From: Jeffrey Schwab <jeff@...>
Date: 2005-12-17T13:03:46+09:00
Subject: Re: Assigning a block to a variable in Ruby

ajmayo@my-deja.com wrote:
> Technically, Ruby is really an 'incremental compiler'. That there is no
> final code generation step actually doesn't change this.
> 
> Normally we make the following distinctions
> 
> An INTERPRETER reads source code and processes each symbol in the code
> each time it encounters the symbol. This is very inefficient, of
> course. Thus a loop with an inner expression will require that the
> inner expression be parsed and transformed into an evaluation stack
> each time it is encountered. Early versions of BASIC were pure
> interpreters, and performance was consequently far removed from native
> code.
> 
> An INCREMENTAL COMPILER (most scripting languages fall into this
> category) transforms source into an internal structure with a symbol
> table and expression stacks etc as it reads the source. Subsequently
> this structure is modified, but not discarded, if the source is changed
> or additional source code is fed into the system. (The first popular
> example of an incremental compiler was Microsoft QuickBasic, as far as
> I can recall. At the time, this was a miracle to behold). The internal
> representation can then be 'executed', usually by what is in effect a
> virtual machine within the system.
> 
> A TRUE COMPILER normally operates as a batch process. All the tasks
> that an incremental compiler performs are done once on the whole source
> tree. Subsequently, the code generator emits code either for the native
> hardware architecture or for a virtual machine. Source code changes
> require that the entire batch process be repeated.
> 
> There are also 'just in time' compilers for virtual machines which can,
> at runtime, evaluate 'hotspots' in the code which would benefit from
> further compilation - more technically, translation - into native
> machine code.
> 
> It is because modern scripting languages are incremental compilers and
> not merely interpreters that their performance is so good. If they were
> pure interpreters they would be hundreds of times slower.
> 
> Possibly Wikipedia could benefit from some clarification in its
> explanation.
> 

Don't forget "dynamic translation," a la Smalltalk.  Code is compiled 
for a virtual machine, but all VM instructions (not just hot spots) are 
translated into native instructions the first time they are encountered.