TeachScheme, ReachJava

1
TeachScheme, ReachJava

• Stephen Bloch, Adelphi University, Garden City,
  NY
• John Clements, Cal Poly, San Luis Obispo, CA
• Kathi Fisler, Worcester Polytechnic Institute,
  Worcester, MA
• Matthew Flatt, University of Utah, Salt Lake
  City, UT
• Shriram Krishnamurthi, Brown University,
  Providence, RI
• Viera K. Proulx, Northeastern University, Boston,
  MA
• Supported by NSF grant 0618543
• Related work by Matthias Felleisen, Robby
  Findler,Kathy Gray, Eli Barzilay, et al

2
What is a problem?

• What is the meaning of life?
• Not an objective question
• How old am I?
• An objective question, which requires information
  you dont have
• What is the smallest prime number?
• An objective, fully-specified question, but once
  its answered once, its over

3
What is a problem?

• What is the Celsius equivalent of 45F?
• Again, once its answered, its over
• What is the Celsius equivalent of 87F?
• Ditto, but closely related
• What is the Celsius equivalent of ___ in
  Fahrenheit?
• Generalizes infinitely many questions

4
What is a problem?

• Question has a single answer
• Problem large (or infinite) family of questions,
  sharing some features differing in others
• Program general rule for solving a problem, with
  input for the features that differ
• Computation a program working on particular
  input to produce particular results

5
Introductions

• Who are you?
• What kind of school? students?
• Background in teaching CS
• Background in Scheme, Java, etc?

6
This week

• Youre primarily students, not teachers
• Get the student experience of this approach
  only faster
• Occasional breaks to talk pedagogy

7
What is computer science?

• microscope science about microbes
• telescope science about stars
• Its not about the tool, its about what you can
  study using the tool!
• computer science about information and the
  manipulation thereof

8
What is a beginning CS course about?

• How to use Microsoft Office et al
• Survey of the CS profession
• A language that will get students jobs
• Concepts, habits, methodologies of programming
• Four completely different courses, each valuable
  for different reasons and audiences
• 3 useful for students about to graduate, or
  about to get internships and summer jobs
• 4 more useful as a first course. Let's call it
  "CS1"

9
What is CS1 about?

10
What you teach

• blah algorithm blah variable blah function blah
  data type blah object blah re-use blah
  methodology blah testing blah design blah
  composition blah refinement blah abstraction

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What they hear

• blah blah blah ) blah blah return blah
  this.x x blah public static void main
  (String args) blah / blah // blah ltintgt blah
  if ( blah JOptionPane blah class Posn(int x,y)

12
Programming languagesa necessary evil

• Necessary to write real programs that really run
  on real computers
• Evil distract students from the important stuff
• Will be obsolete in a few years anyway

13
How to minimize language IDE?

• Introduce features one at a time
• Avoid "black magic" never ask students to write
  anything they can't understand now
• Introduce features only as necessary to teach an
  important concept
• Corollary any feature that doesn't help teach a
  CS1 concept shouldn't be mentioned in CS1
• Corollary need enforced language subsets so
  students don't accidentally use language features
  they haven't seen
• For GUI, robotics, networking, etc. need
  interface between student code (in subset) and
  library code (in full language)

14
What Java do we need? When?

• Realistically, most of us must cover a certain
  amount of Java by the end of CS2
• Not obvious that the best way to do this is Java
  from start of CS1
• Instead, teach concepts habits first, then
  complicated language

15
Alternatives to Java-first

• Leading alternatives Alice, Python, Scheme
• All three start w/simpler syntax while students
  master concepts
• Difference Alice Python usually taught
  imperatively Scheme usually functionally
• (Functional-first Python has potential)

16
Why functional?

• Simple, familiar semantic model
• 345 gt 320 gt 23
• Ease of writing test suites
• each test is usually one stmt, rather than"set
  up", "call method", "check side effects"
• tests order-independent
• no worry about aliasing, equals() vs. , etc.
• If testing is easy, students may actually do it!

17
But what about OOP?

• Students need to learn OOP, and an OO language,
  by 2nd or 3rd semester
• OOP is a terrific way to organize multi-KLOC
  programs
• First-term students don't write multi-KLOC
  programs
• OOP's benefits aren't apparent in first term
  students see only the overhead
• (Challenge write a short, OO C/Java program
  that wouldn't be simpler without OO)

18
Main points of TSRJ

• Multi-language approach to CS1/CS2
• Start in Scheme, develop concepts habits
• Switch to Java after 2-4 months
• Step-by-step design recipe in both languages
• concrete questions products at each step
• test-first methodology
• strong emphasis on data types
• shape of data determines shape of code test
  cases
• DrScheme development environment
• beginner-friendly
• interactive testing and experimentation
• enforces language subsets w/appropriate messages

19
Lets try DrScheme

• DrScheme
• Interactions pane (bottom half)
• Literal expressions
• 7
• "hello world"
• copy and paste picture from Web browser
• true
• Try www.picturingprograms.com/pictures/

20
Operating on pictures

• (reflect-vert paste-picture-from-Web)
• Same with reflect-horiz, rotate-cw, rotate-ccw,
  rotate-180
• (above picture1 picture2)
• Same with beside, overlay
• These three actually accept two or more pictures.
• Syntax rule ( operation pic1 )
• Terminology expression, function, argument,
  value, literal

21
Composing functions

• gt (beside (reflect-horiz ))
• Exercise write expressions to produce

22
Scheme syntax

• All expressions fully parenthesizedno order of
  operations to memorize
• All functions (both built-in and user-defined)
  are prefix, inside parentheses
• Some functions have arity 1, some 2, some"2 or
  more", etc. as appropriate.

23
Definitions pane

• Editable and savable
• "Run" button evaluates all expressions in order,
  replacing old Interactions
• Try7"hello world"copied image from Web(above
  (rotate-cw picture) (rotate-ccw picture))

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The Stepper

• Type several nested expressions into Definitions
  pane
• Click "Step" button at top
• See each sub-expression replaced with its value,
  one by one
• Valuable for students who never really "got"
  algebra

25
Variable definitions

• (define calendar )
• (above calendar (reflect-vert calendar))
• Define another variable to hold a different
  picture copied from the Web write some
  expressions using both.
• Note no declared data typedata have types,
  variables don't

26
Building images from scratch

• (rectangle 34 21 "solid" "blue")
• (circle 18 "outline" "green")
• (ellipse 34 55 "outline" "purple")
• (triangle 22 "solid" "pink")
• (star 7 20 10 "solid" "blue")
• (text "Hello there" 18 "blue")
• Try your own variations
• (define blue-star (star 7 20 10 "solid" "blue"))
• Try modifying combining these using above,
  overlay, reflect-vert, rotate-cw, etc.

27
Terminology

• Three kinds of literals image (pasted from Web
  browser), number, and string.
• Data types

28
Function contracts

• reflect-vert image -gt image
• reflect-horiz, rotate-cw, rotate-ccw,
  rotate-180 similar
• beside image image -gt image
• above, overlay similar
• rectangle number(width) number(height)
  string(solid/outline) string(color) -gt image
• Summarizes what a function takes in and returns,
  in a brief, standard notation
• Note semicolon for comment-to-end-of-line

29
More image functions

• crop-bottom image number(pixels) -gt image
• crop-top, crop-left, crop-right similar
• image-width image -gt number
• image-height similar
• place-image image(foreground) number(x)
  number(y) image(background) -gt image
• text string(text) number(size) string(color)
  -gt image

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Defining functions

• gt (define (mirror picture)
• (beside picture (reflect-horiz picture)))
• (mirror calendar)
• (mirror (rotate-cw (triangle 10 "solid" "blue")))
• Note no declared parameter or return types
  again, data have types
• Try (mirror calendar hacker) -- wrong number of
  arguments!
• Try (mirror 7) -- wrong type of argument!

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Exercise

• Define a function four-square that takes in an
  image and produces a 2x2 arrangement of it like

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Worked Exercise

• Define a function counterchange that takes in two
  images and produces a 2x2 arrangement like
• My answer
• (define (counterchange topleft topright)
• (above (beside topleft topright)
• (beside topright topleft)))

33
Testing functions

• With English descriptions
• (mirror calendar) "should be a calendar on the
  left, and a right-left-reflected calendar on the
  right"
• With check-expect
• (check-expect (mirror calendar) (beside
  calendar (reflect-horiz calendar)))
• Put either or both in Definitions window hit Run
  see what happens.
• Try with wrong function definition!

34
Design recipes

• We could write counterchange in an ad-hoc way,
  but in general we use
• Step-by-step recipes to get from English-language
  description to working, tested code
• One recipe for functions, one for data types, one
  for abstractions, one for event-driven animations

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How to accomplish anything

1. Figure out what you want to do
2. Do it
3. Check that you did it right

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How to write a function/method

• Figure out what you want to do
• Contract Specify name, inputs, outputs
• Data definition Identify any new data types in
  the problem
• Examples Write test cases with expected answers
• Do it
• Skeleton Write boilerplate code for function
  definition
• Inventory Write available expressions, their
  types, and (later) their values for a non-trivial
  test case
• Body Fill in details based on problem to solve
• Check that you did it right
• Testing Run test cases

37
The design recipe in practice

• contract
• counterchange image (topleft) image(topright)
  -gt image
• data definition problem only involves images,
  which are predefined
• test cases
• (check-expect (counterchange calendar schemelogo)
• (above (beside calendar schemelogo)
• (beside schemelogo calendar)))
• (check-expect (counterchange book (ellipse 30 10
  "outline" "pink"))
• (above (beside book (ellipse 30 10 "outline"
  "pink"))
• (beside (ellipse 30 10 "outline" "pink")
  book))

38
The design recipe in practice

• skeleton (inserted before examples)(define
  (counterchange topleft topright) )
• inventory(define (counterchange topleft
  topright) topleft an image
  topright an image )
• body (fill in something real, using expressions
  from inventory)(define (counterchange topleft
  topright) topleft an image
  topright an image (above (beside
  topleft topright) (beside topright
  topleft)))
• testing hit "Run" and see whether tests pass

39
The result

• counterchange image (topleft) image(topright)
  -gt image
• (check-expect (counterchange calendar schemelogo)
• (above (beside calendar schemelogo)
• (beside schemelogo calendar)))
• (check-expect (counterchange book (ellipse 30 10
  "outline" "pink"))
• (above (beside book (ellipse 30 10 "outline"
  "pink"))
• (beside (ellipse 30 10 "outline" "pink")
  book))
• (define (counterchange topleft topright)
  topleft an image topright
  an image (above (beside topleft topright)
  (beside topright topleft)))

40
Testing again

• Change the definition so that it's wrong run the
  tests again and see what happens.

41
Design recipes as pedagogy

• Note "test-first" methodology (a la XP) identify
  special cases before writing code
• Use as grading rubric partial credit for each
  step
• First steps are in comments non-threatening,avoi
  ds "blank page syndrome"
• Each step has concrete questions and concrete
  products
• I don't help with step N until I see step N-1
• Same steps apply in Java, C, etc.(but more
  complicated)
• Know-it-all students usually try to skip the
  recipeuntil Chapter 12 of HtDP, or Chapter 23
  of PP

42
Exercises

• Write contracts and test cases, but no
  definitions, for
• a function named copies-beside that takes in a
  number and an image, and produces that many
  copies of the image side by side
• a function named pinwheel that takes in an image
  produces a 2x2 rotated thing like
• a function named checkerboard2 that takes in two
  color names produces a 2x2 checkerboard in
  those colors

43
Exercises

• Write skeletons definitions for pinwheel and
  checkerboard2 (you don't know how to do
  copies-beside yet)
• Do all the steps to define lollipop, which takes
  in two numbers and a color name, and creates a
  picture of a lollipop w/specified radius, stick
  length, and color

44
Exercises

• Do all the steps to define diamond, which takes
  in a color name and a number, and creates a
  picture of a diamond w/specified color and side
  length.

45
Discussion break

• How is this different from what you've done in
  the past?
• How much explaining would it take for your
  students?
• I have a lot of mathophobic students, so I start
  with images rather than numbers and algebra. The
  prefix notation doesn't throw them, because they
  don't already "know" the "right" notation for
  operating on images.