Notation: literals, keywords, delimiters, punctuation
n
x
e1 * e2
e1 - e2
e1 < e2
if e1 then e2 else e3
f(e1, e2, …)
if n < 1 then
1
else
n * fact(n - 1)

How can we couple notation to abstract syntax?
!63
signature
sorts Exp Var
constructors
Var : ID -> Exp
n
x
e1 * e2
e1 - e2
e1 < e2
if e1 then e2 else e3
f(e1, e2, …)
if n < 1 then
1
else
n * fact(n - 1)
Notation: literals, keywords, delimiters, punctuation

Context-Free Grammars
!64
grammar
non-terminals N0 N1 N2 …
terminals T0 T1 T2 …
productions
N0 = S1 S2 …
…
Non-terminals (N): syntactic categories
Terminals (T): words of sentences
Productions: rules to create sentences
Symbols (S): non-terminals and terminals

Well-Formed Sentences
!65
The family of sentences L(G) deﬁned by context-free
grammar G is inductively deﬁned as follows:

A terminal T is a sentence in L(G)(T)

If N0 = S1 S2 … is a production in G and  
if w1, w2, … are sentences in L(G)(S1), L(G)(S2), …,  
then w1 w2 … is a sentence in L(G)(N0)

Well-Formed Sentences
!66
if n < 1 then
1
else
n * fact(n - 1)
grammar
non-terminals Exp Var
productions
Exp = IntConst
Exp = Id
Exp = Exp "*" Exp
Exp = Exp "-" Exp
Exp = Exp "<" Exp
Exp = "if" Exp "then" Exp "else" Exp
Exp = Id "(" {Exp ","}* ")"

What is the relation between concrete and abstract syntax?
!67
if n < 1 then
1
else
n * fact(n - 1)
If(
, Int("1")
, Times(
Var("n")
)
)
grammar
productions
Exp = IntConst
Exp = Id
Exp = Exp "*" Exp
Exp = Exp "-" Exp
Exp = Exp "<" Exp
Exp = Id "(" {Exp ","}* ")"
?
?
signature
sorts Exp Var
constructors
Var : ID -> Exp

Context-Free Grammars with Constructor Declarations
!68
sorts Exp Var
context-free syntax
Exp.Int = IntConst
Exp.Var = Id
Exp.Times = Exp "*" Exp
Exp.Minus = Exp "-" Exp
Exp.Lt = Exp "<" Exp
Exp.If = "if" Exp "then" Exp "else" Exp
Exp.Call = Id "(" {Exp ","}* ")"
signature
sorts Exp Var
constructors
Var : ID -> Exp
grammar
productions
Exp = IntConst
Exp = Id
Exp = Exp "*" Exp
Exp = Exp "-" Exp
Exp = Exp "<" Exp
Exp = Id "(" {Exp ","}* ")"

Context-Free Grammars with Constructor Declarations
!69
sorts Exp Var
context-free syntax
Exp.Int = IntConst
Exp.Var = Id
Exp.Call = Id "(" {Exp ","}* ")"
Abstract syntax: productions deﬁne
constructor and sorts of arguments
Concrete syntax: productions deﬁne
notation for language constructs

Abstract syntax
- Production defines constructor, argument sorts, result sort

- Abstract from notation: lexical elements of productions

Concrete syntax
- Productions define context-free grammar rules

Some details to discuss
- Sequences

- Lexical syntax

- Ambiguities

- Converting text to tree and back (parsing, unparsing)
!70
CFG with Constructors defines Abstract and Concrete Syntax

Encoding Sequences (Lists)
!72
sorts Exp Var
context-free syntax
Exp.Int = IntConst
Exp.Var = Id
Exp.Call = Id "(" ExpList ")"
ExpList.Nil =
ExpList = ExpListNE
ExpListNE.One = Exp
ExpListNE.Snoc = ExpListNE “," Exp
printlist(merge(list1,list2))
Call("printlist"
, [Call("merge", [Var("list1")
, Var("list2")])]
)

Sugar for Sequences and Optionals
!73
context-free syntax
Exp.Call = Id "(" {Exp ","}* ")"
printlist(merge(list1,list2))
Call("printlist"
, [Call("merge", [Var("list1")
, Var("list2")])]
)
context-free syntax
// automatically generated
{Exp “,"}*.Nil = // empty list
{Exp “,”}* = {Exp ","}+
{Exp “,"}+.One = Exp
{Exp “,"}+.Snoc = {Exp ","}+ "," Exp
Exp*.Nil = // empty list
Exp* = Exp+
Exp+.One = Exp
Exp+.Snoc = Exp+ Exp
Exp?.None = // no expression
Exp?.Some = Exp // one expression

Normalizing Lists
!74
rules
Snoc(Nil(), x) -> Cons(x, Nil())
Snoc(Cons(x, xs), y) -> Cons(x, Snoc(xs, y))
One(x) -> Cons(x, Nil())
Nil() -> []
Cons(x, xs) -> [x | xs]
context-free syntax
// automatically generated
{Exp “,"}*.Nil = // empty list
{Exp “,”}* = {Exp ","}+
{Exp “,"}+.One = Exp
{Exp “,"}+.Snoc = {Exp ","}+ "," Exp
Exp*.Nil = // empty list
Exp* = Exp+
Exp+.One = Exp
Exp+.Snoc = Exp+ Exp
Exp?.None = // no expression
Exp?.Some = Exp // one expression

Using Sugar for Sequences
!75
module Functions
imports Identifiers
imports Types
context-free syntax
Dec = FunDec+
FunDec = "function" Id "(" {FArg ","}* ")" "=" Exp
FunDec = "function" Id "(" {FArg ","}* ")" ":" Type "=" Exp
FArg = Id ":" Type
Exp = Id "(" {Exp ","}* ")"
let function power(x: int, n: int): int =
if n <= 0 then 1
else x * power(x, n - 1)
in power(3, 10)
end
module Bindings
imports Control-Flow Identifiers Types Functions Variables
sorts Declarations
context-free syntax
Exp = "let" Dec* "in" {Exp ";"}* "end"
Declarations = "declarations" Dec*

Context-Free Syntax vs Lexical Syntax
!77
let function power(x: int, n: int): int =
if n <= 0 then 1
else x * power(x, n - 1)
in power(3, 10)
end
Mod(
Let(
[ FunDecs(
[ FunDec(
"power"
, [FArg("x", Tid("int")), FArg("n", Tid("int"))]
, Tid("int")
, If(
Leq(Var("n"), Int("0"))
, Int("1")
, Times(
Var("x")
, Call(
"power"
, [Var("x"), Minus(Var("n"), Int("1"))]
)
)
)
)
]
)
]
, [Call("power", [Int("3"), Int("10")])]
)
)
phrase structure
lexeme / token
separated by layout
structure not relevant
not separated by layout

Character Classes
!78
lexical syntax // character codes
Character = [65]
Range = [65-90]
Union = [65-90] / [97-122]
Difference = [0-127] / [1013]
Union = [0-911-1214-255]
Character class represents choice from a set of characters

Sugar for Character Classes
!79
lexical syntax // sugar
CharSugar = [a]
= [97]
CharClass = [abcdefghijklmnopqrstuvwxyz]
= [97-122]
SugarRange = [a-z]
= [97-122]
Union = [a-z] / [A-Z] / [0-9]
= [48-5765-9097-122]
RangeCombi = [a-z0-9_]
= [48-579597-122]
Complement = ~[nr]
= [0-255] / [1013]
= [0-911-1214-255]

Literals are Sequences of Characters
!80
lexical syntax // literals
Literal = "then" // case sensitive sequence of characters
CaseInsensitive = 'then' // case insensitive sequence of characters
syntax
"then" = [116] [104] [101] [110]
'then' = [84116] [72104] [69101] [78110]
syntax
"then" = [t] [h] [e] [n]
'then' = [Tt] [Hh] [Ee] [Nn]

Identiﬁers
!81
lexical syntax
Id = [a-zA-Z] [a-zA-Z0-9_]*
Id = a
Id = B
Id = cD
Id = xyz10
Id = internal_
Id = CamelCase
Id = lower_case
Id = ...

Lexical Ambiguity: Longest Match
!82
lexical syntax
Id = [a-zA-Z] [a-zA-Z0-9_]*
context-free syntax
Exp.Var = Id
Exp.Call = Exp Exp {left} // curried function call
ab
Mod(
amb(
[Var("ab"),
Call(Var("a"), Var("b"))]
)
)

abc
Lexical Ambiguity: Longest Match
!83
Mod(
amb(
[ amb(
[ Var("abc")
, Call(
amb(
[Var("ab"), Call(Var("a"), Var("b"))]
)
, Var("c")
)
]
)
, Call(Var("a"), Var("bc"))
]
)
)
lexical syntax
Id = [a-zA-Z] [a-zA-Z0-9_]*
context-free syntax
Exp.Var = Id

abc def ghi
Lexical Restriction => Longest Match
!84
lexical syntax
Id = [a-zA-Z] [a-zA-Z0-9_]*
lexical restrictions
Id -/- [a-zA-Z0-9_] // longest match for identifiers
context-free syntax
Exp.Var = Id
Call(Call(Var("abc"), Var("def")), Var("ghi"))
Lexical restriction: phrase cannot be followed by character in character class

if def then ghi
Lexical Ambiguity: Keywords overlap with Identiﬁers
!85
lexical syntax
Id = [a-zA-Z] [a-zA-Z0-9_]*
context-free syntax
Exp.Var = Id
Exp.Call = Exp Exp {left}
Exp.IfThen = "if" Exp "then" Exp
amb(
[ Mod(
Call(
Call(Call(Var("if"), Var("def")), Var("then"))
, Var("ghi")
)
)
, Mod(IfThen(Var("def"), Var("ghi")))
]
)

ifdef then ghi
Lexical Ambiguity: Keywords overlap with Identiﬁers
!86
amb(
[ Mod(
Call(Call(Var("ifdef"), Var("then")), Var("ghi"))
)
, Mod(IfThen(Var("def"), Var("ghi")))
]
)
lexical syntax
Id = [a-zA-Z] [a-zA-Z0-9_]*
context-free syntax
Exp.Var = Id

Reject Productions => Reserved Words
!87
lexical syntax
Id = [a-zA-Z] [a-zA-Z0-9_]*
Id = "if" {reject}
Id = "then" {reject}
"if" "then" -/- [a-zA-Z0-9_]
context-free syntax
Exp.Var = Id
if def then ghi IfThen(Var("def"), Var("ghi"))

ifdef then ghi
Reject Productions => Reserved Words
!88
parse error
lexical syntax
Id = [a-zA-Z] [a-zA-Z0-9_]*
Id = "if" {reject}
Id = "then" {reject}
"if" "then" -/- [a-zA-Z0-9_]
context-free syntax
Exp.Var = Id

Real Numbers
!89
lexical syntax
RealConst.RealConstNoExp = IntConst "." IntConst
RealConst.RealConst = IntConst "." IntConst "e" Sign IntConst
Sign = "+"
Sign = "-"

Ambiguity
!91
t1 != t2 / format(t1) = format(t2)
Add
VarRef VarRef
“y”“x”
Mul
Int
“3”
Add
VarRef
VarRef
“y”
“x”
Mul
Int
“3”
3 * x + y

Disambiguation Filters
!93
parse ﬁlter
Disambiguation Filters [Klint & Visser; 1994], [Van den Brand, Scheerder, Vinju, Visser; CC 2002]

Associativity and Priority
!94
context-free syntax
Expr.Int = INT
Expr.Add = "Expr" + "Expr" {left}
Expr.Mul = "Expr" * "Expr" {left}
context-free priorities
Expr.Mul > Expr.Add
Recent improvement: safe disambiguation of operator precedence [SLE13, Onward15, Programming18]
Add
VarRef VarRef
“y”“x”
Mul
Int
“3”
Add
VarRef
VarRef
“y”
“x”
Mul
Int
“3”
3 * x + y

context-free syntax
Exp.Add = Exp "+" Exp
Exp.Sub = Exp "-" Exp
Exp.Mul = Exp "*" Exp
Exp.Div = Exp "/" Exp
Exp.Eq = Exp "=" Exp
Exp.Neq = Exp "<>" Exp
Exp.Gt = Exp ">" Exp
Exp.Gte = Exp ">=" Exp
Exp.Lte = Exp "<=" Exp
Exp.And = Exp "&" Exp
Exp.Or = Exp "|" Exp
Ambiguous Operator Syntax
!95

Disambiguation by Associativity and Priority Rules
!96
context-free syntax
Exp.Add = Exp "+" Exp {left}
Exp.Sub = Exp "-" Exp {left}
Exp.Mul = Exp "*" Exp {left}
Exp.Div = Exp "/" Exp {left}
Exp.Eq = Exp "=" Exp {non-assoc}
Exp.Neq = Exp "<>" Exp {non-assoc}
Exp.Gt = Exp ">" Exp {non-assoc}
Exp.Lt = Exp "<" Exp {non-assoc}
Exp.Gte = Exp ">=" Exp {non-assoc}
Exp.Lte = Exp "<=" Exp {non-assoc}
Exp.And = Exp "&" Exp {left}
Exp.Or = Exp "|" Exp {left}
{ left:
Exp.Mul
Exp.Div
} > { left:
Exp.Add
Exp.Sub
} > { non-assoc:
Exp.Eq
Exp.Neq
Exp.Gt
Exp.Lt
Exp.Gte
Exp.Lte
} > Exp.And
> Exp.Or

Disambiguation by Associativity and Priority Rules
!97
context-free syntax
Exp.Add = Exp "+" Exp {left}
Exp.Sub = Exp "-" Exp {left}
Exp.Mul = Exp "*" Exp {left}
Exp.Div = Exp "/" Exp {left}
Exp.Eq = Exp "=" Exp {non-assoc}
Exp.Neq = Exp "<>" Exp {non-assoc}
Exp.Gt = Exp ">" Exp {non-assoc}
Exp.Lt = Exp "<" Exp {non-assoc}
Exp.Gte = Exp ">=" Exp {non-assoc}
Exp.Lte = Exp "<=" Exp {non-assoc}
Exp.And = Exp "&" Exp {left}
Exp.Or = Exp "|" Exp {left}
{ left:
Exp.Mul
Exp.Div
} > { left:
Exp.Add
Exp.Sub
} > { non-assoc:
Exp.Eq
Exp.Neq
Exp.Gt
Exp.Lt
Exp.Gte
Exp.Lte
} > Exp.And
> Exp.Or
1 + 3 <= 4 - 35 & 12 > 16
And(
Leq(
Plus(Int("1"), Int("3"))
, Minus(Int("4"), Int("35"))
)
, Gt(Int("12"), Int("16"))
)

Testing Syntax Deﬁnitions
with SPT
98
Are you deﬁning the right language?

Test Suites
!99
module example-suite
language Tiger
start symbol Start
test name
[[...]]
parse succeeds
test another name
[[...]]
parse fails
SPT: SPoofax Testing language

Success is Failure, Failure is Success, …
!100
module success-failure
language Tiger
test this test succeeds [[
1 + 3 * 4
]] parse succeeds
test this test fails [[
1 + 3 * 4
]] parse fails
test this test fails [[
1 + 3 *
]] parse succeeds
test this test succeeds [[
1 + 3 *
]] parse fails

Test Cases: Valid
!101
Language
valid program
test valid program
[[...]]
parse succeeds

Test Cases: Invalid
!102
test invalid program
[[...]]
parse fails
invalid program
Language

Test Cases
!103
module syntax/identifiers
language Tiger start symbol Id
test single lower case [[x]] parse succeeds
test single upper case [[X]] parse succeeds
test single digit [[1]] parse fails
test single lc digit [[x1]] parse succeeds
test single digit lc [[1x]] parse fails
test single uc digit [[X1]] parse succeeds
test single digit uc [[1X]] parse fails
test double digit [[11]] parse fails

Test Corner Cases
!104
Language

Testing Structure
!105
module structure
language Tiger
test add times [[
21 + 14 + 7
]] parse to Mod(Plus(Int(“21"),Times(Int("14"),Int("7"))))
test times add [[
3 * 7 + 21
]] parse to Mod(Plus(Times(Int(“3"),Int("7")),Int("21")))
test if [[
if x then 3 else 4
]] parse to Mod(If(Var("x"),Int("3"),Int("4")))
The fragment did not parse to the expected ATerm. Parse result was:
Mod(Plus(Plus(Int("21"),Int("14")),Int("7"))) Expected result was:
Mod(Plus(Int(21), Times(Int(14), Int(7))))

Testing Ambiguity
!106
Note : this does not actually work in Tiger, since () is a
sequencing construct; but works fine in Mini-Java
module precedence
language Tiger start symbol Exp
test parentheses
[[(42)]] parse to [[42]]
test left-associative addition
[[21 + 14 + 7]] parse to [[(21 + 14) + 7]]
test precedence multiplication
[[3 * 7 + 21]] parse to [[(3 * 7) + 21]]

Testing Ambiguity
!107
module precedence
language Tiger start symbol Exp
test plus/times priority [[
x + 4 * 5
]] parse to Plus(Var("x"), Times(Int("4"), Int("5")))
test plus/times sequence [[
(x + 4) * 5
]] parse to Times(
Seq([Plus(Var("x"), Int("4"))])
, Int("5")
)

Syntax Engineering in
Spoofax
108

Compiler Construction | Lecture 2 | Declarative Syntax Definition

Developing syntax definition
- Define syntax of language in multiple modules

- Syntax checking, colouring

- Checking for undefined non-terminals

Testing syntax definition
- Write example programs in editor for language under def

- Inspect abstract syntax terms

‣ Spoofax > Syntax > Show Parsed AST

- Write SPT test for success and failure cases

‣ Updated after build of syntax definition
!110
Syntax Engineering in Spoofax

Declarative Syntax
Deﬁnition: Summary
111

Syntax definition
- Define structure (decomposition) of programs

- Define concrete syntax: notation

- Define abstract syntax: constructors

Using syntax definitions (next)
- Parsing: converting text to abstract syntax term

- Pretty-printing: convert abstract syntax term to text

- Editor services: syntax highlighting, syntax checking, completion
!112
Declarative Syntax Definition

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Compiler Construction | Lecture 2 | Declarative Syntax Definition

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