Part 1 — Maximum
Start with the interpreter with functions, and add a max operator that takes two numbers and returns the larger of them.
Since you must change the Exp datatype, and since different people may change it in different ways, you must update the parse function, which accepts an S-expression and produces an Exp value.
Some examples:
(test (interp (parse `{max 1 2})
(list))
2)
(test (interp (parse `{max {+ 4 5} {+ 2 3}})
(list))
9)
Part 2 — Functions that Accept Multiple Arguments
Extend the interpreter to support multiple or zero arguments to a function, and multiple or zero arguments in a function call.
For example,
{define {area w h} {* w h}}
defines a function that takes two arguments, while
{define {five} 5}
defines a function that takes zero arguments. Similarly,
{area 3 4}
calls the function area with two arguments, while
{five}
calls the function five with zero arguments.
At run-time, a new error is now possible: function application with the wrong number of arguments. Your interp function should detect the mismatch and report an error that includes the words “wrong arity”.
To support functions with multiple arguments, you’ll have to change fd and appE and all tests that use them. When you update the parse function, note that s-exp-match? supports ... in a pattern to indicate zero or more repetitions of the preceding pattern. Beware of putting the multi-argument application pattern too early in parse, since that pattern is likely to match other forms. In addition, you’ll need to update the parse-fundef function that takes one quoted define form and produces a Func-Defn value.
Some examples:
(test (interp (parse `{f 1 2})
(list (parse-fundef `{define {f x y} {+ x y}})))
3)
(test (interp (parse `{+ {f} {f}})
(list (parse-fundef `{define {f} 5})))
10)
(test/exn (interp (parse `{f 1})
(list (parse-fundef `{define {f x y} {+ x y}})))
"wrong arity")
Remember that Plait provides map, which takes a function and a list, and applies the function to each element in the list, returning a list of results. For example, if sexps is a list of S-expressions to parse, (map parse sexps) produces a list of ExprCs by parsing each S-expression.
Part 3 — Function Argument Checking
A function is ill-defined if two of its argument names are the same. To prevent this problem, update your parse-fundef function can detect this problem and report a “bad syntax” error.
For example, (parse-fundef `{define {f x x} x}) must report a “bad syntax” error, while (parse-fundef `{define {f x y} x}) should produce a Func-Defn value.
