type Token $=$

$|$ NOUN

$|$ VERB

$|$ ARTICLE

$|$ ADJECTIVE

$|$ ADVERB

$|$ PREPOSITION

$|$ COMMA

$|$ CONJUNCTION

$|$ EOS $//$ End of Sentence $($period$,$ exclamation point, etc.$)$

$|$ OTHER of string $//$ Could represent and ID in a more complex language, but for now, just a catch$-$all for anything else.

$//$ Member $($of the type$)$ function to get a token from a lexeme $($String$)$

static member tokenFromLexeme $=$ function

$|","->$ COMMA

$|$ "dog" $|$ "cat" $|$ "tree" $->$ NOUN

$|"$a$"|"$an$"|$ "the" $->$ ARTICLE

$|$ "chases" $->$ VERB

$|$ "small" $|$ "tall" $|$ "slow" $|$ "fast" $->$ ADJECTIVE

$|$ "quietly" $|$ "quickly" $->$ ADVERB

$|"$up$"|$ "around" $->$ PREPOSITION

$|"."|"!"|"?"->$ EOS

$|$ "and" $|"$or$"->$ CONJUNCTION

$|$ x $->$ OTHER x $//$ aka, ID

let rec parse theList $=$ theList $|>$ sentence

$//$ :

and sentence lst $=$ lst $|>$ np $|>$ vp $|>$ np $|>$ sentenceTail

and sentenceTail lst $=$

match lst with

$|$ CONJUNCTION :: xs $->$ sentence xs

$|$ EOS :: xs $->$ failwith $"End of sentence marker found, but not at end!

Remaining Tokens $\{$xs$\}"$

$|$ x :: $\_->$ failwithf $"Expected EOS but found: $\{$x$\}"$

$|[]->$ "Done!"

$//***$ This uses the matchToken function that was defined above.

$//$ ::$=$ ARTICLE NOUN

and np lst $=$

let matchNoun lst $=$

match lst with

$|$ NOUN :: xs $->$ xs

$|$ x $->$ failwith $"Expected Noun, but found $\{$x$\}."$

in

match lst with

$|$ ARTICLE :: xs $->$ xs $|>$ adjList $|>$ matchNoun $|>$ pp

$|$ x :: xs $->$ failwithf $"Expected article, but found $\{$x$\}.$

remaining tokens: $\{$xs$\}"$

$|[]->$ failwith "article should not be empty"

$//$ ::$=$ ADJECTIVE $|\backslash$epsi

and adjList $=$

function

$|$ ADJECTIVE :: xs $->$ xs $|>$ adjTail

$|$ xs $->$ xs $//\backslash$epsi is permitted, so just resolve to what was passed if no other match.

$//$ ::$=$ COMMA $|\backslash$epsi

and adjTail $=$

function

$|$ COMMA :: xs $->$ xs $|>$ adjList

$|$ xs $->$ xs $//\backslash$epsi is permitted, so just resolve to what was passed if no other match.

$//$ ::$=$ PREPOSITION $|\backslash$epsi

and pp $=$

function

$|$ PREPOSITION :: xs $->$ xs $|>$ np

$|$ xs $->$ xs $//\backslash$epsi is permitted, so just resolve to what was passed if no other match.

$//$ ::$=$ ADVERB VERB $|$ VERB

and vp lst $=$

let matchVerb lst $=$

match lst with

$|$ VERB :: xs $->$ xs

$|$ x $->$ failwithf $"Expected Verb, but found $\{$x$\}."$

in

match lst with

$|$ VERB :: xs $->$ xs

$|$ ADVERB :: xs $->$ xs $|>$ matchVerb

$|$ x :: xs $->$ failwithf $"Expected Verb Phrase, but found $\{$x$\}.$

Remaining tokens: $\{$xs$\}"$

$|[]->$ failwith "Unexpected end of input while processing Verb Phrase."

open System.Text.RegularExpressions

let main $()=$

let mapTokens $=$ Array.toList $>>$ List.map Token.tokenFromLexeme

$//$ This is very $".$NET" specific. Split is part of the $.$NET API.

$//$ KEEPS DELIMIER: let getTokenList $($str: string$)=$ Regex.Split$($str$.$Trim$(),$ @$"(?<=\backslash$s$+)")|>$ mapTokens

let getTokenList $($str: string$)=$ Regex.Split$($str$.$Trim$(),$ @$"\backslash$s$+")|>$ mapTokens

$(*$ Begin Parsing Process $*)$

let startParsing str $=$

$//$ Display our list of tokens...

printfn $$"$

Initial String: $\%$s$\{$str$\}"$

$//$ Try to parse the list of tokens and display the results.

try

let tokenList $=$ getTokenList str

printfn $"Tokens Before Parsing: $\%$A$\{$tokenList$\}"$

let parsedList $=$ parse tokenList

if $($parsedList$.$Length $>0)$ then

printfn $"Parsing Failed because we have extra tokens! $\%$A$\{$parsedList$\}"$

printfn $"Extra Tokens:$\backslash$t$\%$A$\{$parsedList$\}"$

else

printfn "Done!"

$//$ If an exception $("$failwith$")$ is thrown, display the error message.

with Failure msg $->$

printfn $"Error: $\%$s$\{$msg$\}"$

$//$ Get the user input and start parsing

let getUserInput $()=$

printf "Enter $($Space Delimited$)$ String

$=>"$

$//$ A case where it's easier to use the $.$NET ReadLine as opposed to the more restrictive OCaml native variant.

System.Console.ReadLine$()$

in

$//$ Get the user input and start parsing

getUserInput $()|>$ startParsing $|>$ ignore $//$ Just ignore the result, as we are just printing results above.

main $()$