This talk is about the world's most popular programming language... Java? Python? COBOL? No... Excel! Excel somehow hits the sweet spot between being powerful enough to run entire domains like finance and insurance, but also easy enough to attract a huge audience. Why is that? In this talk we'll explore some of the mechanisms that make spreadsheets so powerful, while implementing a few interesting mathematical problems in spreadsheets. While doing that, you might learn some (functional) programming concepts too, as we'll talk about sorting, dynamic programming and lambdas.

1. Functional programming in Excel
Felienne Hermans
@Felienne

4. Spreadsheets are code

5. 1) Used for similar problems

7. But why in a spreadsheet?

9. 2) Formulas are Turing complete

15. In summary: both the activities,
complexity and problems are the same

16. Spreadsheets are code
In summary: both the activities,
complexity and problems are the same

17. And not just a programming
language!

18. The next language
to learn
Resistance is futile!

19. Stukje Bret Victor hier
live programming

20. pure functional

21. pure functional

22. Lingua franca
of computing

23. Let’s
go!
Lego Adventure by Austin Kirk CC-BY-2.0

24. Functionalprogramming isprogrammingwithout assignmentstatements

25. Functionalprogramming isprogrammingwithout assignmentstatements
Example:
Squares
of
integers

29. List
of
inputs

30. List
of
inputs
Function definition

31. List
of
inputs
Function definition
Map

33. A bit more
exciting

34. Sum of all
squares
over 30

40. I’d like to
directly multiply
two ranges!

41. L This is not that,
it first intersects
and then
multiplies

42. Ctrl-shift-entercreates “array formula”

43. Ctrl-shift-entercreates “array formula”

44. Ctrl-shift-entercreates “array formula”
This cell now
holds an
entire array
It just shows
1 element

45. If I give the
formula space,
we get the
whole array

47. Hit ctrl-shirt-
enter to make
an array
formula

48. The first 5
elements of
the array
Input * Input

49. We can
reduce the
array too!

50. And filter

58. What is the
datatype of
DangerZone?

59. What is the
datatype of
DangerZone?
It is a
Maybe Int!

62. + is lifted and
applied to
only the ints

63. Let’s do some
real
programming!

64. Money in
a maze

65. Money in
a maze
Recursive
solution:

66. Money in
a maze
Recursive
solution:
V(O15)=

67. Money in
a maze
Recursive
solution:
V(O15)=

68. Money in
a maze
Recursive
solution:
V(O15)=

69. Recursive
solution:
V(O15)=
MAX( V(O14),V(N15) )
+ 33
Money in
a maze

71. Money in
a maze
Any
problems?

72. Any
problems?
Money in
a maze

73. Works,
but is
expensive
Money in
a maze

74. We can use
‘dynamic
programming’
Money in
a maze

76. We save the
maximum
value possible
at a field

89. Symbols will
represent path
direction

97. Only if the cell
below is “^”
Otherwise “”

103. Spreadsheets are code

104. So if spreadsheets are code, can we
apply software engineering methods?

105. In software, tools have been created to
help programmers understand code

114. Because spreadsheets are so similar to
code, users can benefit from SE methods

115. Like “code smells”
for spreadsheet
formulas

116. “Long method”

117. “Too many
parameters”

118. Conditional
complexity

119. Code
clones

120. Code
clones

123. If you say smells, you say…

124. If you say smells, you say refactoring

125. So, we built BumbleBee:
a refactoring tool for spreadsheets

126. So, we built BumbleBee:
a refactoring tool for spreadsheets

133. The transformations are entirely
programmable, with a small language

134. Available from my website:
felienne.com/bumblebee

135. And of course, if you say refactoring,
you say …

136. And of course, if you say refactoring,
you say testing

138. These test formulas can be exploited

153. Available from my website:
felienne.com/expector

166. More info?
• www.perfectxl.com
• www.felienne.com
• www.spreadsheetlab.org
Want to connect?
• @felienne / mail@felienne.com