talk-parser-combinators/presentation.html at master · sir-wabbit/talk-parser-combinators · GitHub

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name: inverse
layout: true
class: center, middle, inverse
---
#Parser Combinators

---
layout: false
.left-column[
  ## About me
]
.right-column[
* Working on information extraction from social media and newswire.
* Code mostly in Python, hack in functional languages (Scala, Haskell, F#, etc).
]

---
layout: false
.left-column[
  ## Options
  ### Cargo cult parsing
]
.right-column[
* Cargo cult parsing: the use of "magic" regular expressions -- often cut and pasted directly from Google search results.
* Regular expressions:
  + Available in almost every language
  + WYSIWIG as compared to WYSIWIGIYULR(k) (or your personal favourite grammar class).
  + Completely inadequate for fundamentally irregular languages (work "most of the time").
]

---
layout: false
.left-column[
  ## Options
  ### Cargo cult parsing
  ### Parser generators
]
.right-column[
* Have large upfront tax (LL, LR, LALR).
* Hard to integrate, aren't in-language.
* An arduous and clumsy affair.
* Require separate tools, hard to modify.
* But in the end you get efficient and powerful parsers.
]

---
layout: false
.left-column[
  ## Options
  ### Cargo cult parsing
  ### Parser generators
  ### Hand-written parsers
]
.right-column[
* Bugs!
* Easy to tweak.
* Difficult to develop and test.
* Difficult to optimize.
]

---
layout: false
.left-column[
  ## Options
  ### Cargo cult parsing
  ### Parser generators
  ### Hand-written parsers
  ### Parser combinators
]
.right-column[
* Modular, TDD-friendly.
* User-friendly default error messages (depends).
* Closely resemble normal functions (sometimes are normal functions).
* Use of closures borders on pathological.
]

---
.left-column[
  ## What is a parser?
]
.right-column[

]

---
.left-column[
  ## What is a parser?
]
.right-column[
Take a string, produce a value:

```scala
type Parser[+T] = String => T
```

- No way to indicate failure.
- Does not compose.

```scala
def seq[A, B](pa: Parser[A], pb: Parser[B]): Parser[(A, B)] =
  { s0: String =>
    ???
  }
```

]

---
.left-column[
  ## What is a parser?
]
.right-column[
Take a string, produce a value **together with the remaining part of the string**:

```scala
type Parser[+T] = String => (T, String)

def seq[A, B](pa: Parser[A], pb: Parser[B]): Parser[(A, B)] =
  { s0: String =>
    val (a, s1) = a(s0)
    val (b, s2) = b(s1)
    ((a, b), s2)
  }
```

Still no way to handle a failure.

]

---
.left-column[
  ## What is a parser?
]
.right-column[
Take a string, produce **an optional** value together with the remaining part of the string:

```scala
type Parser[+T] = String => Option[(T, String)]

def seq[A, B](pa: Parser[A], pb: Parser[B]): Parser[(A, B)] =
  { s0: String =>
    for {
      (a, s1) <- a(s0)
      (b, s2) <- b(s1)
    } yield ((a, b), s2)
  }
```

What about ambiguous grammars?

]

---
.left-column[
  ## What is a parser?
]
.right-column[
Take a string, produce **a list of** results:

```scala
type Parser[+T] = String => List[(T, String)]

def seq[A, B](pa: Parser[A], pb: Parser[B]): Parser[(A, B)] =
  { s0: String =>
    for {
      (a, s1) <- a(s0)
      (b, s2) <- b(s1)
    } yield ((a, b), s2)
  }
```

What if we want better error reporting?
]

---
.left-column[
  ## What is a parser?
]
.right-column[
Take a string, produce **either an error or some value with the remainding part of the string**:

```scala
type ParserError = String
type Parser[+T] = String => Either[ParserError, (T, String)]

def seq[A, B](pa: Parser[A], pb: Parser[B]): Parser[(A, B)] =
  { s0: String =>
    for {
      (a, s1) <- a(s0).right
      (b, s2) <- b(s1).right
    } yield ((a, b), s2)
  }
```

How about keeping the surface-level representation of the parsed value?
]

---
.left-column[
  ## What is a parser?
]
.right-column[
Take a string, produce **an optional value with the remainding part of the string and the consumed part of the string**:

```scala
type Parser[+T] = String => Option[(String, T, String)]

def seq[A, B](pa: Parser[A], pb: Parser[B]): Parser[(A, B)] =
  { s0: String =>
    for {
      (c1, a, s1) <- a(s0)
      (c2, b, s2) <- b(s1)
    } yield (c1 + c2, (a, b), s2)
  }
```

What if we want to parse an array of bytes?
]

---
.left-column[
  ## What is a parser?
]
.right-column[

```scala
type Parser[E, +T] = Stream[E] => Option[(Vector[E], T, Stream[E])]

def seq[E, A, B]
(pa: Parser[E, A], pb: Parser[E, B]): Parser[E, (A, B)] =
  { s0: Stream[E] =>
    for {
      (c1, a, s1) <- a(s0)
      (c2, b, s2) <- b(s1)
    } yield (c1 ++ c2, (a, b), s2)
  }
```
]

---
.left-column[
  ## What is a parser?
]
.right-column[

What did all of our examples have in common?

```scala
type S[+T] // Stream.
type R[+T] // Result Monad (Applicative).
type P[E, +T] = S[E] => R[(T, S[E])]
```

Take an input stream, produce some monadic value containing pairs of values and stream tails.
]

---
.left-column[
  ## Frameworks
]
.right-column[

<table>
  <tr><th>Name</th> <th>Easy</th> <th>Docs</th><th>Modifiablity</th> <th>Speed</th> </tr>
  <tr><td>WabbitParse</td> <td>+++</td> <td>---</td> <td>+++</td> <td>---</td></tr>
  <tr><td>Parboiled2</td> <td>++</td> <td>+</td> <td>---</td> <td>+++</td></tr>
  <tr><td>Fastparse</td> <td>++</td> <td>++</td> <td>-</td> <td>++</td></tr>
  <tr><td>GLL-combinators</td> <td>++</td> <td>+</td> <td>+</td> <td>+</td> </tr>
  <tr><td>scala-parser-combinators</td> <td>+++</td> <td>+++</td> <td>-</td> <td>--</td> </tr>
  <tr><td>Hand-written</td> <td>--</td> <td>+++</td> <td>--</td> <td>+++</td> </tr>
  <tr><td>Parser generators</td> <td>---</td> <td>+++</td> <td>---</td> <td>+++</td> </tr>
</table>

]

---
name: last-page
template: inverse

## The end.


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