This project implements an evaluator for a small subset of Racket, called Beginning Student Language (BSL).

The context-free grammar of BSL is defined as follows:

A BSLProgram is a sequence of BSLdefinitions followed by a BSLexpression.

A BSLDefiition is one of:

• (define (Variable Variable ...) BSLexpression)
• (define-struct Variable (Variable ...))
• (define Variable BSLexpression)

A BSLexpression is one of:

• PythonNumber | Num(PythonNumber)
• true | Boolean(true)
• false | Boolean(false)
• Variable | Var(String)
• (FunctionName BSLExpression ...)
• (PrimitiveName BSLExpression ...)
• (if BSLExpression BSLexpression BSLexpression)
• (and BSLExpression BSLExpression BSLExpression ...)

A Variable: is a Token (that is not a PrimitiveName or a Literal token or a PythonNumber)
A FunctionaName: is a Token (that is not a PrimitiveName or a Literal token or a PythonNumber)
A PrimitiveName is one of:

• /
• <
• =

The LiteralTokens are:

• define
• define-struct
• if
• and

A Token: is a sequence of characters not including '(', ')' and whitespace (' ') All whitespace between tokens and ( and ) is ignored.

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R E A D E R: -> P-expression reads one S-expression from STDIN

An S-expression has the following textual representation:

• Token
• OpenParen

An OpenParen is: '(' followed by Seq

A Seq is one of:

• ')'
• S-expression followed by Seq

An P-expression is one of:

• Atom
• [Listof P-expression]

An Atom is:

• Any sequence of characters

S-expression	P-expression
Token	Atom
OpenParen	Listof P-expression

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P A R S E R: P-expression -> AST

An AST is one of:

• BSLDef(String, [Listof String])
• IsClsDef
• BSLExpr
• ComparisonDef([Num ... -> bool or float or complex or int], [bool of float or complex or int -> Value] Value) %% but not Structure

A BSLDef is one of:

• StructDef
• ConstructorDef
• SelectorDef
• PredicateDef
• FunctionDef(BSLExpr)

An IsClsDef is one of:

• IsBooleanDef

A BSLExpr is one of:

• And(BSLlist)
• Boolean(bool)
• FunctionApplication(String, BSLlist)
• If(BSLlist)
• Num((int or complex or float))
• Variable(String)

A BSLlist is wrapper for the python list

Notes:
The parser will create an AST for P-expression whose original S-expression satisfies the context-free grammar of BSL.

• EX: A parser will not catch the following error: ['define', ['add', 'x', 'x', 'z'], ['+', 1, 3]] because the repetition of 'x' is not context free.
  
• EX: It will catch the following error: ['define', ['f', 'x', 1, 'y'], 42] because 1 is not a variable
  
• EX: It will catch the following error: ['define', ['f', 'x', '+', 'y'], 42] because '+' is not a variable
  

S-expression	AST
PythonNumber	Num(PythonNumber)
true	Boolean(true)
false	Boolean(false)
Variable	Var(String)

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I N T E R P R E T E R: AST -> Value

A Value is one of:

• Boolean(bool)
• Num(PythonNumber)
• Structure(String, [Listof (String, Value)])

For StructDef and FuncDef, eval_internal does not exist. For AST that come from BSLexpr, the following table explains eval_internal:

Expression ASTs	Value
And	Boolean
Boolean	Boolean
Num	Num
FunctionApplication	Value
If	Value
Variable	Value

Values produced from calling the apply method on some Definition

Definition	Value
ComparisonDef	Num/Boolean
IsClsDef	Boolean
PredicateDef	Boolean
SelectorDef	Value
ConstructorDef	Structure
FuncDef	Value