Write You a Haskell Stephen Diehl

Extensible Operators
Haskell famously allows the definition of custom infix operators, and extremely useful language feature
although this poses a bit of a challenge to parse! ere are several ways to do this and both depend on
two properties of the operators.
• Precedence
• Associativity
1. e first is the way that GHC does is to parse all operators as left associative and of the same
precedence, and then before desugaring go back and “fix” the parse tree given all the information
we collected after finishing parsing.
2. e second method is a bit of a hack, and involves simply storing the collected operators inside
of the Parsec state monad and then simply calling buildExpressionParser on the current state
each time we want to parse and infix operator expression.
To do later method we set up the AST objects for our fixity definitions, which associate precedence and
associativity annotations with a custom symbol.
data FixitySpec = FixitySpec
{ fixityFix :: Fixity
, fixityName :: String
} deriving (Eq, Show)
data Assoc
= L
| R
| N
deriving (Eq,Ord,Show)
data Fixity
= Infix Assoc Int
| Prefix Int
| Postfix Int
deriving (Eq,Ord,Show)
Our parser state monad will hold a list of the at Ivie fixity specifications and whenever a definition is
uncounted we will append to this list.
data ParseState = ParseState
{ indents :: Column
, fixities :: [FixitySpec]
} deriving (Show)
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