Lately, I've been thinking a bit about the way language design influences library design. My line of thought started out inspired by some of the recent conversations about closures in Java, but it ended up also touching on dynamic typing and a few other 'modern' language features. This will end up being more than one post, but I thought I'd record some of it in my blog, with the hope that it might shed some light for somebody, somewhere.
To motivate this discussion, I'll use as an example a simple C implementation of a string-interning function, intern_string. If you're not familiar with the concept of interning, the premise is that interning two objects ensures that if they have the same value, they also have the same identity. In the case of C strings, interning ensures that if `strcmp(internstring(a), internstring(b)) == 0 holds true, then internstring(a) == internstring(b)` also holds true. Since it effectively means that each string value is only stored one time, this technique can reduce memory requirements. It also gives you a cheap string equality comparison: checking two interned strings for equality reduces to a pointer comparison, which is about as fast as it gets.
Given a hash table that compares keys by value, implementing the function string_intern is fairly simple. In the following code code, intern_table is a hash table that maps strings to themselves. hash_ref, hash_set, and hash_has are functions that manipulate the hash table:
int hash_has(hash_table_t ht, char *key) - Returns TRUE or FALSE, depending on whether or not the key key is found in the hash table ht.char *hash_ref(hash_table_t ht, char *key) - Returns the value bound to the key key by the hash table ht. If the key is not found, behavior is undefined.char *hash_set(hash_table_t ht, char *key, char *value) - Binds the value value to the key key in the hash table ht. If the key is already present, the existing value is overwritten. This function returns value.
Note the critical assumption that the hash_* accessors implement key comparison by value sementics, strcmp, rather than identity semantics, ==.
hash_table_t intern_table;
char *intern_string(char *str)
{
if (hash_has(intern_table, str))
return hash_ref(intern_table, str);
char *interned_str = strdup(str);
hash_set(intern_table, interned_str, interned_str);
return interned_str;
}
The first step of intern_string is to check to see if the intern table already contains a string with the value of the new string. If the new string is already in the intern table, then the function returns the copy that's in the table. Otherwise, a new copy of the incoming string is created and stored in the hash table. In either case, the string returned is in the the intern table. This logic ensures that every time intern_string is called with a str of the same value, it returns the same exact string.
If you haven't guessed already, the problem with this implementation of intern_string lies in the dual calls to hash_has and hash_ref. Both calls involve searching the hash table for the key: hash_has to determine if the key exists, and hash_ref to retrieve the key's value. This means that in the common case, interning a string that's already been interned, this implementaion searches the hash table twice. Double work.
Fixing this problem involves changing the calling conventions for hash_ref. One of the simplest ways to do this involves defining a specific return value that hash_ref can return in the 'key not found' case. For strings in C, NULL is a logical choice. This change to hash_ref makes it possible to remove the double search by eliminating the explicit hash_has check:
hash_table_t intern_table;
char *intern_string(char *str)
{
char *interned_str = hash_ref(intern_table, str);
if (interned_str == NULL)
{
interned_str = strdup(str);
hash_set(intern_table, interned_str, interned_str);
}
return interned_str;
}
For this string interning, this change to hash_ref interface works fairly well. We know that we'll never store a hash key with a NULL value, so we know that NULL is safe to use for signaling that a key was not found. Were this ever to change, this version of hash_ref doesn't return enough information to distinguish between the 'key not found' case and the 'NULL value' case. Both would return NULL. To fix this, hash_ref needs to be extended to also return a seperate value that indicates if the key was found. This can be done in C by having hash_ref return the 'key found' flag as a return value, and also accept a pointer to a buffer that will contain the key's value, if it's found:
hash_table_t intern_table;
char *intern_string(char *str)
{
char *interned_str;
if (!hash_ref(intern_table, str, &interned_str))
{
interned_str = strdup(str);
hash_set(intern_table, interned_str, interned_str);
}
return interned_str;
}
This is probably about as good as you can get in straight C. It easily distinguishes between the 'no-value' and 'no-key' cases, it's relatively clear to read, and it uses the common idioms of the language. However, C is a relatively sparse language. If you're willing to switch to something a bit more expressive, you have other choices.
One example of this is a choice that's particularly well supported by dynamically typed languages. With a language that can identify types at runtime, it becomes possible for hash_ref to return values of a different type if the key is not found. This value can be distinguished from other return values by virtue of the run time type identification supported by the language. In one such language, Scheme, this lets us implement intern-string like this:
(define *intern-table* (make-hash :equal))
(define (intern-string str)
(let ((interned-str (hash-ref *intern-table* str 42)))
(cond ((= interned-str 42)
(hash-set! *intern-table* str str)
str)
(#t
interned-str)))))
If you prefer C/JavaScript-style syntax, it looks like this:
var intern_table = make_hash(EQUAL);
function intern_string(str)
{
var interned_str = hash_ref(intern_table, str, 42);
if (interned_str == 42)
{
hash_set(intern_table, str, str);
return str;
}
return interned_str;
}
In this case, hash_ref has been extended with a third argument: a default return value if the key is not found. The above code uses this to have hash_ref return a number in 'no value' case, and it's the type itself of this return value that ensures its distinctness. This is a common dynamic language idiom, but for a moment, consider what it would look like in C:
hash_table_t intern_table;
char *intern_string(char *str)
{
char *interned_str = hash_ref(intern_table, str, (char *)42);
if (interned_str == (char *)42)
{
interned_str = strdup(str);
hash_set(intern_table, interned_str, interned_str);
}
return interned_str;
}
At first, this actually seems like it might a plausible implementation of intern_string. My guess is that it might even work most of the time. Where this implementation gets into trouble is the case when an interned string might reasonably be located at address 42. Because C is statically typed, When hash_ref returns, all it returns is a pointer. The caller cannot distinguish between the 'valid string at address 42' return value and the 'no-key' return value. This is basically the same problem as the case where we overloaded NULL to signal 'no-key'.
The way the dynamically typed language solved this problem is worth considering. When a dynamically typed language passes a value, what it's really doing is returning a pointer along with a few extra bits describing the type of the object being pointed to. (Runtime implementations might vary, but that's the gist of many.) Using dynamic typing to distinguish between those two possible cases really amounts to using those few extra type bits to contain 'another' return value, one holding information on whether or not the key was found. This is exactly what our 'best' C implementation does explicitly with a return value and a reference value. The dynamic typing isn't necessarily adding any expressive power, but it is giving another, concise means of expressing what we're trying to say.