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+---
+title: On the Design of a Parser
+published: 2016-01-12
+tags: Thermoprint
+---
+
+The concrete application we’ll be walking through is a naive parser for [bbcode](https://en.wikipedia.org/wiki/BBCode)
+-- more specifically the contents of the directory `bbcode` in the
+[git repo](git://git.yggdrasil.li/thermoprint#rewrite).
+
+In a manner consistent with designing software as
+[compositions of simple morphisms](https://en.wikipedia.org/wiki/Tacit_programming) we start by determining the type of
+our solution (as illustrated by the following mockup):
+
+~~~ {.haskell}
+-- | Our target structure -- a rose tree with an explicit terminal constructor
+data DomTree = Element Text (Map Text Text) [DomTree]
+             | Content Text
+             deriving (Show, Eq)
+                         
+type Errors = ()
+
+bbcode :: Text -> Either Errors DomTree
+-- ^ Parse BBCode
+~~~
+
+Writing a parser capable of dealing with `Text` directly from scratch would be unnecessarily abstruse, we’ll be using
+the [attoparsec](https://hackage.haskell.org/package/attoparsec/docs/Data-Attoparsec-Text.html) family of parser
+combinators instead (the exact usage of attoparsec is out of scope -- we'll just mockup the types).
+
+~~~ {.haskell}
+data Token = BBOpen Text
+               | BBClose Text
+                   | BBStr Text
+
+tokenize :: Parser [Token]
+
+type Error' = ()
+
+runTokenizer :: Text -> Either Error' [Token]
+~~~
+
+We have now reduced the Problem to `[Token] -> DomTree`.
+We quickly see that the structure of the problem is that of a
+[fold](https://hackage.haskell.org/package/base/docs/Data-Foldable.html).
+
+Having realised this we now require an object of type `DomTree -> Token -> DomTree` to recursively build up our target
+structure.