Another episode in series of Functional Thinking in Scala. This episode relates to Referential Transparency, Substitution model, context dependence and error handling in Scala.

1. 1
Thinking Functional in
Scala
Vikas Hazrati
Knoldus Software LLP

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agenda
●
Introduction
●
Referential
Transparency
●
Substitution Model
●
Exception Handling

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agenda
●
Introduction
●
Referential
Transparency
●
Substitution Model
●
Exception Handling

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impure
Scala is an impure functional programming
language

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no side effects
Mutation
I/O
Exceptions
printing to console
as well

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side-effect
If the function does something other than just providing
result
➔
Modifying a variable
➔
Modifying a data structure in place
➔
Setting a field on an object
➔
Throwing an exception or halting with an error
➔
Printing to the console or reading user input
➔
Reading from or writing to a file
➔
Drawing on the screen

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benefits of a pure function
Easy to
Test
Reuse
Parallelise
Generalize
Reason

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a pure function
●
A function f with input type A and output type B (written in Scala as
a single type: A => B , pronounced “ A to B ” or “ A arrow B ”) is a
computation that relates every value a of type A to exactly one
value b of type B such that b is determined solely by the value of a .
●
Any changing state of an internal or external process is irrelevant
to computing the result f(a)
●
A function intToString having type Int => String will take every
integer to a corresponding string. Furthermore, if it really is a
function, it will do nothing else

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a pure function
Referential
Transparency
Substitution
Model
Not context
dependent

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agenda
●
Introduction
●
Referential
Transparency
●
Substitution Model
●
Exception Handling

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referential transparency
What is the difference between 2 code blocks?

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referential transparency
What is the difference between 2 code blocks?
If the expression can be replaced by its value and
vice versa AND nothing changes then it is called
Referential Transparency.
This model of substituting values for expressions
is called Substitution Model.

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agenda
●
Introduction
●
Referential
Transparency
●
Substitution Model
●
Exception Handling

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substitution model
What do you expect the output to be?

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substitution model
What do you expect the output to be?

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substitution model
Now, let us replace money2 with referntial transparency. Hence money2 becomes
Money1.add(20)
Your sum1 is Money(60)
Your sum2 is Money(90)
Why?

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substitution model
Ok, let us change the way Money works
What do you expect the output to be?

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substitution model
Ok, let us change the way Money works
What do you expect the output to be?
Your sum1 is GoodMoney(40)
Your sum2 is GoodMoney(40)

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substitution model
And now lets apply the referential transparency
What do you expect the output to be?

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another example

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another example
Ideally we should be able to call
any listIngredients with coffee

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side effect
What is the side effect here ?

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Functional Exception Handling

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Exceptions are not type safe
What is the datatype of this ?
How would the caller know that there is an exception which can occur?
How was it better in Java?

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Exception break RT
Exceptions introduce context dependence
What would be the output?
What would be the output once we introduce RT?

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Exception break RT
After introducing RT
What would be the output?

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How should we work with Exceptions?

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sentinel value
●
Return a Sentinel value
Sentinel value
Silent error propagation
Boilerplate code
Typed methods would not know value
Demands special handling at caller end

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other options?
Explicit return that function may not always have
a value
What would you do?

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other options?
Explicit return that function may not always have
a value
What would you do? Option

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option
When exception could be thrown, wrap the
result in an Option

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wrapping exception throwing
api's
●
Lifting methods
f(A=>B)
def lift[A,B](f: A => B): Option[A] => Option[B] = _ map f
Example : val k = lift(math.abs)

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wrapping exception throwing
api's
●
Any number of parameters

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converting non option methods

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return exceptions
Sometimes we do want to tell the caller that an
exception has occured and pass the exception

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return exceptions
Sometimes we do want to tell the caller that an
exception has occured and pass the exception
Either

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Let us wrap our method
println(divideWithError(10,1).fold(a=>a,b=>b+1))
println(divideWithError(10,0).fold(a=>a,b=>b+1))

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That's It !
;)