1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
|
\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}
|
