Towards More
Natural Functional
Programming Languages
Brad A. Myers
Human-Computer Interaction Institute
School of Computer Science
Carnegie Mellon University
http://www.cs.cmu.edu/~bam
bam@cs.cmu.edu
1
The User Interface of Programming Languages
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ICFP’02
Programming is a human activity
Want to improve the ability of people to program
It makes sense to look at the human side
Brad Myers
CMU - HCI Institute
2
“Millions for compilers but hardly a penny for understanding
human programming language use. Now, programming
languages are obviously symmetrical, the computer on one
side, the programmer on the other. In an appropriate
science of computer languages, one would expect that half
the effort would be on the computer side, understanding
how to translate the languages into executable form, and
half on the human side, understanding how to design
languages that are easy or productive to use.... The human
and computer parts of programming languages have
developed in radical asymmetry.”
— Allen Newell and Stuart Card, 1985
ICFP’02
Brad Myers
CMU - HCI Institute
3
What is “Usability”?
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Usability = “The effectiveness, efficiency, and
satisfaction with which users can achieve tasks in
a particular environment of a product.”
Components:
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Learnability: Easy to learn so users can get started
rapidly.
Effectiveness: Experts can use it effectively and with
high productivity.
Low Error rate: Users make few errors.
Satisfaction: Pleasant to use. No frustrations for users.
 Similar to motivations for functional languages
ICFP’02
Brad Myers
CMU - HCI Institute
4
Why Human Computer Interaction?
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The field of Human Computer Interaction studies
how to improve and evaluate usability
Data, knowledge that can guide designs
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Techniques for evaluating usability
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ICFP’02
To make systems more usable
So claims can be substantiated
So improvements can be made
Brad Myers
CMU - HCI Institute
5
Who are the Programmers?
• Not just professional programmers
anymore

By 2005, 55 million end-user programmers
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ICFP’02
Compared to only 2.75 million professional
programmers
Brad Myers
CMU - HCI Institute
6
Design of New Languages
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How make design decisions?
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Based on math, logic, type theory
Designer’s intuition or sense of aesthetics
Similarity to other languages
 But many have known problems
Key concept:

If you care about usability:
Can leverage off of what is known and
what can be learned about people
to guide design decisions
ICFP’02
Brad Myers
CMU - HCI Institute
7
What we are doing...
Studying the People
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Examples of Problems
The men and women here raise your hands!
if ((isMan x) && (isWoman x)) then (raise_hands x) else ()
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(This issue with “and” applies to other natural
languages as well.)
if
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Buy a paint that is not red or blue
(((not (isRed ) || isBlue)) x) then buy x else
()
Research shows that these differences between
natural languages and computer languages hurt
understanding
ICFP’02
Brad Myers
CMU - HCI Institute
9
My Research Goals
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Make programming significantly easier to learn
and more effective for non-professional
programmers and beginners
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Try to provide a more objective basis for usability
decisions for programming language design
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ICFP’02
Apply results of Empirical Studies of Programmers,
Psychology of Programming, and Human-Computer
Interaction to programming language design
New studies
Design new programming languages and
environments based on these results
Brad Myers
CMU - HCI Institute
10
Multiple Criteria

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Focusing on learnability and naturalness for
beginners
Less emphasis:
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ICFP’02
Scalability
Provability
Efficiency
Mathematical or Logic properties
Similarity to other familiar languages
etc.
Brad Myers
CMU - HCI Institute
11
Gentle Slope Systems
Difficulty
of
Use
Program Complexity and Sophistication
Gentle Slope Systems
Programming in C++
MFC
Difficulty
of
Use
Program Complexity and Sophistication
Gentle Slope Systems
Programming in C++
Functional
Languages?
MFC
Difficulty
of
Use
UI libraries
Program Complexity and Sophistication
Gentle Slope Systems
Visual Basic
Programming in C++
Functional
Languages?
MFC
Difficulty
of
Use
C++ Programming
UI libraries
Basic
Program Complexity and Sophistication
Gentle Slope Systems
Visual Basic
Programming in C++
Functional
Languages?
MFC
Difficulty
of
Use
C Programming
UI libraries
Basic
Program Complexity and Sophistication
My Goal
What is “Natural Programming”?

Attempt to make programming closer to the way
people think
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Make programming “more natural”
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First, have to find out how people think about
algorithms, data structures, etc.
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Note: Not “natural language”
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ICFP’02
Still creating a formal language
Brad Myers
CMU - HCI Institute
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Why Might Being Natural be Good?
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“Programming is the process of transforming a
mental plan into one that is compatible with the
computer.”
— Jean-Michel Hoc
So process might be easier if transformation is
smaller
Closeness of mapping
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ICFP’02
"The closer the programming world is to the problem
world, the easier the problem-solving ought to be.…
Conventional textual languages are a long way from that
goal." — Green and Petre
Brad Myers
CMU - HCI Institute
18
Why Might Being Natural be Good?

Example:
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ICFP’02
Inserting item into 3rd place of high score list
Conventional Languages:
 Loop, starting at end of array, shuffle items down,
then insert
Brad Myers
CMU - HCI Institute
19
Why Might Being Natural be Good?

Directness (as in “Direct Manipulation”)
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“Distance between one's goals and the actions required
by the system to achieve those goals.”
— Hutchins, Hollan and Norman
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Example:
vs.
VB: Let Shape1.FillColor = &H00FF00FF&
ML: SetColor ( Shape1, 0x00FF00FF )
ICFP’02
Brad Myers
CMU - HCI Institute
20
Background Research

Empirical Studies of Programmers, Psychology of
Programming, and HCI results not being used in
the design of new languages
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Summarized in our comprehensive tech report
—
ICFP’02
30 years of research on what makes languages hard to
learn and error-prone
Java / C# looping, etc.
John Pane and Brad Myers, “Usability Issues in the Design of
Novice Programming Systems” TR# CMU-CS-96-132. Aug, 1996.
http://www.cs.cmu.edu/~pane/cmu-cs-96-132.html
Brad Myers
CMU - HCI Institute
21
Examples of Problems Identified
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Promote Locality and Avoid Hidden Dependencies
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Type definitions often are far from the use
Code readability is of key importance
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Don’t try to reduce keystrokes if makes less readable
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Inheritance and object-oriented design are very
difficult
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Beware of misleading appearances
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ICFP’02
When novices and experts mis-read code
Avoid subtle distinctions in syntax
E.g., a=b vs. a==b; () vs. [] vs. {}; >= vs. => vs. ->
Brad Myers
CMU - HCI Institute
22
More Examples
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People expect consistency with external
representations and usage (math, English, etc.):
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People will search for an analogue in their experience that
is similar to the syntax
“and”; a=a+1; a=2 vs. 2=a; 1<a<10;
ML: ~ is unary negative, - for subtraction: 5 - ~2
So, if different meaning, should have different presentation
Significant difficulties in finding bugs due to invisible
data, dependencies, and control flow
ICFP’02
Brad Myers
CMU - HCI Institute
23
HCI Methods for Analyzing Languages
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Analyze languages as user interfaces
Green’s Cognitive Dimensions
— Green and Petre, 1996, “Usability Analysis of VP Environments: A
‘Cognitive Dimensions Framework’. Journal of VL&C, 7(2): 131-174
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13 dimensions
Nielsen’s heuristic analysis principles
— Nielsen, J., Usability Engineering. 1993, Boston: Academic
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ICFP’02
Press
10 principles
Can also perform usability studies for specific
issues
Brad Myers
CMU - HCI Institute
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Consistency
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Both a Cognitive Dimension and a Heuristic
Analysis principle
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ICFP’02
C++ uses the word "static" to mean at least 3
different things
In C++, can use int a,b; to define globals or locals,
but not as procedure parameters
 Should be able to copy code and use the same code
elsewhere
In Visual Basic, to assign something you use “=”
unless is an object, in which case you use “Set” and “=”
"foo = 15" vs. "Set foo = object“
ML: “fun f x = 0”
vs.
Brad Myers
CMU - HCI Institute
“case e of x => 0”
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Error-Proneness
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HCI Principle = Prevent errors
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ICFP’02
C and C++ array bounds errors
Requiring the "break" in each branch of C, C++ switch
statements causes many errors (still in Java, fixed in C#)
Small typos can result in compilable programs that
perform incorrectly, e.g., "=" for "==” or
"x-=3” vs. "x=-3“ or
fun f(SOME _)=... (a constructor pattern) vs.
fun f(SOME_)=... (a variable)
Brad Myers
CMU - HCI Institute
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Good Error Messages
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Should be: clear, helpful, precise, constructive
Not “syntax error”
In C++, so much flexibility, compiler often doesn’t
know where error is
Similar problems with type inference systems
SML/NJ:
stdIn:30.1-30.4 Error: operator and operand don't agree [tycon mismatch]
operator domain: ?.t
operand:
?.t
in expression:
f B
ICFP’02
Brad Myers
CMU - HCI Institute
27
Closeness of Mapping
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HCI principle = Speak the User's Language
Expressions of algorithms close to the way users
think of them
Also, syntactic Issues:
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ICFP’02
C++ uses "void" to mean "none", "char" to mean 8-bit
number, ...
Visual Basic uses "Dim" to declare variables and "wend"
to end while loops
Arrays start at 0 whereas people think of counting from 1
Case sensitivity
Brad Myers
CMU - HCI Institute
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Viscosity
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Resistance to local change
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ICFP’02
To change parameters of a function in C++, have to edit
.h file and .cpp file, plus all call sites
Changing an “if” statement into a “do” statement was
difficult in early structure editors
VLs are very difficult due to layout issues
 May have to reposition all lines and boxes to make
room and neaten resulting drawing
 May need to disconnect and reconnect many wires
Need for correct indenting may make Haskell programs
resistant to editing
 But good editor can help
Brad Myers
CMU - HCI Institute
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Less is More
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HCI principle (“keep it simple”)
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ICFP’02
C, C++ have 16 levels of precedence that have to be
memorized, some of which are left-associative and
some are right-associative. Consider:
a=b+=c=+d*e+++f==g
which is a legal statement in C++ and C
Deep nesting in functional languages
 “Too many parentheses”
Brad Myers
CMU - HCI Institute
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Help the user get started with the system
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Small things should be simple
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Programs that do small things must still
often be very large, e.g., creating a
window containing a single red rectangle
The 2-pages needed in Motif to do “Hello World”
 “zero” lines in Visual Basic
In Java, it still requires:
class HelloWorldApp {
public static void main(String[] args) {
System.out.println("Hello World!");
}
}
Note 3 kinds of parentheses, 9 special words
ML: print "Hello
World!"
Brad Myers
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ICFP’02
CMU - HCI Institute
31
Other Issues
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ICFP’02
Many more, see:
http://www.cs.cmu.edu/~NatProg/langeval.html
You can send me examples from each other’s
systems!
But these are mainly good for analysis
Given a design question, how answer it?
Brad Myers
CMU - HCI Institute
32
Our Research
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Lots of gaps in prior research on people and
programming
Develop knowledge that can be used in design
Ph.D. thesis of John Pane
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http://www.cs.cmu.edu/~pane/thesis/
Evaluate:
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ICFP’02
Available at:
How people express algorithms and think about tasks
Vocabulary and notations used
Related to the HCI principles of “know the user”,
“task analysis”, and “closeness of mapping”
Brad Myers
CMU - HCI Institute
33
Our Studies so far
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How people naturally express programming concepts
and algorithms
1) Nine scenes from PacMan
2) Transforming and calculating data in a spreadsheet
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Specific issues of language design
3) Selecting specific objects from a group (“and”, “or”, “not”)
ICFP’02
Brad Myers
CMU - HCI Institute
34
Experimental Design
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Question should not bias the answer
So use pictures instead of textual descriptions
Concentrate on kids, non-programmers
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ICFP’02
Subjects should not be “tainted” by existing
programming languages
Tested that the results generalize to adults and
programmers
Brad Myers
CMU - HCI Institute
35
Study 1
Usually Pacman moves like this.
Now let's say we add a wall.
Pacman moves like this.
Not like this.
Do this: Write a statement that summarizes
how I (as the computer) should move Pacman
in relation to the presence or absence of other
things.
Second Study
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Whether similar results from other domains and
with adults
Developed 11 questions with scenarios using
spreadsheets
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ICFP’02
To test database access and operations
More conventionally “computational”
Brad Myers
CMU - HCI Institute
37
Example Question, 2nd Study
Question 4
• Describe in detailed steps what the computer should do to
categorize these people into 2 groups of ‘Gold’ and ‘Black’.
First
name
1 Sandra
2 Bill
3 Cindy
4 Tom
5 Bill
6 Whitney
7 Michael
8 Jay
9 David
10 Will
No.
ICFP’02
Last
name
Bullock
Clinton
Crawford
Cruise
Gates
Houston
Jordan
Leno
Letterman
Smith
Group
Brad Myers
CMU - HCI Institute
First
name
1 Sandra
2 Bill
3 Cindy
4 Tom
5 Bill
6 Whitney
7 Michael
8 Jay
9 David
10 Will
No.
Last
name
Bullock
Clinton
Crawford
Cruise
Gates
Houston
Jordan
Leno
Letterman
Smith
Group
Gold
Gold
Gold
Gold
Black
Gold
Gold
Black
Black
Gold
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Results
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Rule-based style
“If PacMan loses all his lives, its game over.”
 Some use of Constraint style:
“Pacman cannot go through a wall.”
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Aggregate operations instead of iterations
“The monsters turn blue and run away”
“Subtract 20,000 from all elements in Round 2”
— These tend to eliminate control structures
ICFP’02
Brad Myers
CMU - HCI Institute
39
More Results
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The words “AND” and “THEN” often used for
sequencing instead of as a logical operator
“The monsters turn color and start to back up.”
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Boolean expression (AND, OR) not common
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Usually had mutually exclusive rules
“If I press the up arrow, PacMan goes up.
If I press the down arrow, PacMan goes down, …”
General case first, then exceptions
“When you encounter a ghost, it should kill you.
But if you get a big pill first you can eat them.”
ICFP’02
Brad Myers
CMU - HCI Institute
40
Yet More Results
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Most arithmetic used natural language style
“When PacMan eats a big dot, the score goes up 100.”
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Operations suggest data as lists, not arrays
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Objects normally moving
“If PacMan hits a wall, he stops.”
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so objects remember their own state
2/3 of the first study subjects drew pictures
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ICFP’02
People don’t make space before inserting
Usually to define the initial state
Brad Myers
CMU - HCI Institute
41
Third Study: Select Objects from a Group
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Concentrate on a known problematic area
Use of AND, OR, NOT
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ICFP’02
Often eliminated from Web searching
 Newsweek reports that less than 6% of users
manage to use “and”, “or”, “+”, “-”
Still dominant in all programming languages
First: generate queries given results
Then, answer queries
Form-based and Textual formats
Order was counter-balanced
Brad Myers
CMU - HCI Institute
42
Generate Queries
ICFP’02
Brad Myers
CMU - HCI Institute
43
Answer Queries
ICFP’02
Brad Myers
CMU - HCI Institute
44
Results
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Using “unless” did not help accuracy
“select the objects that are blue unless the objects are square”
vs.
“select the objects that match blue and not square
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“And” was a Boolean conjunction sometimes
“select the objects that match blue and circle”
vs.
“select the objects that match blue and the objects that match circle”
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Precedence of “not” varied
“select the objects that match not red and square”
64% interpreted as “(not red) and square”
“select the objects that match not triangle or green”
67% interpreted as “not (triangle or green)”
ICFP’02
Brad Myers
CMU - HCI Institute
45
More results
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2-D forms helped for generation
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94% correct with match forms, vs.
85% correct with text (p<.0001)
(blue and not square) or (circle and not green)
ICFP’02
Brad Myers
CMU - HCI Institute
46
Implications for New Languages
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For increased usability for novices:
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ICFP’02
Use event-based style for dynamic events
Work to minimize the need for control structures and
variables
Provide operations on groups of objects
Data structures that combine the capabilities of
lists + arrays + sets
Support simple arithmetic in natural language style
(“add 1 to score”)
Avoid the use of the word “and” altogether
Brad Myers
CMU - HCI Institute
47
New Language and System: HANDS
Human-centered Advances for Novices to Develop Software
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ICFP’02
Video
Brad Myers
CMU - HCI Institute
48
Properties of HANDS
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Goal: Allow children age 10 to create
interactive games and simulations
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Event based computation model
Metaphor of agent manipulating cards
All data is visible as properties of the cards
All operations work on singletons or lists
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Programming in the small (in the tiny)
No distinction in syntax
Can generate lists on the fly with queries
Minimize need for control structures
Minimize need for local variables
ICFP’02
Brad Myers
CMU - HCI Institute
49
More Properties of Hands
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Verbose Language
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No precedence
Does use parentheses
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But just one kind!
Environment provide lots of help with syntax and
graphics
Tries to be extremely consistent, and also apply
other HCI rules
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ICFP’02
Easier to read
For example, combines IF, CASE (switch), and
COND (from Lisp) into one construct
Brad Myers
CMU - HCI Institute
50
Conclusions
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Much more research needed on the human side
of programming
Usability of languages and environments can be
improved
Claims about usability can be tested
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ICFP’02
Languages can be evaluated using HCI principles and
techniques
If you want a useable and learnable programming
language, there are data and techniques available
that can help.
Brad Myers
CMU - HCI Institute
51
Credits
Support for this research has come in part from the
National Science Foundation under
Grant No. IRI-9900452
and
Grant No. IIS-9817527
For more information, see:
http://www.cs.cmu.edu/~NatProg
ICFP’02
Brad Myers
CMU - HCI Institute
52
Towards More
Natural Functional
Programming Languages
Brad A. Myers
Human-Computer Interaction Institute
School of Computer Science
Carnegie Mellon University
http://www.cs.cmu.edu/~bam
http://www.cs.cmu.edu/~NatProg
bam@cs.cmu.edu
53