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\documentclass{beamer}
\usepackage{pgfpages}
\title[Thermoprint]{Thermoprint -- A Toolset for Interacting With Character Based Printers}
\subtitle[]{A Project in Advanced Functional Programming}
\author{Gregor Kleen}
\date{WiSe 2015--16}
\beamertemplatenavigationsymbolsempty
\usetheme{Montpellier}
\usecolortheme{dove}
\setbeameroption{show notes on second screen}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[tt=false]{libertine}
\usepackage{tikz}
\usetikzlibrary{positioning,shapes}
\usepackage{varwidth}
\newsavebox\IBox
\newenvironment{resizefig}[1][\textwidth]{\gdef\figWidth{#1}\begin{lrbox}{\IBox}\varwidth{\textwidth}\centering}{\endvarwidth\end{lrbox}\resizebox{\figWidth}{!}{\usebox\IBox}}
\usepackage{listings}
\lstset{language=Haskell}
\lstset{basicstyle=\ttfamily,breaklines=true,postbreak=\raisebox{0ex}[0ex][0ex]{\ensuremath{\color{gray}\hookrightarrow\space}}}
\begin{document}
\frame{\titlepage}
\frame{\tableofcontents}
\section{Motivation}
\begin{frame}
\frametitle{How to Talk to a Line Thermal Printer}
\begin{description}
\item[\texttt{ESC/POS}]
\begin{itemize}
\item An industry standard
\item Control-\alert{bytes}
\end{itemize}
\end{description}
\emph{Preparing printouts manually is ridiculous.}\\
Therefore: \alert{semantic} document format
\end{frame}
\section{Project Structure}
\begin{frame}
\begin{figure}
\begin{resizefig}[0.8\textwidth]
\begin{tikzpicture}[node distance=4cm]
\tikzstyle{package} = [rectangle, draw=black!80]
\node[package] (spec) [] {thermoprint-spec};
\node[package] (tprint) [below of=spec] {tprint};
\node[package] (client) [right of=tprint] {thermoprint-client};
\node[package] (webgui) [below of=tprint] {thermoprint-webgui};
\node[package] (tp-bbcode) [left of=tprint] {thermoprint-bbcode};
\node[package] (bbcode) [above of=tp-bbcode] {bbcode};
\node[package] (server) [right of=spec] {thermoprint-server};
\draw[->] (client) to (spec);
\draw[->] (tprint) to (client);
\draw[->] (webgui) to (client);
\draw[->] (tprint) to (tp-bbcode);
\draw[->] (webgui) to (tp-bbcode);
\draw[->] (tp-bbcode) to (bbcode);
\draw[->] (tp-bbcode) to (spec);
\draw[->] (server) to (spec);
\draw[->, dashed] (server) -- (client);
\end{tikzpicture}
\label{fig:cabal_dep_graph}
\caption{Dependency graph}
\end{resizefig}
\end{figure}
\note[item]{\texttt{theromprint-spec}: defines API \& document format}
\note[item]{\texttt{bbcode}: parser for bbcode}
\note[item]{\texttt{tp-bbcode}: parses bbcode Document Object Model to document format}
\note[item]{\texttt{client}: derivation of client for API defined in \texttt{thermoprint-spec}}
\note[item]{\texttt{server}: implementation of server for API defined in \texttt{thermoprint-spec} (mostly database Create Read Update \& Delete)}
\note[item]{\texttt{webgui}: uses \texttt{tp-bbcode}, \texttt{client} and (a patched version of) \texttt{threepenny}}
\note[item]{\texttt{tprint}: uses \texttt{tp-bbcode}, \texttt{client} and \texttt{optparse-applicative}}
\note[item]{\texttt{server} uses \texttt{client} only for testing (also covers \texttt{client})}
\end{frame}
\section{An Introduction to Servant}
\begin{frame}[fragile]
\frametitle{An API}
\begin{lstlisting}
-- GET /date
type MyAPI = "date" :> Get '[JSON] Date
-- GET /time/:tz
:<|> "time"
:> Capture "tz" Timezone
:> Get '[JSON] Time
myAPI :: Proxy MyAPI
myAPI = Proxy
\end{lstlisting}
\note[item]{\texttt{:<|>} combines two alternative apis into one (isomorphic to \texttt{(,)})}
\note[item]{\texttt{:>} composes parts of url: literals (\texttt{-XOverloadedStrings}), captures, query arguments, endpoints\\also isomorphic to \texttt{(,)} -- binds stronger than \texttt{:<|>}}
\note[item]{\texttt{'[JSON]} is a type of the kind \texttt{[JSON]} -- acceptable encodings carried within the type (\texttt{-XDataKinds})}
\end{frame}
\begin{frame}[fragile]
\frametitle{A Server}
\begin{lstlisting}
server :: Server MyAPI
server = getDate :<|> getTimeForTZ
where
getDate :: EitherT ServantErr IO Date
getDate = liftIO getCurrentDate
getTimeForTZ :: Timezone
-> EitherT ServantErr IO Time
getTimeForTZ = liftIO . getTimeAtTZ
main :: IO ()
main = run 8000 $ serve myAPI server
serve :: HasServer layout => Proxy layout -> Server layout -> Application
\end{lstlisting}
\note[item]{\texttt{Application} \& \texttt{run} from wai and warp}
\note[item]{\texttt{HasServer} maps api type to handler type via associated type family \& recursion}
\note[item]{servant provides utilities for changing monad -- \texttt{:<|>} is associative under (effectively) monad functors}
\end{frame}
\begin{frame}[fragile]
\frametitle{A Client}
\begin{lstlisting}
getCurrentDate :: EitherT ServantError IO Date
getTimeAtTZ :: Timezone
-> EitherT ServantError IO Time
(getCurrentDate :<|> getTimeAtTZ) = client myAPI url
where url = BaseUrl Http "localhost" 8000
client :: HasClient layout
=> Proxy layout
-> BaseUrl
-> Client layout
\end{lstlisting}
\note[item]{\texttt{Client} is associated type family of \texttt{HasClient}}
\note[item]{\texttt{:<|>} on left side of \texttt{=} is pattern match}
\end{frame}
\section{On the Difficulties Existentially Quantified Configuration Introduces}
\begin{frame}[fragile]
\frametitle{Thermoprints Use of Existential Quantification}
\begin{lstlisting}
type QueueManager t = StateT Queue t STM ()
-- Not really
intersection :: [QueueManager t] -> QueueManager t
idQM :: QueueManager t
data QMConfig m where
QMConfig :: (MonadTrans t, …) => QueueManager t -> (t IO) :~> m -> QMConfig m
\end{lstlisting}
\pause
Can we build \texttt{intersection} and \texttt{idQM} for \texttt{QMConfig}?
\note[item]<1>{\texttt{QueueManager}s manage queues.}
\note[item]<1>{\texttt{QMConfig} makes configuration nicer for the user (methinks)}
\note[item]<1>{\texttt{:$\sim$>} means monad functor in servant-speak}
\note[item]<2>{Combining \texttt{QueueManager}s is obviously useful. Why not \texttt{QMConfig}s?}
\end{frame}
\begin{frame}[fragile]
\frametitle{Difficulty in combining \texttt{QMConfig}s}
\begin{lstlisting}
combineMgrs :: Monad (t (t' IO)) => QueueManager t -> QueueManager t' -> QueueManager (ComposeT t t')
combine :: {-# constraints #-} QMConfig m -> QMConfig m -> QMConfig m
\end{lstlisting}
\note[item]{\texttt{QMConfig} carries constraints -- we need them for the result}
\note[item]{\texttt{QMConfig} carries no information about its transformer in its type}
\note[item]{Therefore we cannot construct a constraint for \texttt{combine}}
\note[item]{We \emph{might} be able to have \texttt{QMConfig} carry the necessary constraints for \texttt{combine} but thats difficult.\\Might be possible with quantified constraints (available on hackage), but not easily -- no partially applied types in haskell}
\end{frame}
\end{document}
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