This Plait code requires debugging. Explain why the error is ocurring and return a fully functional solution.

ERROR: String: expected an expression, found a type in: String

CODE:

#lang plait

;; Define a type for a single digit $(0-9)$

type digit $=0|1|2|3|4|5|6|7|8|9$

;; Define a type for three digits $($a sequence of $3$ digits$)$

type three$-$digits $=[$digit digit digit$]$

;; Define a type for four digits $($a sequence of $4$ digits$)$

type four$-$digits $=[$digit digit digit digit$]$

;; Define a type for the area code $(787$ or $939)$

type area$-$code $=787|939$

;; Define a type for the phone number $($format: xxx$-$xxx$-$xxxx$)$

type phone$-$number $=$ String

;; Function to validate the phone number

$($: validate$-$phone$-$number $($String $->$ Boolean$))$

$($define $($validate$-$phone$-$number phone$)$

$($define phone$-$pattern $($regexp $"^\backslash \backslash (787\backslash \backslash |939\backslash \backslash )-[0-9]\{3\}-[0-9]\{4\}$$$"))$

$($regexp$-$match? phone$-$pattern phone$))$

;; Test cases

$($validate$-$phone$-$number $"787-123-4567")$ ;; should return #t $($true$)$

$($validate$-$phone$-$number $"939-987-6543")$ ;; should return #t $($true$)$

$($validate$-$phone$-$number $"800-123-4567")$ ;; should return #f $($false$)$

$($validate$-$phone$-$number $"787-12-4567")$ ;; should return #f $($false$)$