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.

makeExprParser term table builds an expression parser for terms term 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)