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diff --git a/books/Little Schemer b/books/Little Schemer new file mode 100644 index 0000000..394de5b --- /dev/null +++ b/books/Little Schemer @@ -0,0 +1,108 @@ +============================ +The Little Schemer +============================ + +:by: Daniel Friedman and Matthias Felleisen +:Edition: Fourth (4rth) + +See also `Scheme </k/software/scheme/>`__. + +I read this book before starting on a scheme/physics project. I had programmed +in scheme previously as an algebra/analysis tool, but never really sat down +and got comfortable with the language. Working through all the examples +has made me *much* more comfortable with this style of programming. Despite +the humble tone and ambitions of the book I think I learned deeply. + +The first 7 chapters were very straight forward, the end of chapter 8 took +some more thought and I'm not sure how happy I am with the description of +collectors and continuations. + +For a better description of the Y-combinator, see these `course notes +<http://dangermouse.brynmawr.edu/cs245/ycomb_jim.html>`__. + +This book is followed by `The Seasoned Schemer </k/books/seasonedschemer/>`__ +and The Reasoned Schemer. + +Preface Definitions +------------------------ +This primitive function is required for most of the functions in the book:: + + (define atom? + (lambda (x) + (and (not (pair? x)) (not (null? x))))) + +Laws +----------------------- +Law of Car + The primitive *car* is defined only for non-empty lists. + +Law of Cdr + The primitive *cdr* is defined only for non-empty lists. The *cdr* of any + non-empty list is always another list. + +Law of Cons + The primitive *cons* takes two arguments. The second argument to *cons* + must be a list. The result is a list. + +Law of Null? + The primitive *null?* is defined only for lists. + +Law of Eq? + The primitive *eq?* takes two arguments. Each must be a non-numeric atom. + +Commandments +------------------------ + +The First Commandment + When recurring on a list of atoms, *lat*, ask two questions about it: + *(null? lat)* and **else**. When recurring on a number, *n*, ask two + questions about it: *(zero? n)* and **else**. + + When recurring on a list of S-expressions, *l*, ask three questions + about it: *(null? l)*, *(atom? (car l))*, and **else**. + +The Second Commandment + Use *cons* to build lists. + +The Third Commandment + When building a list, describe the first typical element, and then + *cons* it onto the natural recursion. + +The Fourth Commandment + Always change at least one argument while recurring. It must be changed to + be closer to termination. The changing argument must be tested in the + termination condition: + + when using *cdr*, test termination with *null?* and + + when using *sub1*, test termination with *zero?*. + +The Fifth Commandment + When building a value with +, always use 0 for the value of the terminating + line, for adding 0 does not change the value of an addition. + + When building a value with x, always use 1 for the value of the terminating + line, for multiplying by 1 does not change the value of a multiplication. + + When building a value with cons, always consider () for the value of the + terminating line. + +The Sixth Commandment + Simplify only after the function is correct. + + +The Seventh Commandment + Recur on the subpart that are of the same nature: + + * on the sublists of a list. + * on the subexpressions of an arithmetic expression. + +The Eighth Commandment + Use help functions to abstract from representations. + +The Ninth Commandment + Abstract common patterns with a new function. + +The Tenth Commandment + Build functions to collect more than one value at a time. + diff --git a/books/Seasoned Schemer b/books/Seasoned Schemer new file mode 100644 index 0000000..0bc6346 --- /dev/null +++ b/books/Seasoned Schemer @@ -0,0 +1,139 @@ +============================ +The Seasoned Schemer +============================ + +:by: Daniel Friedman and Matthias Felleisen +:Edition: First (1st) + +See also `Scheme </k/software/scheme/>`__. This book is a sequel +to `The Little Schemer`_; The Reasoned Schemer is a paralel exploration of +logical programming. + +One of the things I liked about learning a programming language this way, or +maybe just about scheme in general, is the seperation between the specification +and implementations. Usually when I start learning a new language I try to +break it as fast as possible and I am most interested in how certain little +tricky bits are handled (are the file handles cross platform? does it catch +infinite recursion? what kind of errors are thrown when? how big are the +primitives and simple objects/user defined data types?). These are the +imporant day to day issues and are a good basis for choosing a language to get +work done in, but it's kind of like searching for anti-aliasing in digital +photos or scanning the edges of a wall for painting mistakes. Sometimes the +big picture is the whole point and it's worth putting up with small flaws. + +.. _The Little Schemer: /k/books/littleschemer/ + +Issues/Omissions +-------------------------- +The Y combinator function is never defined in this book, I had to copy it out of +`The Little Schemer`_; + + (define Y + (lambda (thing) + ((lambda (le) + ((lambda (f) (f f)) + (lambda (f) (le (lambda (x) ((f f) x)))))) + thing))) + +Also ``eqlist?``:: + + (define eqlist? + (lambda (a b) + (cond + ((and (null? a) (null? b)) #t) + ((or (null? a) (null? b)) #f) + ((and (atom? (car a)) (atom? (car b))) + (and (eqlist? (cdr a) (cdr b)))) + ((or (atom? (car a)) (atom? (car b))) #f) + (else (and (eqlist? (car a) (car b)) (eqlist? (cdr a) (cdr b))))))) + +MIT/GNU Scheme doesn't seem to have ``letcc`` or ``try``; I stuck with +``call-with-current-continuation``: + + (call-with-current-continuation (lambda (hook) ...) + ; is the same as + (letcc hook (...)) + + ; as noted in the book (p. 89) + (try x a b) + ; is the same as + (letcc success + (letcc x + (success a)) + b) + ; is the same as + (call-with-current-continuation + (lambda (success) + (begin + (call-with-current-continuation + (lambda (x) + (success a))) + b))) + +When reimplementing scheme at the end of the book, I'm kind of miffed that the +(letcc ...) definition basically just uses letcc, because magic North Pole +compasses seem like the most interesting part. + +Notes +----------------- +Y-bang is the "applicative-order imperative Y combinator":: + + (define Y-bang + (lambda (f) + (letrec + ((h (f (lambda (arg) (h arg))))) + h))) + +At one point I wondered:: + + Is there any language/interpreter which, when it runs into an undefined + value, lets you define it on the spot? Would be great for learners. + +MIT/GNU Scheme, of course, has this feature in the error REPL. But I never +noticed it. + +The Next 10 Commandments +-------------------------- + +The Eleventh Commandment + Use additional arguments when a function needs to know what other + arguments to the function have been like so far. + +The Twelfth Commandment + Use (letrec ..) to remove arguments that do not change for + recursive applications. + +The Thirteenth Commandment + Use (letrec ...) to hide and protect functions. + +The Fifteenth Commandment + Use (let ...) to name the values of repeated expressions in a function + definition if they may be evaluated twice for one and same use of the + function. And use (let ...) to name the values of expressions (without + set!) that are re-evaluated every time a function is used. + +The Sixteenth Commandment + Use (set! ...) only with names define in (let ...)s + +The Seventeenth Commandment + Use (set! x ...) for (let ((x ..)) ..)) only if there is at least one + (lambda .. between it and the (let ..), or if the new value for x is a + function that refers to x. + +The Eighteenth Commandment + Use (set! x ...) only when the value that x refers to is no longer needed. + +The Nineteenth Commandment + Use (set! ...) to remember valuable things between two distinct uses of a + function. + +The Twentieth Commandment + When thinking about a value created with (letcc ...), write down the + function that is equivalent but does not forget. Then, when you use it, + remember to forget. + +**I love that last sentence!** + + + + |