But let's not stop here. What does the dot \hinline{(.)} actually mean?
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It's just a function (the \emph{prefix} version of \hinline{.})! Here is the type signature:
\begin{haskellcode}
(.) :: (b -> c) -> (a -> b) -> a -> c
\end{haskellcode}
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\textbf{Exercise:} Implement it! It's really just one line! Remember the mathematical definition.
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Solution:
\begin{haskellcode}
(.) :: (b -> c) -> (a -> b) -> a -> c
(.) f g x = f (g x)
\end{haskellcode}