# Solved: calculator

In the fascinating world of computing, calculators play an invaluable part. Despite the simplicity of what might first come to mind as calculators – devices for performing arithmetic operations – they can be some of the most complex and intricate devices in existence. In particular, the development of a calculator involves careful considerations of algorithms, user interface and performance optimizations. One such example is in Haskell programming language, where functional programming concepts are leveraged to elegantly solve calculation problems.

In this article, we’ll explore the making of a basic calculator using Haskell. We shall disentangle the intricacies of functional programming, detail the important Haskell libraries and functions involved, and unravel the path to a solution in a step-by-step process.

## Functional Programming in Calculator Design

Functional programming takes a different approach to programming than traditional imperative languages. The essence of functional programming is that programs are constructed by applying and composing functions, contrasted with imperative programming, which heavily relies on changes in state. Haskell, as a purely functional language, encourages programmers to implement systems in a declarative and high-level style. It’s well-suited for the creation of calculators due to its support for complex number manipulations, list comprehensions, and high-order functions.

Consider the arithmetic operation of addition. In Haskell, one could write a function to perform this operation as follows:

```addFunction :: Num a => a -> a -> a
addFunction x y = x + y
```

The first line is referred to as the type signature. It provides a high level of assurance that functions perform as intended, catching most errors at compile-time.

## Haskell Libraries for Calculator Functionality

One of Haskell’s strengths is its extensive collection of libraries which are powerful tools for crafting elegant and efficient solutions. To create a simple calculator, an understanding of two main libraries is crucial: the Prelude and the Text.Read libraries.

The Prelude in Haskell is the default library that gets imported into your Haskell programs. It provides a generous set of functions for handling lists, manipulating characters, and working with basic types such as integers and floating point numbers.

On the other hand, the Text.Read library offers the “readMaybe” function, which is particularly useful in reading user inputs in a calculator program. It provides safe partial functions for converting strings into Haskell variables.

With these libraries, the core functionality of a calculator can be written relatively easily.

## Building a Calculator: Code Breakdown

Getting down to the brass tacks, let’s delve into the nitty gritty of coding a very basic calculator. The following Haskell script allows a user to input a string of a simple arithmetic operation, and returns the result if the operation is valid:

```import Text.Read
import Data.Maybe

calculate :: String -> Maybe Float
calculate exprString =
case words exprString of
_ -> Nothing
```

The script starts by importing the Text.Read library (for user input) as well as the Data.Maybe library, which provides the “Maybe” type that is used to signify potentially unsuccessful computations.

The calculate function transforms an input string into a ‘Maybe Float’, using pattern matching to identify the operation and the ‘liftA2’ function from the standard Prelude library to apply the operation to the parsed numbers. This function encapsulates the core features of the calculator and demonstrates the beauty of functional programming in deriving solutions.

## Expanding Calculator Functionality

The basic calculator we’ve developed can be expanded upon with additional features like parentheses handling, advanced mathematical operations, or even storing variables. Further exploration into Haskell and its packages will reveal a myriad of ways to make the calculator more sophisticated, practical and user-friendly.

By understanding the core concepts and libraries in Haskell as well as the step-by-step construction of a simple calculator, one can appreciate the elegance of functional programming in tackling computational problems. This duality of simplicity and complexity is what makes calculator development in Haskell not just an interesting problem, but also a worthwhile journey.

Happy coding and exploring!

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