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@pcameronpresley
Level Up on FP
Rebuilding LINQ
Cameron Presley
Cameron@TheSoftwareMentor.com

@pcameronpresley
What is LINQ?
Fundamentals
Implementing the Trinity
3

@pcameronpresley 6
Which Delivery Teams
Have More
Than 7 Members?

@pcameronpresley
“Set of operations that works on data
independent of the source
In addition, these operations are type safe,
allowing the compiler to catch your
mistakes sooner
9

@pcameronpresley
What Makes LINQ Work?
1. Func
2. Generics
3. Extension Methods
11

@pcameronpresley
Getting Functional with Funcs!
12

@pcameronpresley
Background
◦ Introduced in C# 3.0
◦ Type that when invoked returns a value
◦ Requires 0 or more parameters
13

@pcameronpresley
Func<DeliveryTeam, bool>
Func<DeliveryTeam, string>

@pcameronpresley
Writing General Code
Generics
18

@pcameronpresley
Background
◦ Allows for type parameters for various
constructs
◦ Great for when the type doesn’t matter
for implementation
19

@pcameronpresley
When Generics Make Sense
When you’re writing the third structure in a
row…
Logic is the same, only the types are
different
23

@pcameronpresley 24
Extending Type Functionality
Extension Methods

@pcameronpresley
Background
◦ Extends functionality to types without
having to recompile or creating a new
type
◦ Great for when working with types you
don’t have access to
25

@pcameronpresley 27
You Could Have
Invented LINQ!

@pcameronpresley
“The most common extension methods
are the LINQ standard query operators
that add query functionality to the
existing System.Collections.IEnumerable
and
System.Collections.Generic.IEnumerable
<T> types.
28
MS Docs on Extension Methods

@pcameronpresley
Design Guidelines
◦ Extend the IEnumerable type
◦ Same signatures of LINQ
◦ Same properties as LINQ
30

@pcameronpresley
Changing Types with Map
31

@pcameronpresley
Background
Changes a collection of one type to
another by using a mapper function
Maintains the length of the collection
32

@pcameronpresley 33
Given a collection of DeliveryTeams,
we want to see the names, so we write the following

@pcameronpresley 35
we want to see the TeamLeads, so we write the following

@pcameronpresley 39
we want to know the number of team members on each team

@pcameronpresley 43
Any reason we need to constraint this to the DeliveryTeam type?

@pcameronpresley 44
Select Imp from GitHub

@pcameronpresley 45
Do we need to limit to
List<TSource> and List<TResult>?

@pcameronpresley
“Use Map to Transform the
Collection Into The Data You
Need
46

@pcameronpresley
Keeping What’s Needed with Keep
47

@pcameronpresley
Background
Returns a subset of a collection based on a
predicate
Length is between 0 and original length
Doesn’t return items that weren’t in the
list
Maintains order
48

49
Which DeliveryTeams have more than 7 team members?

50
Which DeliveryTeams have Dana as a TeamLead?

@pcameronpresley 54
Any reason why this would be
limited to DeliveryTeam?

@pcameronpresley 56
Where Implementation from GitHub

@pcameronpresley
“Use Keep to Keep Data That
You Want
57

@pcameronpresley
Going from many to one with
Reduce
58

@pcameronpresley
Background
Takes a collection, an initial value, and a
combiner method, returns a single value
59

@pcameronpresley 60
How many team members do we have in total?

@pcameronpresley 65
Any reason why this should only
work on DeliveryTeam?

@pcameronpresley 67
Aggregate Imp from GitHub

@pcameronpresley
“Use Reduce to Transform a
Collection to a Single Value by
First Identifying the Initial
Value, Followed by Writing
the Combining Logic
68

@pcameronpresley
Reducing LINQ to Reduce
69
No, Seriously…

@pcameronpresley
Breaking Down The Problem
◦ Determine the Initial Value
◦ Determine how to combine the initial
value with the next element
70

@pcameronpresley 71
What’s the initial value?
What should we return if the source is empty?
Enumerable<TResult>.Empty()

@pcameronpresley
Setting Up Map via Reduce
72
1. Covert item from T to TResult
2. Append the converted item to values

@pcameronpresley
Converting from T to TResult
73

@pcameronpresley
Appending to values
74

@pcameronpresley
Redoing Keep as Reduce
75

@pcameronpresley 76
What’s the initial value?
What should we return if the source is empty?
Enumerable<TSource>.Empty()

@pcameronpresley
Setting Up Keep via Reduce
77
1. Check if the item meets the criteria
2. If the item meets, append to values
%%

@pcameronpresley
Checking if an item is valid
78

@pcameronpresley
Appending to values
79

@pcameronpresley
What Did We Cover?
What is LINQ?
Fundamentals (Func, Generics, Extension
Methods)
Implementing Map, Keep, Reduce
80

@pcameronpresley
LINQ isn’t magic
81

@pcameronpresley
Composition of Funcs,
Generics, Extension Methods
82

@pcameronpresley
Building bigger programs
from smaller programs
83

@pcameronpresley
Questions?
Cameron@TheSoftwareMentor.com
84

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Level Up Your Functional Programming Skills with LINQ

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