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digitalmars.D.learn - TDPL dictionary example - error


I just joined here, so sorry if this has been posted before.

I'm reading TDPL and the example on page 8 doesn't compile.  I'm using the
latest GDC with GCC 4.4.5.  I've checked the errata, but nothing for this

import std.stdio;
import std.string;

void main(){

     size_t[char[]] dictionary;
    foreach(line; stdin.byLine()){
              foreach(word; splitter(strip(line))){
                          if(word in dictionary) continue;
                    auto newID = dictionary.length;
                          dictionary[word] = newID;
                    writeln(newID, '\t', word);

This is the error:
dictionary.d:12: Error: associative arrays can only be assigned values with
immutable keys, not char[]

If i use immutable keys, it works, but it defeats the purpose.  So what's
wrong with the code?
Dec 23 2010