From: ajmayo@...
Date: 2005-12-16T23:22:44+09:00
Subject: Re: Assigning a block to a variable in Ruby

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.