Haskell: Hello World program using ghci

Lets start writing simple program, which calculates simple interest.

Let Principal = P, Rate = R% per annum (p.a.) and Time = T years. Then,

Simple Interest = (P x R x T)/100

Step 1: Open any text editor and type following code, save it as hello.hs. Haskell files stored with .hs extension.

hello.hs

p = 100000
r = 1.10
t = 5

interest = p * t * r *0.01

Step 2: Go to the directory, where your hello.hs is located and type the command ghci.

Prelude> :load hello.hs
[1 of 1] Compiling Main             ( hello.hs, interpreted )
Ok, modules loaded: Main.
*Main> 
*Main> interest
5500.000000000001

‘:load’ command is used to load Haskell file. You can use :l in short cut to load Haskell files (:l hello.hs).

Observe above code snippet, when the file is loaded, Haskell changes the prompt from prelude to *Main. You can use the variables defined in hello.hs in *Main prompt.

Why the prompt changes from Prelude to Main?

When you load a file into GHCi, if the file has module definition in it, then the Prelude is changes to the module name. In the absence of a name for the module it is called the 'Main' module. So you got Main prompt here. I will explain about modules later.

*Main> p
100000.0
*Main> 
*Main> t
1.1
*Main> 
*Main> r
5.0
*Main> 
*Main> interest
5500.000000000001
*Main> 

How to reload a Haskell file?

Most of the time, you may change the contents Haskell file, in that case you need to reload the file, you can do that by using :reload command. You can use :r as shortcut of :reload.

*Main> :reload
Ok, modules loaded: Main.
*Main> :r
Ok, modules loaded: Main.

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