author | Tony Garnock-Jones <tonyg@kcbbs.gen.nz> |
Thu, 31 Mar 2005 01:25:02 +1200 | |
changeset 0 | ea4e1a00864c |
permissions | -rw-r--r-- |
0
ea4e1a00864c
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Tony Garnock-Jones <tonyg@kcbbs.gen.nz>
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1 |
[comment -*- outline -*- ] |
ea4e1a00864c
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2 |
[html <div class="essay_body">] |
ea4e1a00864c
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Tony Garnock-Jones <tonyg@kcbbs.gen.nz>
parents:
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ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
Tony Garnock-Jones <tonyg@kcbbs.gen.nz>
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4 |
*** A process language with locations and reflection |
ea4e1a00864c
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Tony Garnock-Jones <tonyg@kcbbs.gen.nz>
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ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
Tony Garnock-Jones <tonyg@kcbbs.gen.nz>
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6 |
**** The Process Calculus |
ea4e1a00864c
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Tony Garnock-Jones <tonyg@kcbbs.gen.nz>
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7 |
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ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
Tony Garnock-Jones <tonyg@kcbbs.gen.nz>
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8 |
Locations are nested, as in the distributed join calculus |
ea4e1a00864c
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("Join"). Locations and channels are identified (unlike in |
ea4e1a00864c
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Tony Garnock-Jones <tonyg@kcbbs.gen.nz>
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Join). Channels/locations are restricted with normal scope |
ea4e1a00864c
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extrusion. Communication happens between locations transparently - any |
ea4e1a00864c
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unguarded receive and any unguarded send on the same channel will |
ea4e1a00864c
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occur, no matter the structure of the locations involved. |
ea4e1a00864c
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ea4e1a00864c
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15 |
The main challenge at the moment is to come up with a notion of |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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definition site for a channel, so that we have a known location for |
ea4e1a00864c
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sent messages to migrate to. This is where Join can help out: each |
ea4e1a00864c
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18 |
Join definition both provides a location and a receiver |
ea4e1a00864c
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definition. The Chemical Abstract Machine semantics for Join provides |
ea4e1a00864c
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two equivalent views of a collection of processes, one in which the |
ea4e1a00864c
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hierarchic nesting of locations is reflected in the syntax, and one in |
ea4e1a00864c
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which each location is shown in a flat multiset of locations, with the |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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[i path] to each location annotating the location: |
ea4e1a00864c
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ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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@code |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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m[n[P] | Q] | o[n[R]] (hierarchy shown) |
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ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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P@mn | Q@m | R@on (exposed processes shown) |
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, |
ea4e1a00864c
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30 |
Locations are the unit of reflection granularity. Locations can be |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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lifted to a description of the corresponding collection of processes, |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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and such a description can be dropped to instantiate the described |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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processes. Locations are written |
ea4e1a00864c
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ea4e1a00864c
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@code |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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m[P] |
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ea4e1a00864c
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38 |
where 'm' is a channel acting as a 'location tag' for this location. |
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ea4e1a00864c
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40 |
Processes P, Q: |
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ea4e1a00864c
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42 |
@code |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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43 |
0 stop |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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(x)P new |
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P | Q parallel composition |
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46 |
µA.P process variable binding |
ea4e1a00864c
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A process variable reference |
ea4e1a00864c
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48 |
x<M> output |
ea4e1a00864c
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x(M).P input |
ea4e1a00864c
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x[P] location |
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lift x lift |
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drop x drop (aka eval) |
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53 |
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Messages M are a possibly empty sequence of names. |
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Structural equivalence: |
ea4e1a00864c
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57 |
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58 |
@code |
ea4e1a00864c
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59 |
m[P] | m[Q] === m[P | Q] |
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m[x<M>] === x<M> |
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m[lift x] === lift x |
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m[0] === 0 |
ea4e1a00864c
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64 |
Reduction: |
ea4e1a00864c
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65 |
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66 |
@code |
ea4e1a00864c
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67 |
m not used as a location tag in Q |
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68 |
------------------------------------------- |
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69 |
m[P] | Q | lift m --> Q | LIFT(m,P) |
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70 |
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71 |
where LIFT(m,P) expands into a process description of the process P, |
ea4e1a00864c
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72 |
the channel for interfacing to which is placed on the channel m. |
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73 |
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74 |
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75 |
@code |
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76 |
P --> P' |
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77 |
---------------------- |
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78 |
m[P] --> m[P'] |
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79 |
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80 |
@code |
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81 |
-------------------------- |
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82 |
drop x --> DROP(x) |
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83 |
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84 |
where DROP(x) expands into a process which examines the process |
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85 |
description of a process P that answers to requests on channel x, and |
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86 |
instantiates P. |
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87 |
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88 |
Note that lift and drop are asymmetric: lift collects the description |
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89 |
for an entire location, but drop instantiates the provided description |
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90 |
into the current location. |
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91 |
|
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92 |
Note also that output and lift are continuationless and may freely |
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Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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93 |
move between locations, where input and drop have continuations |
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Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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94 |
(implicit, in the case of drop) and may not leave the confines of |
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Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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|
95 |
their location. Messages (names) are thus mobile, while processes are |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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96 |
immobile without explicit use of lift and drop. |
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97 |
|
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98 |
|
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99 |
**** Uses of Lift |
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Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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100 |
|
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101 |
Lift can be used to interrupt a running collection of processes for |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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|
102 |
inspection in an interactive debugger: |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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103 |
|
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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104 |
@code |
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105 |
m[P] | lift m | m(p).debuggerFactory<"breakpoint",p> |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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106 |
|
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107 |
Lift can (sort-of) be used for error handling - each location can be |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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|
108 |
used as a 'catch' to which the error condition is 'thrown': |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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109 |
|
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110 |
@code |
ea4e1a00864c
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111 |
m[P | m<"error description">] | m(e).(lift m | m(p).debuggerFactory<e,p>) |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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112 |
|
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113 |
Nested handlers can be defined: |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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114 |
|
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115 |
@code java |
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Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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116 |
try { |
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Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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117 |
CODE |
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118 |
} catch (e, p) { |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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119 |
HANDLER |
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120 |
} |
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Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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121 |
|
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122 |
becomes: |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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123 |
|
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Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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124 |
@code |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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125 |
(exn)( exn[CODE] | exn(e).(lift exn | exn(p).HANDLER) ) |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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126 |
|
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127 |
so long as |
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Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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128 |
|
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129 |
@code java |
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130 |
throw e |
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131 |
|
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132 |
is encoded as |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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133 |
|
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134 |
@code |
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Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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135 |
exn<e> |
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136 |
|
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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|
137 |
for the innermost defined 'exn' name. See below for some problems with |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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|
138 |
this approach. |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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139 |
|
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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|
140 |
**** Variants and challenges |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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141 |
|
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|
142 |
***** Try/Finally clauses |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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143 |
|
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|
144 |
"try/finally" clauses are a challenge, since [code m[0] === 0] and we |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
Tony Garnock-Jones <tonyg@kcbbs.gen.nz>
parents:
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|
145 |
want to detect when [code m[]] has 'stopped'. One way around it might |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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|
146 |
be to change the syntax for processes to have instead |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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147 |
|
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148 |
@code |
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149 |
x[P].Q |
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150 |
|
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151 |
for locations, with an altered structural equivalence: |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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152 |
|
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153 |
@code |
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154 |
m[P].Q | m[P'].Q' === m[P | P'].(Q | Q') |
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155 |
m[x<M>].P === m[0].P | x<M> |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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156 |
m[lift x].P === m[0].P | lift x |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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|
157 |
m[0].P =/= 0 (this rule is replaced by a new reduction relation) |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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158 |
|
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|
159 |
and altered reduction rules: |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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160 |
|
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161 |
@code |
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162 |
m not used as a location tag in Q |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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|
163 |
------------------------------------------------- |
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Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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|
164 |
m[P].P' | Q | lift m --> Q | LIFT(m,P,P') |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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165 |
|
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|
166 |
where LIFT(m,P,P') expands into a process description of the processes |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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|
167 |
P and P', the channels for interfacing to which (p and p' |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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|
168 |
respectively) are placed on the channel m as: [code m<p,p'>] |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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169 |
|
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170 |
|
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171 |
@code |
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|
172 |
P --> P' |
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173 |
-------------------------- |
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174 |
m[P].Q --> m[P'].Q |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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175 |
|
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176 |
|
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177 |
@code |
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178 |
m not used as a location tag in Q |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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179 |
------------------------------------- |
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180 |
m[0].P | Q --> P | Q |
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Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
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181 |
|
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***** Dynamic environments |
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183 |
|
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184 |
The current exception handler ('exn' above) is a dynamically-scoped |
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entity in most languages, but in the discussion of error-handling |
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186 |
above it is [i lexical] since there is no clear notion of dynamic |
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scope. This is unacceptable for use as an error-reporting mechanism. |
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188 |
|
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189 |
Note that the current continuation is a dynamic entity as well! Since |
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190 |
the current continuation is explicitly passed around in encodings of |
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191 |
lambda calculi into π, there's nothing stopping us passing around |
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192 |
arbitrary other dynamic entities at the same time, so long as our |
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encoding is uniform. This would give rise to a [code |
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194 |
call-with-current-exception-handler] by analogy with [code |
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195 |
call-with-current-continuation]. |
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196 |
|
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197 |
Traditional continuation-passing-style: |
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198 |
|
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199 |
@code scheme |
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(lambda (x) (+ x 1)) --> (lambda (k x) (+ k x 1)) |
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201 |
|
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202 |
Extended continuation-and-exception-handler-passing-style: |
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203 |
|
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@code scheme |
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(lambda (x) (+ x 1)) --> (lambda (k h x) (+ k h x 1)) |
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206 |
|
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207 |
This could be generalised to a (perhaps implicit at the implementation |
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208 |
level) collection of arbitrary dynamic state. |
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209 |
|
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210 |
***** Error or exception reporting, revisited |
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211 |
|
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212 |
A dynamic-extent error-reporting channel can be combined with the use |
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213 |
of [code lift] to give a more acceptable form of exception |
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214 |
handling. When throwing an exception, care must be taken to select an |
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215 |
appropriate channel; procedural code can use [code |
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216 |
call-with-current-error-channel] of course, but raw process code needs |
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217 |
to be more explicit about the target of the thrown exception. |
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218 |
|
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219 |
There are still a few issues: "stack traces" are still missing from |
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220 |
the picture, for instance. An exception thrown by some library code in |
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221 |
a different lexical scope will have to be careful to provide |
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222 |
self-describing restarts as part of the error report message sent down |
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223 |
the error-reporting channel at the time of the throw. This is |
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224 |
straightforward in a procedural situation, but in the general case it |
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225 |
is less obvious how to do this. |
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226 |
|
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227 |
One approach might be for each ongoing computation to be partitioned |
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228 |
into a fresh location: |
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229 |
|
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230 |
@code |
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231 |
µLOOP . |
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232 |
service(k,h,message) . |
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233 |
LOOP | |
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234 |
(loc) loc[ BODY OF SERVICE | |
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235 |
h<"error description", k, loc> ]) |
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236 |
|
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237 |
When an error is signalled, the location can be passed (with some |
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238 |
suitable self-describing convention) along with the error report to |
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239 |
the waiting handler on the other side of the report channel. The |
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240 |
location need not be lifted at the time of the throw; whether it was |
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241 |
lifted at all might be a policy decision made by the error-report |
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242 |
receiver at the other side. Often the location might be lifted into |
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243 |
the debugger along with the location that sent the message to the |
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244 |
library code that caught the error. |
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245 |
|
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246 |
***** Capabilities |
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247 |
|
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248 |
Channels, so long as they are unforgeable, can be used as capabilities |
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249 |
in a few different ways. One relies on an equivalence relation between |
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250 |
names and a comparison process |
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251 |
|
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252 |
@code |
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253 |
[x=y]{P,Q} |
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254 |
|
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255 |
which reduces to [code P] if [code x] and [code y] are the same name, |
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256 |
and [code Q] otherwise. Names (without processes attached to them |
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257 |
necessarily) can then be used as permissions. |
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258 |
|
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259 |
The dynamic environment idea detailed above can then be used to carry |
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260 |
around a collection of permissions. A function [code |
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261 |
call-with-current-permission-map] can be provided, allowing a dynamic |
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262 |
permissions check in server code. Some means of replacing the current |
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263 |
permission map within a piece of code can also be provided, allowing |
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264 |
code to enlarge or reduce its permission map within some scope. |
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265 |
|
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266 |
***** A variation on comm |
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267 |
|
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268 |
The current comm rules (implied above) are not well pinned down since |
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269 |
there's a structural equivalence rule equating [code m[x<M>]] with |
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270 |
[code x<M>]. This means that [code lift m] may or may not capture the |
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271 |
output on [code x]. |
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272 |
|
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273 |
We need some way of forcing unguarded sends out of a location at the |
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274 |
time of lifting that location. One way to approach that might be to |
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275 |
redefine the lift operation to partition the location's contents: |
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276 |
|
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277 |
@code |
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278 |
m not used as a location tag in Q |
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279 |
no sends on any channel unguarded in P |
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280 |
------------------------------------------------- |
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281 |
m[P].P' | Q | lift m --> Q | LIFT(m,P,P') |
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282 |
|
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283 |
[b Alternative:] there are still problems, since now outputs are |
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284 |
opaque to the reflection primitive, lift. A better way might be to try |
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285 |
to attach the output messages to the restriction itself, thus |
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286 |
modelling the message queue directly. The problem then is making any |
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287 |
unguarded inputs find the messages from the relevant restriction! |
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288 |
|
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289 |
***** Dynamic Environments of a different kind |
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290 |
|
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291 |
To get lookup on "local services" (eg. java.lang.String etc) perhaps |
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292 |
lift and drop should be augmented with a second argument: ports equal |
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293 |
(via name equality) to this argument would be replaced with a special |
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294 |
piece of syntax in the lift, and in the drop, the special piece of |
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295 |
syntax would be replaced with the value of the argument. This lets you |
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296 |
provide eg. a sandbox environment or whatever. |
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297 |
|
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298 |
|
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299 |
**** Integrating lambda with π |
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300 |
|
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301 |
See also the "blue calculus" by G. Boudol. |
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302 |
|
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303 |
Thinking about evaluating Scheme using an operational semantics: |
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304 |
evaluate each position in a combination until the positions are values |
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305 |
rather than expressions. Then apply the combination. |
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306 |
|
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307 |
@code scheme |
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308 |
(let ((x (lambda (y) (+ y 1)))) |
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309 |
(x (x 2))) |
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310 |
;==> macroexpands to |
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311 |
((lambda (x) (x (x 2))) |
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312 |
(lambda (y) (+ y 1))) |
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313 |
;--> |
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314 |
((lambda (y) (+ y 1)) ((lambda (y) (+ y 1)) 2)) |
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315 |
;--> |
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316 |
((lambda (y) (+ y 1)) (+ 2 1)) |
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317 |
;--> |
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318 |
((lambda (y) (+ y 1)) (#<primitive+> 2 1)) |
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319 |
;--> |
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320 |
((lambda (y) (+ y 1)) 3) |
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321 |
;--> |
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322 |
(+ 3 1) |
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323 |
;--> |
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324 |
(#<primitive+> 3 1) |
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325 |
;--> |
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326 |
4 |
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327 |
|
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328 |
***** Syntax |
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329 |
|
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330 |
Recommend an A-normal form - [code let x = M N in x] (??) - to provide |
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331 |
linear form. Note that's a let, not a letrec. I guess this is pretty |
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332 |
similar to Boudol's Blue Calculus? |
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333 |
|
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334 |
Also, how about Matthias' well-formedness constraint to ensure |
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335 |
appropriate use of input capability? How does the input capability |
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336 |
move with lift and drop? Does the location for the port move with the |
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337 |
process?? |
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338 |
|
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339 |
Processes P, Q: |
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340 |
|
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341 |
@code |
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342 |
0 stop |
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343 |
(x)P new |
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344 |
P | Q parallel composition |
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345 |
µA.P process variable binding |
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346 |
A process variable reference |
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347 |
x<M> output |
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348 |
x(M).P input |
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349 |
x[P].Q location |
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350 |
lift x lift |
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351 |
drop x drop (aka eval) |
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352 |
|
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353 |
Messages <M>: |
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354 |
|
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355 |
Bindings (M): |
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356 |
|
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357 |
Expressions E, F: |
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358 |
|
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359 |
Values V, W are |
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360 |
|
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361 |
The rules, then, are |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
Tony Garnock-Jones <tonyg@kcbbs.gen.nz>
parents:
diff
changeset
|
362 |
|
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
Tony Garnock-Jones <tonyg@kcbbs.gen.nz>
parents:
diff
changeset
|
363 |
@code |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
Tony Garnock-Jones <tonyg@kcbbs.gen.nz>
parents:
diff
changeset
|
364 |
\x . |
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
Tony Garnock-Jones <tonyg@kcbbs.gen.nz>
parents:
diff
changeset
|
365 |
|
ea4e1a00864c
Initial version, from TLA arch@eighty-twenty.org--2004/smalltalk-tng--main--0--version-0
Tony Garnock-Jones <tonyg@kcbbs.gen.nz>
parents:
diff
changeset
|
366 |
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