CS302: Paradigms of Programming Variations on the Scheme Interpreter - - PowerPoint PPT Presentation

CS302: Paradigms of Programming Variations on the Scheme Interpreter

Manas Thakur

Feb-June 2020

A feat accomplished

• What we have seen:
• The kernel of an Eval/Apply Interpreter based on the Environment

Model for a (good enough) subset of Scheme

• Add the implementation of environment data structures:
• We get a working interpreter!

2

Graduate to Post-Graduate

• We can use the knowledge gained to experiment with

programming language technology in several ways:

• 1. Extend the language with new syntactic constructs
• What’s the disadvantage of using macros for this?
• 2. Explore various design choices in the space of programming

languages

• 3. Design new programming languages themselves!
• That even introduce novel paradigms of programming!!

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4

• 1. Extending languages with new syntactic sugar

Varargs in Scheme

• Desire: Eliminate the need to “fix” the number of parameters

that a procedure takes

• Have we seen such procedures?
• Hint: We saw some in the first class itself!
• Need changes at three levels:
• Syntax
• Semantics
• Implementation

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(+ x x) (+ x y z) (* (+ 2 3) (+ 5 7 8) z)

Choosing syntax for a new construct

• Considerations:
• Do we need a keyword or a symbol?
• ?, :, if
• How much would we be constraining the programmers as a

result of our choice?

• Underscores
• Can we introduce it unambiguously?
• Problems with ambiguity?
• Hyphens, if-else chaining, …

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if (c1) S1 else if (c2) S2 else S3

Syntax and Semantics for Varargs

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(define foo (lambda (x . y) (+ x (sum-list y)))) (define (sum-list (lambda (l) (foldr + l 0))) > (foo 2 5 7 9)

• x takes the first argument
• y takes the rest of the arguments
• y has to be a list.

What happens
 without the ‘.’?

Changes in the interpreter

• Standard way of pairing up actual argument values with formal

parameter variables:

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(define (pair-up (lambda (vars vals) (cond ((eq? vars ‘()) (cond ((eq? vals ‘()) ‘()) (else (error “TMA”)))) ((eq? vals ‘()) (error “TFA”)) (else (cons (cons (car vars) (car vals)) (pair-up (cdr vars) (cdr vals)))))))

Changes in the interpreter (Cont.)

• Pairing up actual argument values with formal parameter

variables with varargs:

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(define (pair-up (lambda (vars vals) (cond ((eq? vars ‘()) (cond ((eq? vals ‘()) ‘()) (else (error “TMA”)))) ((symbol? vars) (cons (cons vars vals) ‘())) ((eq? vals ‘()) (error “TFA”)) (else (cons (cons (car vars) (car vals)) (pair-up (cdr vars) (cdr vals)))))))