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+---
+title: On the Architecture of a tool-set for interacting with character-oriented printers
+published: 2015-12-25
+tags: Thermoprint
+---
+
+# Motivation
+
+Some time ago I bought a cheap Chinese
+[thermoprinter](https://en.wikipedia.org/wiki/Thermal_printing) off eBay.
+As expected the printers firmware is really awkward to use (including binary
+control codes used to switch between char sets such as bold, italic, underlined,
+etc.).
+The obvious solution was to write a library to parse a more sensible
+representation and send it to be printed.
+
+Since there might, at some point, be other users wanting to print to my
+acquisition the architecture is intended to be present a somewhat usable
+interface to the uninitiated.
+
+# Implementation
+
+## Location
+
+Recently I created a new branch in
+[thermoprint](https://git.yggdrasil.li/thermoprint) called
+[rewrite](https://git.yggdrasil.li/thermoprint?h=rewrite).
+
+## Architecture Overview
+
+The new macroscopic architecture I´m currently aiming for is quite similar to
+the old one:
+
+ * A server intended to run on the machine connected to my cheap printer talking
+   directly to the printer on one end and serving a
+   [json api](https://hackage.haskell.org/package/servant) on the other.
+ * A (hopefully) tiny cli tool for debugging and personal use.
+ * A website (it will probably end up being based on
+   [yesod](https://hackage.haskell.org/package/yesod)) presenting a web interface
+   similar to the cli tool.
+
+## Features
+
+Features I intend to implement include:
+
+  * A parser for a bbcode-dialect which should be used in both the cli tool and the
+    website (it will probably end up using
+    [attoparsec](https://hackage.haskell.org/package/attoparsec)) -- bbcode as
+    presented on [Wikipedia](https://en.wikipedia.org/wiki/BBCode) is a proper
+    superset of the feature-set of my cheap Chinese printer.
+  * Reasonable test coverage using
+    [QuickCheck](https://hackage.haskell.org/package/QuickCheck),
+    [HUnit](http://hackage.haskell.org/package/HUnit).
+   
+    Automatic testing with [cabal](https://www.haskell.org/cabal/) facilitated by
+    [hspec](https://hackage.haskell.org/package/hspec).
+  * Support and server-side storage for drafts.
+  * The Website should provide some richer formats than bbcode which will
+    probably find inclusion in the payload datastructure such as lists,
+    checklists, tables, etc.
+
+    The cli-tool should be able to use these too (the input will probably end up
+    being json-formatted).
+
+## Work so far
+
+### Prototype
+
+I already have a prototype.
+It's quite bug-ridden and has recently developed serious problems actually
+printing after working satisfactorily for a few weeks.
+
+It also does not include a web-interface and I am quite unsatisfied with the
+overall code quality.
+
+The [685 lines of code](http://cloc.sourceforge.net/) can be found in the
+[repo](https://git.yggdrasil.li/thermoprint?h=master) as well.
+
+### Rewrite
+
+Currently the [rewrite](https://git.yggdrasil.li/thermoprint?h=rewrite) contains a
+single file of moment -- spec/src/Thermoprint/Printout.hs -- wherein we define
+the payload for the api -- our take on a structured document format (somewhat
+inspired by
+[pandoc](http://hackage.haskell.org/package/pandoc-types/docs/Text-Pandoc-Definition.html)):
+
+~~~ {.haskell}
+-- | A 'Printout' is a sequence of visually seperated 'Paragraph's
+type Printout = Seq Paragraph
+
+-- | A 'Paragraph' is a non-seperated sequence of 'Chunk's
+type Paragraph = Seq Chunk
+
+-- | We introduce both 'Chunk' and 'Paragraph' mainly to allow 'Raw'.
+-- 
+-- Were we to disallow 'Raw', 'Block' would be identical to 'Paragraph'
+data Chunk = Cooked Block -- ^ text semantically structured to be rendered in accordance with the display format of printer
+           | Raw ByteString -- ^ direct instructions to the printer
+           deriving (Generic, NFData, Show, CoArbitrary)
+
+-- | 'Block' is the entry point for our structured document format
+data Block = Line Line -- ^ a single 'Line' of text
+           | VSpace Integer -- ^ vertical space of height equivalent to 'Integer' lines
+           | NewlSep (Seq Block) -- ^ A sequence of 'Block's seperated by newlines
+           deriving (Generic, NFData, Show, CoArbitrary)
+
+{- | A 'Line' is one of:
+
+  * a single word
+  * horizontal space equivalent to the width of 'Integer' `em`.
+  * a sequence of words seperated by spaces
+
+We don't export all constructors and instead encourage the use of 'text'.
+-} 
+data Line = Word Text
+          | HSpace Integer
+          | SpaceSep (Seq Line)
+          deriving (Generic, NFData, Show, CoArbitrary)
+~~~
+
+(The code is verbatim as of 8307d7e).
+
+<!--  LocalWords:  Thermoprint thermoprint json api cli yesod bbcode attoparsec
+ -->
+<!--  LocalWords:  superset QuickCheck HUnit hspec datastructure repo pandoc
+ -->
+<!--  LocalWords:  haskell ByteString NFData CoArbitrary VSpace
+ -->
+<!--  LocalWords:  NewlSep HSpace SpaceSep
+ -->