#PARSEC HASKELL UPGRADE#
#PARSEC HASKELL CODE#

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. A monadic parser combinator library (by haskell). Term = pure eval factor many (pure (,) (char '*' char '/') factor) Compare parsec vs megaparsec and see what are their differences. 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. 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. Next, we need to combine the `Parser Int` from our first `term` and `Parser ` into a single `Parser Int`. We already learned about Haskell's support for regular expressions back. Thus, applying `many` to `Parser (Char, Int)` type gives us a parser of type `Parser `. Parsing a file, or data of various types, is a common task for programmers. `many` function from Applicative module is applied here to tell Parsec to keep parsing `(Char, Int)` as long as it can.

when p m if p then m else return () main do args <- getArgs when ( null args) ( putStrLn 'No args specified') and then use it with the same effect as the original expression.

#PARSEC HASKELL CODE#

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)`. The purity of Haskell code makes it easy to fuse chains of functions together, allowing for performance benefits. 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.

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.

Parsec haskell