This class will be held in the location of our labs, MI 302.
DrScheme can do some strange things. If you type
(eval '(+ 1 2)) in the interactions (bottom) window,
you will get back
3, as expected. However, if you
type the exact same expression,
(eval '(+ 1 2)), into the definitions window, DrScheme gives
an error basically saying that
+ is undefined. What gives?
The long answer is documented here (note: this is technically the documentation for a later version of Scheme called Racket, but the exposition is better than in the documentation for our older version).
The short answer is that the definitions window actually creates a Scheme
module, and the interactions window is executed outside of that module.
So the behavior of
eval is different because they are in different
namespaces. Roughly speaking,
eval can't see the bindings of the
module it's currently in.
In case you actually do want to do something like this, there is a
Putting this in the definitions window just attaches a name -
(define ns (namespace-anchor->namespace nsanchor))
ns - to the module's namespace. Then you can pass this namespace
eval and something like
(eval '(+ 1 2) ns)
will actually work - in either window.
(Try to remember this when we talk about namespaces from the point of view of the compiler in a few weeks.)
These readings are not required; however, they will be very useful to review this material and/or gain more depth.
Reading: SICP Section 1.3. (You can skip the unnumbered subsections on the half-interval method, Newton's method, etc.)
Exercises: Code up solutions to exercises 1 and 2 and print them out. Add your written solution to #3 and hand in all three as a hard copy. Due at the beginning of class on Friday.
(make-adder n)that returns a function (lambda expression) which takes one argument and adds n to it.
((make-adder 5) 3)→
(double f)which takes a 1-argument function
fand produces a one-argument function (lambda expression) that takes an argument
(f (f x)). So for instance
(double sqrt)produces a function to find 4th roots.
(double double)? What does it produce?