04a-syntax

Module 04a: Syntax (grammars, ASTs, and parsers)

Write your team names here:
You may again choose whoever you want to start as the driver. Write your choice here:

Remember:

Be sure that your module loads into GHCi with no errors before turning it in.
Write in complete sentences, with capital letters and punctuation. Presentation matters!

{-# LANGUAGE GADTSyntax #-}

import Data.Char   -- so you can use the 'isDigit' function later

Some examples

Example 1

<mirror> ::= '.'
           | 'L' <mirror> 'R'

Example strings that match <mirror>:

"."
"L.R"
"LL.RR"

Example strings that do not match <mirror>:

"Q"
"LR"
"LL.RRR"

Example 2

<tree> ::= '#'
         | '(' <tree> ')'
         | '(' <tree> <tree> ')'

Example strings that match <tree>:

"(##)"
"((#)(#(##)))"

Example strings that do not match <tree>:

"((#)"
"(###)"

Example 3

<digit>   ::= '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9'
<natural> ::= <digit> | <digit> <natural>
<integer> ::= <natural> | '-' <natural>

Example strings that match <integer>:

"0023"
"-25"

Example strings that do not match <integer>:

"x27"
"--25"
"4-2"

For each of <mirror>, <tree>, and <integer>, give three more examples of strings that match, and three more examples that do not match.
What does | mean? (Note for this question and the next: this is not Haskell syntax! Just say what you think these notations mean based on the examples above.)
What is the difference between something in single quotes (like ’*’) and something in angle brackets (like <tree>)?

The things in single quotes are usually called terminals, and the things in brackets are called nonterminals. These sorts of definitions are known as (context-free) grammars, written in Backus-Naur Form or Backus Normal Form (BNF), named for John Backus and Peter Naur.

In what context was BNF first developed?
What else are John Backus and Peter Naur known for?

Now, back to your regularly scheduled grammars…

Does <natural> match the empty string ""? Why or why not?
An alternative, equivalent way to define <natural> is as follows:
```
  <natural> ::= <digit>+
```
Given that this is equivalent to the original definition, what do you think + means?

Technically this sort of + notation was not included in the original form of BNF, but it is a common extension.

<natural> ::= <digit>* would match all the same strings as <digit>+, but also matches the empty string. What do you think * means in this context?
Describe how to modify the definition of <natural> so it does not allow unnecessary leading zeroes. That is, "203" should still match but "0023" should not; however, "0" should still be a valid <natural>. If you wish, you can also introduce more definitions or modify definitions besides <natural>.
Write down a grammar (as concisely as possible) that matches all these strings:
```
  "(XX)Y"
  "(XXXX)YZ"
  "(XX)ZZZZZ"
  "(XXXXXX)YZZ"
```
…but does not match any of these strings:
```
  "()Y"
  "(XXX)YZ"
  "(X)ZZZ"
  "(XX)YY"
```

Mirror, mirror

ROTATE ROLES and write the name of the new driver here:

{-
   <mirror> ::= '.'
              | 'L' <mirror> 'R'
-}

data Mirror where
  Middle :: Mirror
  Layer  :: Mirror -> Mirror
  deriving Show

prettyMirror :: Mirror -> String
prettyMirror Middle    = "."
prettyMirror (Layer m) = "L" ++ prettyMirror m ++ "R"

parseMirror :: String -> (Mirror, String)
parseMirror ('.' : rest) = (Middle, rest)
parseMirror ('L' : rest) =
  case parseMirror rest of
    (m, 'R' : rest') -> (Layer m, rest')

Write down three different example values of type Mirror.
Try calling prettyMirror on your example Mirror values above, and record the results.
For this language, how are the concrete syntax (represented by the grammar <mirror>) and abstract syntax (represented by the data type Mirror) different?
Try calling parseMirror on five different example inputs. An example input can be any String. Try to pick a variety of examples that show the range of behavior of parseMirror. Record the results here.
Describe the behavior of parseMirror. Your answer should refer to the grammar <mirror>.
Why does parseMirror return a (Mirror, String) pair instead of just a Mirror? (Hint: if you are not sure, try writing a function parseMirror2 :: String -> Mirror which behaves the same as parseMirror but does not return the extra String.)
Modify parseMirror so that it has type String -> Maybe (Mirror, String) and never crashes. Instead of crashing on inputs that do not match <mirror>, it should return Nothing. Call your modified function parseMirrorSafe and write it below.