1.
Functional programming in Excel
Felienne Hermans
@Felienne

2.
Spreadsheets are code

3.
1) Used for similar problems

4.
But why in a spreadsheet?

5.
2) Formulas are Turing complete

6.
In summary: both the activities,
complexity and problems are the same

7.
Spreadsheets are code
In summary: both the activities,
complexity and problems are the same

8.
And not just a programming
language!

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The next language
to learn
Resistance is futile!

10.
Stukje Bret Victor hier
live programming

11.
pure functional

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pure functional

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Lingua franca
of computing

14.
Let’s
go!
Lego Adventure by Austin Kirk CC-BY-2.0

15.
Functionalprogramming isprogrammingwithout assignmentstatements

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Functionalprogramming isprogrammingwithout assignmentstatements
Example:
Squares
of
integers

17.
List
of
inputs

18.
List
of
inputs
Function definition

19.
List
of
inputs
Function definition
Map

20.
A bit more
exciting

21.
Sum of all
squares
over 30

22.
I’d like to
directly multiply
two ranges!

23.
L This is not that,
it first intersects
and then
multiplies

24.
Ctrl-shift-entercreates “array formula”

25.
Ctrl-shift-entercreates “array formula”

26.
Ctrl-shift-entercreates “array formula”
This cell now
holds an
entire array
It just shows
1 element

27.
If I give the
formula space,
we get the
whole array

28.
Hit ctrl-shirt-
enter to make
an array
formula

29.
The first 5
elements of
the array
Input * Input

30.
We can
reduce the
array too!

31.
And filter

32.
What is the
datatype of
DangerZone?

33.
What is the
datatype of
DangerZone?
It is a
Maybe Int!

34.
+ is lifted and
applied to
only the ints

35.
Let’s do some
real
programming!

36.
Money in
a maze

37.
Money in
a maze
Recursive
solution:

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

39.
Money in
a maze
Recursive
solution:
V(O15)=

40.
Money in
a maze
Recursive
solution:
V(O15)=

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

42.
Money in
a maze
Any
problems?

43.
Any
problems?
Money in
a maze

44.
Works,
but is
expensive
Money in
a maze

45.
We can use
‘dynamic
programming’
Money in
a maze

46.
We save the
maximum
value possible
at a field

47.
Symbols will
represent path
direction

48.
Only if the cell
below is “^”
Otherwise “”

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Spreadsheets are code

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

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

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

53.
Like “code smells”
for spreadsheet
formulas

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“Long method”

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“Too many
parameters”

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Conditional
complexity

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Code
clones

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Code
clones

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If you say smells, you say…

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If you say smells, you say refactoring

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So, we built BumbleBee:
a refactoring tool for spreadsheets

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So, we built BumbleBee:
a refactoring tool for spreadsheets

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The transformations are entirely
programmable, with a small language

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Available from my website:
felienne.com/bumblebee

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

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

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These test formulas can be exploited

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Available from my website:
felienne.com/expector

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