Copyright  © 2017–present Mark Karpov 

License  BSD 3 clause 
Maintainer  Mark Karpov <markkarpov92@gmail.com> 
Stability  experimental 
Portability  nonportable 
Safe Haskell  SafeInferred 
Language  Haskell2010 
A helper module to parse expressions. It can build a parser given a table of operators.
Since: 1.0.0
Documentation
This data type specifies operators that work on values of type a
. An
operator is either binary infix or unary prefix or postfix. A binary
operator has also an associated associativity.
InfixN (m (a > a > a))  Nonassociative infix 
InfixL (m (a > a > a))  Leftassociative infix 
InfixR (m (a > a > a))  Rightassociative infix 
Prefix (m (a > a))  Prefix 
Postfix (m (a > a))  Postfix 
TernR (m (m (a > a > a > a)))  Rightassociative ternary. Rightassociative means that
The outer monadic action parses the first separator (e.g. Example usage:

:: MonadPlus m  
=> m a  Term parser 
> [[Operator m a]]  Operator table, see 
> m a  Resulting expression parser 
builds an expression parser for terms
makeExprParser
term tableterm
with operators from table
, taking the associativity and
precedence specified in the table
into account.
table
is a list of [Operator m a]
lists. The list is ordered in
descending precedence. All operators in one list have the same precedence
(but may have different associativity).
Prefix and postfix operators of the same precedence associate to the left
(i.e. if ++
is postfix increment, than 2++
equals 1
, not 3
).
Unary operators of the same precedence can only occur once (i.e. 2
is
not allowed if 
is prefix negate). If you need to parse several prefix
or postfix operators in a row, (like C pointers—**i
) you can use this
approach:
manyUnaryOp = foldr1 (.) <$> some singleUnaryOp
This is not done by default because in some cases allowing repeating prefix or postfix operators is not desirable.
If you want to have an operator that is a prefix of another operator in the table, use the following (or similar) wrapper (Megaparsec example):
op n = (lexeme . try) (string n <* notFollowedBy punctuationChar)
makeExprParser
takes care of all the complexity involved in building an
expression parser. Here is an example of an expression parser that
handles prefix signs, postfix increment and basic arithmetic:
expr = makeExprParser term table <?> "expression" term = parens expr <> integer <?> "term" table = [ [ prefix "" negate , prefix "+" id ] , [ postfix "++" (+1) ] , [ binary "*" (*) , binary "/" div ] , [ binary "+" (+) , binary "" () ] ] binary name f = InfixL (f <$ symbol name) prefix name f = Prefix (f <$ symbol name) postfix name f = Postfix (f <$ symbol name)