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

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Hello!

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What is LINQ?
Fundamentals
Implementing the Trinity
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Which Delivery Teams
Have More
Than 7 Members?

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“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
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Why LINQ?
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What Makes LINQ Work?
1. Func
2. Generics
3. Extension Methods
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Getting Functional with Funcs!
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Background
◦ Introduced in C# 3.0
◦ Type that when invoked returns a value
◦ Requires 0 or more parameters
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Func<DeliveryTeam, bool>
Func<DeliveryTeam, string>

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Writing General Code
Generics
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Background
◦ Introduced in C# 2.0
◦ Allows for type parameters for various
constructs
◦ Great for when the type doesn’t matter
for implementation
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When Generics Make Sense
When you’re writing the third structure in a
row…
Logic is the same, only the types are
different
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Extending Type Functionality
Extension Methods

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Background
◦ Introduced in C# 3.0
◦ 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
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You Could Have
Invented LINQ!

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“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.
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MS Docs on Extension Methods

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Design Guidelines
◦ Extend the IEnumerable type
◦ Same signatures of LINQ
◦ Same properties as LINQ
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Changing Types with Map
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Background
Changes a collection of one type to
another by using a mapper function
Maintains the length of the collection
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Given a collection of DeliveryTeams,
we want to see the names, so we write the following

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Given a collection of DeliveryTeams,
we want to see the TeamLeads, so we write the following

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Given a collection of DeliveryTeams,
we want to know the number of team members on each team

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Any reason we need to constraint this to the DeliveryTeam type?

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Select Imp from GitHub

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

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“Use Map to Transform the
Collection Into The Data You
Need
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Keeping What’s Needed with Keep
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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
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Which DeliveryTeams have more than 7 team members?

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Which DeliveryTeams have Dana as a TeamLead?

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Any reason why this would be
limited to DeliveryTeam?

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Where Implementation from GitHub

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“Use Keep to Keep Data That
You Want
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Going from many to one with
Reduce
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Background
Takes a collection, an initial value, and a
combiner method, returns a single value
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How many team members do we have in total?

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Any reason why this should only
work on DeliveryTeam?

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Aggregate Imp from GitHub

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“Use Reduce to Transform a
Collection to a Single Value by
First Identifying the Initial
Value, Followed by Writing
the Combining Logic
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Reducing LINQ to Reduce
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No, Seriously…

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Breaking Down The Problem
◦ Determine the Initial Value
◦ Determine how to combine the initial
value with the next element
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What’s the initial value?
What should we return if the source is empty?
Enumerable<TResult>.Empty()

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

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Converting from T to TResult
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Appending to values
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Redoing Keep as Reduce
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What’s the initial value?
What should we return if the source is empty?
Enumerable<TSource>.Empty()

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Setting Up Keep via Reduce
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1. Check if the item meets the criteria
2. If the item meets, append to values
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Checking if an item is valid
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Appending to values
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What Did We Cover?
What is LINQ?
Fundamentals (Func, Generics, Extension
Methods)
Implementing Map, Keep, Reduce
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LINQ isn’t magic
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Composition of Funcs,
Generics, Extension Methods
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Building bigger programs
from smaller programs
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Questions?
Cameron@TheSoftwareMentor.com
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