# Adding string matching to chez-docs

I recently wrote a little library, `chez-docs`, to make accessing documentation easier while learning Chez Scheme (blog post). The main procedure, `doc`, in `chez-docs` only returns results for exact matches with `proc`.1 To aid in discovery, I’ve added a procedure, `find-proc`, that provides exact and approximate matching of search strings.

### Levenshtein Distance

My initial thought was that I should approach this problem with approximate string matching. After a little searching, I learned that Levenshtein distance was one of the simplest approaches to calculate the distance between two strings. This excellent blog post included a few MATLAB implementations of Levenshtein distance algorithms2 that were relatively easy for me to follow because of my experience with MATLAB and R.

I first implemented the recursive algorithm3 thinking that it would be most natural in Scheme, but it was unacceptably slow. I then implemented the iterative two-row algorithm and found the performance to be sufficiently snappy for my needs.

``````(define (lev s t)
(let* ([s (list->vector (string->list s))]
[t (list->vector (string->list t))]
[m (vector-length s)]
[n (vector-length t)]
[y (list->vector (make-list (add1 n) 0))])
((= i m))
(vector-set! y 0 i)
((= j n))
(let ([c (if (char=? (vector-ref s i) (vector-ref t j)) 0 1)])
(+ c  (vector-ref x j))))))
;; swap x and y
(let ([tmp x])
(set! x y)
(set! y tmp)))
(vector-ref x n)))``````

This is the first time that I’ve used `do` loops in Scheme. In the example below, the looping index `i` is initialized to zero and incremented by one on each pass through the loop. The loop is exited when `(= i 10)`. The (sort of) equivalent syntax in R is `for (i in 0:9) print(i)`.

``````> (do ((i 0 (add1 i)))
((= i 10))
(display (string-append (number->string i) " ")))
0 1 2 3 4 5 6 7 8 9``````

`lev` tallies the numbers of insertions, deletions, and substitutions; a value of zero indicates an exact match.

``````> (map (lambda (x) (lev "head" x)) '("head" "read" "load" "list-head"))
(0 1 2 5)``````

### Exact Substring Matching

`doc` uses `assoc` to find any exact matches of the full string in the list of procedures. After working with the Levenshtein distance, I realized that exact matching of substrings would generally be more useful than fuzzy matching. I wrote the `string-match` procedure to test if a search string is present in the target string.

``````(define (string-match s t)
(define (loop s-list t-sub)
(cond [(null? s-list) #t]
[(< (length t-sub) (length s-list)) #f]
[(char=? (car s-list) (car t-sub))
(loop (cdr s-list) (cdr t-sub))]
[else #f]))
(let* ([s-list-temp (string->list s)]
[starts-with? (char=? (car s-list-temp) #\^)]
[s-list (if starts-with? (cdr s-list-temp) s-list-temp)]
[t-list (string->list t)])
(cond [(and starts-with? (not (char=? (car s-list) (car t-list)))) #f]
[(not (for-all (lambda (x) (member x t-list)) s-list)) #f]
[else (loop s-list (member (car s-list) t-list))])))``````

`member` is the workhorse of `string-match`. It’s an interesting turn for me because when I first started using `member` in my Scheme code I was puzzled by why it didn’t work like `%in%` in R. For example, `(member 2 '(1 2 3))` returns `(2 3)`, but `2 %in% c(1, 2, 3)` returns `TRUE`. Because all values other than `#f` count as `#t` in Scheme, `member` can be used as a predicate, e.g., `(if (member 2 '(1 2 3)) 1 0)` returns `1`. Nonetheless, it wasn’t obvious to me how `member`’s behavior was useful…until I started writing `string-match`. Those experiences make programming fun.

`string-match` returns a boolean value.

``````> (map (lambda (x) (string-match "head" x)) '("head" "read" "load" "list-head"))
(#t #f #f #t)``````

### Procedure Discovery

`find-proc` takes a `search-string` and two optional arguments, `max-results` and `fuzzy?`, which default to `10` and `#f`, respectively.

``````(define find-proc
(case-lambda
[(search-string) (find-proc-helper search-string 10 #f)]
[(search-string max-results) (find-proc-helper search-string max-results #f)]
[(search-string max-results fuzzy?) (find-proc-helper search-string max-results fuzzy?)]))``````

`find-proc-helper` maps either `lev` or `string-match` to the full list of procedures, `proc-list`, and then sorts or filters the results, respectively.

``````(define (find-proc-helper search-string max-results fuzzy?)
(unless (string? search-string)
(assertion-violation "(find-proc search-string)" "search-string is not a string"))
(cond [fuzzy?
(let* ([dist-list (map (lambda (x) (lev search-string x)) proc-list)]
[dist-proc (map (lambda (dist proc) (cons dist proc)) dist-list proc-list)]
[dist-proc-sort (sort (lambda (x y) (< (car x) (car y))) dist-proc)])
(prepare-results dist-proc-sort max-results))]
[else
(let* ([bool-list (map (lambda (x) (string-match search-string x)) proc-list)]
[bool-proc (map (lambda (bool proc) (cons bool proc)) bool-list proc-list)]
[bool-proc-filter (filter (lambda (x) (car x)) bool-proc)])
(prepare-results bool-proc-filter max-results))]))

(define (prepare-results ls max-results)
(let* ([len (length ls)]
[max-n (if (> max-results len) len max-results)])

I first realized that Levenshtein distance might not be very useful for `find-proc` when searching for `head`, a commonly used procedure in R.

``````> (find-proc "head" 5 #t)

However, substring matching points us to the relevant function, `list-head`, in Chez Scheme.

``````> (find-proc "head" 5)

Fuzzy matching is useful, though, for discovery when there are options with similar forms, e.g., `hash-table?` and `hashtable?`.

``````> (find-proc "hash-table?" 3)
("hash-table?")
> (find-proc "hash-table?" 3 #t)
("hash-table?" "hashtable?" "eq-hashtable?")``````

The `^` indicates that only search strings found at the start of the procedure should be returned.

``````> (find-proc "map")
("andmap" "hash-table-map" "map" "ormap" "vector-map")
> (find-proc "^map")
("map")

> (find-proc "file" 3)
> (find-proc "^file" 3)
("file-access-time" "file-buffer-size" "file-change-time")

> (find-proc "let" 5)
("delete-directory" "delete-file" "let*" "let*-values" "let-syntax")
> (find-proc "^let")
("let*" "let*-values" "let-syntax" "let-values" "letrec" "letrec*" "letrec-syntax")``````

Under fuzzy matching, the `^` is included as part of the Levenshtein distance calculation and, thus, should not be included in search strings when using fuzzy matching.

``````> (find-proc "map" 5 #t)
("map" "max" "*" "+" "-")
> (find-proc "^map" 5 #t)
("map" "max" "car" "exp" "memp")``````

1. `proc` is shorthand for procedure, but not all of the items in `chez-docs` are procedures, e.g., `&assertion`.↩︎

2. The MATLAB post linked to implementations of Levenshtein distance in other languages, including Scheme, but the Scheme example was hard for me to follow so I set it aside.↩︎

3. After translating the MATLAB version of the recursive algorithm to Chez Scheme, I realized that a recursive example was available on Rosetta Code.↩︎ 