4 ms·

The content of the $$ is a string so it can be anything including invalid variable names or unknown variables. There is nothing the compiler can do. In C it's t

The content of the $$ is a string so it can be anything including invalid variable names or unknown variables. There is nothing the compiler can do. In C it's totally deterministic.

fiddlerwoaroof 8y ago
This isn't true: a C program could read a pointer address from the user, convert it to an int and store it in the two-star pointer, then dereference it (this is essentially what gdb's `x` command does). All this is essentially a double hashtable lookup (assuming the lexical environment is implemented as a hashtable, but if it's something else, it's similarly trivial): environment[environment["i"]] = 3 People have been compiling extremely dynamic languages since the Lisps, at the very least; and, assembly itself isn't really a "statically typed" language.
- mikeash 8y ago
  Such a program invokes undefined behavior and thus isn't a correct C program.
  - fiddlerwoaroof 8y ago
    Hmm, as far as I can tell, if you cast an appropriately sized unsigned integer to a pointer type and then dereference the pointer, the result is only UB if the integer isn’t the address of an initialized memory location compatible with the target pointer type.
munk-a 8y ago
In C the variable to be dereferenced can be uninitialized or it can result in a segfault. $$ is similar though the potential space indicated by an arbitrarily long string of characters is much larger than the potential space indicated by an integer or long - really I think PHP has an advantage here since it only needs to reference it's internal symbol table which it has full control over vs. memory at large where an erroneous symbol could actually be in a program's data memory and look like it's correct - a typo'd var var would have to be either a valid symbol name or necessarily be known to be bad.
wvenable 8y ago
You just have the compiler add all local variables into a dictionary of pointers keyed on the variable name whenever $$ appears in scope. Then compile all $$ as the dictionary lookup.

The content of the $$ is a string so it can be anything including invalid variable names or unknown variables. There is nothing the compiler can do. In C it's totally deterministic.

fiddlerwoaroof 8y ago

This isn't true: a C program could read a pointer address from the user, convert it to an int and store it in the two-star pointer, then dereference it (this is essentially what gdb's `x` command does). All this is essentially a double hashtable lookup (assuming the lexical environment is implemented as a hashtable, but if it's something else, it's similarly trivial): environment[environment["i"]] = 3 People have been compiling extremely dynamic languages since the Lisps, at the very least; and, assembly itself isn't really a "statically typed" language.

mikeash 8y ago
Such a program invokes undefined behavior and thus isn't a correct C program.
- fiddlerwoaroof 8y ago
  Hmm, as far as I can tell, if you cast an appropriately sized unsigned integer to a pointer type and then dereference the pointer, the result is only UB if the integer isn’t the address of an initialized memory location compatible with the target pointer type.

munk-a 8y ago

In C the variable to be dereferenced can be uninitialized or it can result in a segfault. $$ is similar though the potential space indicated by an arbitrarily long string of characters is much larger than the potential space indicated by an integer or long - really I think PHP has an advantage here since it only needs to reference it's internal symbol table which it has full control over vs. memory at large where an erroneous symbol could actually be in a program's data memory and look like it's correct - a typo'd var var would have to be either a valid symbol name or necessarily be known to be bad.

wvenable 8y ago

You just have the compiler add all local variables into a dictionary of pointers keyed on the variable name whenever $$ appears in scope. Then compile all $$ as the dictionary lookup.