Making Environments Accessible in Common Lisp

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Making Environments Accessible in Common Lisp
By Duane Rettig
June, 2005
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What is An Environment?

• A set of bindings of names to values
• A context within which to work
• Parameterizations of behaviors

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Need for Lexical Environments
(defun foo (x y z) (flet ((bar (x) (1
x)) (bas (y) ( x y))) ( x (bar y)
(baz z))))
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Need for Lexical Environments
(let ((x (foo))) ... (let ((x (bar)))
... ) ... )
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Need for Dynamic Environments
In lisp (defmacro foo (x) (car ,x)) In
a file (defmacro foo (y) (1 ,y)) (defun bar
(x) (isqrt (foo x)))
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Typical Implementation of Environments
(let ((x 10)) ... (symbol-macrolet ((x
'foo)) ... (let ((x 20)) ...
here ))) venv lt (... (x . 20)
... (x .sm-marker . foo) ... (x
. 10) ...)
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Problems with list-style Environments

• Environment has no identity
• Multiple lists are necessary
• Objects not distinguishable as environments
• Accesses can be long for deep contour nesting
• Persistence and contour structure not automatic

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Desired Representation of Lexical Environment
Elements
Old
x
x
x
Search direction
Hash
New
Contours
Search direction
(lexical 20)
x
(symbol-macro foo)
(lexical 10)
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Environments Access Design Goals (part 1)

• Compatible with Common Lisp Spec where possible
• Open Source
• Start with CLtL2 definition, moving away only
  when necessary
• Move the Language forward
• Allow the compiler to be more transparent
• Allow portability of optimizations

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Environments Access Design Goals (part 2)

• Fast (building, access)
• Portable (on several levels)
• Controllable Persistence
• describes the contour forms
• easy to conceptualize the program structure
• allow multiple entry and annotation for multiple
  compiler passes and for debuggers
• Consistent for compilation and interpretation

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Major Departures from Environments in CLtL2

• Entries are bindings, not just names
• Environments have kinds
• Dynamic environment component is added
• Augment-environment can reuse object
• Declarations are done differently
• Define-declaration has different syntax
• Declarations sometimes added after variable
  bindings

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Pictorial View of Environment Objects
e8
lexical namespaces
base
1 3
e7
1 1 1
e6
2 1
e5
1 1
e4
3
e3
2
e2
Dynamic namespaces
1
e1
nil
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Environment Kinds

• Interpreter (for lisps with interpreter)
• Compilation (compilation-walking in 7.0)
• Evaluation
• Compiler (compilation in 7.0)
• Macros-only

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Variables Associated with Environments

• interpreter-environment
• compilation-unit-environment
• compile-file-environment in Allegro CL 7.0
• evaluation-environment

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interpreter-environment Usage

• Only available when an interpreter exists
• Hard to capture - REPL continuously binds

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compilation-unit-environment Usage

• Created when new compile-file or
  with-compilation-unit established
• Can be built with either
• sysmake-compilation-unit-environment or
• sysmake-compile-file-environment (deprecated)

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evaluation-environment Usage

• Read-only feel - definitions taken from
  environment but stored globally

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Extensions Beyond the Spec

• Compiler environments
• Extra arg to macroexpand/macroexpand-1 for
  special operators
• Environments extend to compilation-unit

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To Do

• Split define-declaration to allow redefinitions
  and development
• Add expression-information and expression
  argument to augment-environment
• Encourage CL vendors to make use of this module
• Accept suggestions

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Levels of Portability/Porting

• Level 0
• Level 1
• Level 2
• Level 3
• Level 4

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Levels of Portability/Porting

• Level 0
• Implementation has its own proprietary (open or
  not) representation
• Level 1
• Level 2
• Level 3
• Level 4

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Levels of Portability/Porting

• Level 0 Proprietary
• Level 1
• Compilation and operation of the basic
  functionality
• Level 2
• Level 3
• Level 4

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Levels of Portability/Porting

• Level 0 Proprietary
• Level 1 Basic
• Level 2
• Current environment emulated at the Environments
  Access interface level
• Level 3
• Level 4

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Levels of Portability/Porting

• Level 0 Proprietary
• Level 1 Basic
• Level 2 Emulation
• Level 3
• Environments Access holds the information,
  Proprietary interface queries it
• Level 4

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Levels of Portability/Porting

• Level 0 Proprietary
• Level 1 Basic
• Level 2 Emulation
• Level 3 Transitional
• Level 4
• Environments Access fully integrated into
  compiler, optionally interpreter
• proprietary representation unused or deprecated

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Levels of Portability/Porting

• Level 0 Proprietary
• Level 1 Basic
• Level 2 Emulation
• Level 3 Transitional
• Level 4 Full

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Current Levels of Support
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Current Links

• http//franz.com/support/documentation/7.0/doc/env
  ironments.htm
• (http//www.lispwire.com/entry-proganal-envaccess-
  des)

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Look at the Implementation (switch to Linux)

• Bring up lisps and demonstrate Environment
  building/access
• Look at source code for major functions

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Thank You.