Instead of parsing terms we parse factors as described in our grammar. Here instead of parsing + and - operators we parse * and / instead. Term parser is very similar to expr just like the Term symbol is similar to Expr symbol in our grammar. Term = pure eval factor many (pure (,) (char '*' char '/') factor) It looks like comparing Haskells performance with C (even just.
#Parsec haskell upgrade#
Once again we use `` function to apply `eval` on values wrapped in Applicatives (`Parser`s). A major upgrade to Megaparsec: more speed, more power. A monadic parser combinator library (by haskell). We do that by applying a pure function `eval :: Int -> -> Int` (we'll define this later) that will fold the parsed integers and operators into a single integer by adding/subtracting the integers as appropriate. Compare parsec vs megaparsec and see what are their differences. Next, we need to combine the `Parser Int` from our first `term` and `Parser ` into a single `Parser Int`. If this is undesirable, simply use a data type like data Box a Box a and the state type Box YourStateType to add a level of indirection. Thus, applying `many` to `Parser (Char, Int)` type gives us a parser of type `Parser `. ParsecT s u m a is a parser with stream type s, user state type u, underlying monad m and return type a. `many` function from Applicative module is applied here to tell Parsec to keep parsing `(Char, Int)` as long as it can. The type of `term` as we'll see later is `Parser Int` so `pure (,) (char '+' char '-') term` gives us a parser of type `Parser (Char, Int)`. We use the `` function from `Control.Applicative` to lift a pure function `(,)` to apply it to the character produced by `(char '+' char '-')` and the `term` produced the following `term` parser to wrap them in a tuple. Attoparsec is a parsing combinator library that is aimed particularly at dealing efficiently with network protocols and complicated text/binary file. We use the choice function `` function from `Control.Applicative` to tell Parsec to parse either a `+` or a `-` character (but not both). This is exactly how we defined `Expr` symbol in our grammar. Here we parse a `term` (we'll define it later) that's followed by zero or more (`many`) `(char '+' char '-') term` parts. Now consider `term many (pure (,) (char '+' char '-') term)` part of our parser definition. So `Parser Int` is a parser of `String` that produces an `Int`. Type `Parser` is imported from `` module and represents a parser of `String` type.
First is the `expr :: Parser Int` Parser that corresponds to `Expr` symbol in our grammar.
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