Combining functions

You can write larger Haskell programs by composing simple functions into more complex ones.

Calling functions from within functions

Functions are composed simply by defining a more complex function in terms of simpler functions. This means that the definition of the complex function calls other functions.

For example, let us write a function to compute the price of a purchase given the price of the purchased item and the quantity at which it is purchased:

price :: Float -> Int -> Float
price ip qty = ip * fromIntegral qty

This is already an example of the principle that a more complex function, price, calls simpler functions. In this case, the simpler functions are two predefined functions: the (*) operator and the fromIntegral function. Recall that the fromIntegral conversion is needed to convert the Int quantity to a Float type before it can be multiplied by the item price.

When our business logic evolves, we can introduce a discounted price...