4.1. Defining SLang 1¶
4.1.1. Syntax of SLang 1¶
So far we have examined how to program from a functional programming perspective and how the lambda calculus forms the theoretical foundation for that perspective. We will now shift our focus and consider how we could actually develop a small interpreter for a language based on the lambda calculus. We’ll call that language SLang 1, short for “Simple Language version 1”.
The development process for this interpeter will require our writing a Jison grammar to define the syntax of the language and transform programs in the language into an abstract syntax tree (AST).
How does concrete syntax differ from abstract syntax? In other words, how do parse trees differ from abstract syntax trees?
A parse tree has a leaf node for each token, and an interior node for each non-terminal. Parse trees are useful to represent the structure or syntax of the source program.
However, we are now interested in the meaning of the program, not just its superficial structure. Abstract syntax trees (or ASTs) are parse trees stripped of all nodes that are not essential for later processing (for us: the execution or interpretation process).
In an AST:
Operators appear at interior nodes, not at leaf nodes and their operands become their children.
Chains of unit productions (i.e., productions of the form
<X> ::= <Y>) are collapsed.Lists are flattened.
Syntactic details (semi-colons, parens, etc.) are omitted.
This is illustrated in the example below.
Parse Tree Abstract Syntax Tree
========== ====================
exp *
| / \
term 3 +
/|\ / \
term * factor 4 2
/ /|\
/ / | \
factor ( exp )
| /|\
3 exp + term
| |
term factor
| |
factor 2
|
4
An AST often contains few nodes corresponding to non-terminals. Nonetheless, it contains all of the information that is needed for the interpreter to derive the correct meaning of the input program, that is, to evaluate the program and return its correct value.
Input Parse Tree
===== ==========
{ ____ methodBody _________
x = 0; / / \ \
while (x<10) { { declList stmtList }
x = x+1; | / \
} epsilon stmtList stmt___
y = x*2; / \ / | \ \
} stmtList stmt ID = exp ;
/ \ \ (y) / | \
AST stmtList stmt ... exp * term
=== | / | | \ | |
epsilon ID = exp ; term factor
methodBody (x) | | |
/ \ INTLITERAL factor INT
declList stmtList (0) | (2)
/ | \ ID
assign while assign (x)
/ \ ... / \
ID INT ID *
(x) (0) (y) / \
ID INT
(x) (2)
The concrete syntax of SLang 1 is defined by the following EBNF grammar:
<program> ::= <exp>
<exp> ::= <var_exp> | <fn_exp> | <app_exp> | <papp_exp> | <int>
<fn_exp> ::= fn '(' (<var_exp> (',' <var_exp>)*)? ')' => <exp>
<app_exp> ::= '(' <exp> <exp>* ')'
<papp_exp> ::= <prim_op> '(' <args>? ')'
<args> ::= <exp> (',' <exp>)*
<prim_op> ::= + | * | add1
The SLang 1 “program” (fn (a,b) => b y 3) would result in the following parse tree and AST.
parse tree AST
========== ===
program program
| |
exp app_exp
| / \
_ app_exp ______ fn_exp args
/ | | \ \ / \ / \
( exp exp exp ) [a,b] var_exp var_exp int
_________/ \ \ | | |
/ var_exp int b y 3
______fn_exp___________ \ \
/ / / | | \ \ \ y 3 [ "Program",
/ / / | | \ \ \ [ "AppExp",
fn ( var_exp , var_exp ) => exp [ "FnExp",
| | | ["a","b"],
a b var_exp ["VarExp","b"]],
| [ "args",
b ["VarExp","y"],
["IntExp",3]]]]
The expression in the bottom-right corner of the example above is a representation of the abstract syntax tree as a list of lists.
4.1.2. Concrete Syntax of SLang 1¶
The following problem will help you master the concrete syntax of SLang 1. To earn credit for it, you must complete this randomized problem correctly three times in a row.
4.1.3. More Practice with the Concrete Syntax of SLang 1¶
The following problem will reinforce your mastery of the concrete syntax of SLang 1 by providing more intensive practice. To earn credit for it, you must complete this randomized problem correctly three times in a row.
4.1.4. Abstract Syntax of SLang 1¶
The following problem will help you master the abstract syntax of SLang 1.
4.1.5. Curry in SLang 1¶
The following problem will illustrate the semantics of SLang 1 while helping you review the definition of the curry function.
4.1.6. Semantics of SLang 1¶
The following problem focuses on the semantics of SLang 1.
