# BYU CS classes

### Site Tools

cs330_f2016:rackethof2

### Preparation:

You will again use DrRacket, which you should have downloaded and installed for the previous labs.

### Deliverables:

For this lab, you will need to implement the following decorator functions in Racket:

• default-parms - decorates a function to add default parameter values
• type-parms - decorates a function to enforce parameter type checking
• new-sin2 - a decorated version of your earlier new-sin that fills in default parameters and enforces correct types for all parameters

You will also chain these together to add both parameter type checking and default parameters to a function.

#### default-parms

(define (default-parms f values) ...)

where f is a function of arbitrary arity, values is a list of default parameter values of the same length as the arity of f, and the result is a function that behaves exactly like f but fills in corresponding default parameter values from the values list if given fewer parameters than expected.

For example, if f takes two parameters,

(define g (default-parms f (list 42 99))

would result in a function g such that (g) behaves the same as (f 42 99), (g 8) behaves the same as (f 8 99), and (g 8 9) behaves the same as (f 8 9).

Hint: You might find it useful to use the Racket function list-tail, which returns the rest of a list after some number of elements. For example, (list-tail (list 1 2 3 4 5) 2) returns the list (3 4 5).

#### type-parms

(define (type-parms f types) ...)

where f is a function of arbitrary arity, types is a list of type predicates (e.g., number?, string?, etc.) of the same length as the arity of f, and the result is a function that behaves exactly like f but first checks each parameter passed in to make sure it is the correct type.

For example, if f takes two parameters,

(define g (type-parms f (list number? string?))

would result in a function g such that (g 0 “hello”) behaves just like (f 0 “hello”), but passing anything but a number for the first parameter or anything other than a string for the second parameter results in an error message, by calling (error “ERROR MSG”).

Hint: You may want to write a small helper function that takes a list of values and a list of type predicates, then makes sure each of the values satisfies the respective type predicate. This is straightforward to write recursively. Can you write it in shorter form by mapping a two-parameter function pairwise on the lists?

#### Chaining Decorators

If default-parms and type-parms are working correctly, you should be able to chain them together like this:

(define g (default-parms
(type-parms
f
(list number? string?))
(list 0 "")))

would result in a function g that fills in any missing parameters with 0 and “” respectively, and then verifies that the passed parameters are a number and a string.

Use your new-sin function from your first assignment, type-parms, and default-parms to create a function called new-sin2 that behaves just like new-sin but

• uses 0 and 'radians as defaults for the parameters, and
• verifies that the first parameter is a number and that the second parameter is a symbol.

This must be created using default-parms and type-parms, not hand-coded.

### Notes:

You again do not need to bulletproof the code to enforce proper inputs. Your code only needs to return correct values given correct inputs.

Note: catching incorrectly typed inputs is part of the specified functionality for the type-parms decorator, so the functions generated by it should specifically detect and report that type of error.

### Hints:

Remember from class that our decorators have the same basic form:

(define (<name of the decorator> f ...)
(lambda args
...
(apply f args)))