Implicit output

Q: How deeply does Unit Testing Principles, Practices, and Patterns cover implicit output?

Defines hidden outputs, identifies them through referential transparency, and contrasts them with explicit method outputs.

As explained in Unit Testing Principles, Practices, and Patterns

A hidden output is an effect of a computation, such as a side effect or exception, that is not represented by its return value.

An implicit output is an effect of a computation that its return value does not represent. Side effects and exceptions are hidden outputs; changing internal or external state is a common example. section 6.3 section 6.6

Recognize implicit outputs by checking whether:

Replacing a method call with its return value changes program behavior, indicating a lack of referential transparency. section 6.3
The method changes internal state, as AddComment does when it adds a comment to a collection. section 6.3
The computation raises an exception or otherwise produces an effect not expressed in its signature. section 6.6

State-changing method

AddComment(string text) returns a Comment, but also appends it to _comments. The returned object therefore does not describe the method's complete output. section 6.3

public decimal CalculateDiscount(Product[] products)
{
    decimal discount = products.Length * 0.01m;
    return Math.Min(discount, 0.2m);
}

Contrast this explicit calculation with state-changing methods: its input and decimal result are fully represented by the signature. section 6.3

How Unit Testing Principles, Practices, and Patterns covers it

Cover of Unit Testing Principles, Practices, and Patterns

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Grokking SimplicityAn effect of a function that changes external state or performs an observable operation rather than returning the result directly.

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Related concepts

return valuealternative to · A return value is the result produced by a function, method, or operation and supplied to the code that invoked it.
DOM updateincludes · A modification to the Document Object Model that changes a document's represented content, structure, or presentation.
global variableused by · A global variable is a variable whose binding is accessible from broad program or module scope according to the language’s name-resolution rules.
mathematical calculationcontrasts with · A mathematical calculation is an operation or expression that derives a result from one or more input values.
actionused by · An action is a choice or operation selected or performed by an actor that can affect a system or environment or produce an outcome.
generationkind of · Generation is the computational production of outputs from supplied inputs or context, often by applying a model to produce predictions or new content.

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