Is Troff Turing complete?
Troff supports both macro definitions using .de
and branching using .if
(see pages 5 and 6 of the Troff user's manual). In these two respects, it is very much like TeX. However, I don't know of highly complex programs written in Troff (unlike say TikZ for TeX). Is Troff Turing complete?
roff
add a comment |
Troff supports both macro definitions using .de
and branching using .if
(see pages 5 and 6 of the Troff user's manual). In these two respects, it is very much like TeX. However, I don't know of highly complex programs written in Troff (unlike say TikZ for TeX). Is Troff Turing complete?
roff
add a comment |
Troff supports both macro definitions using .de
and branching using .if
(see pages 5 and 6 of the Troff user's manual). In these two respects, it is very much like TeX. However, I don't know of highly complex programs written in Troff (unlike say TikZ for TeX). Is Troff Turing complete?
roff
Troff supports both macro definitions using .de
and branching using .if
(see pages 5 and 6 of the Troff user's manual). In these two respects, it is very much like TeX. However, I don't know of highly complex programs written in Troff (unlike say TikZ for TeX). Is Troff Turing complete?
roff
roff
asked Nov 20 '18 at 4:25
theindigamertheindigamer
2161312
2161312
add a comment |
add a comment |
2 Answers
2
active
oldest
votes
ESR's The Art of Unix Programming claims it is:
We'll examine troff in more detail in Chapter 18; for now, it's
sufficient to note that it is a good example of an imperative
minilanguage that borders on being a full-fledged interpreter (it has
conditionals and recursion but not loops; it is accidentally
Turing-complete).
("Accidentally" as opposed to m4
, which is said to be "deliberately Turing-complete".)
add a comment |
Yes, troff is Turing-complete. It supports arbitrary recursion and conditional branching, which is sufficient. It also has registers and various other ways to store data, which gives you another path in again.
Turing completeness doesn't imply that highly complex programs are practical - just that they're theoretically possible, somehow, at some level of remove - and nor does its absence imply that they aren't, so neither troff's being Turing-complete nor the absence of complex programs don't suggest anything much one way or the other about that.
Turing completeness is not, generally, a property that means anything useful for you the user. All it means is that you can simulate a Turing machine with it, not that you'd want to, and not that the output that you'd get from it is anything like what you'd expect to read. The input or output might just be a number, or even the number of times something appears, rather than something useful, and the sorts of machine you end up simulating and their programs are often barely comprehensible to start with.
Many languages and systems are incidentally Turing-complete but not reasonably applicable for any actual programming in that subset (for example, Conway's Game of Life or CSS), and some languages that are useful for real programming are not Turing-complete (for example, Agda). The defining characteristics really are that you can
- keep going forever
- remember as much data as you want
- choose what, if anything, to do next
Often those properties - particularly non-termination - are actually undesirable, possibly including for troff. Outside of theoretical computer science and language design, Turing completeness is not a terribly interesting property virtually of the time, despite being catchy.
Yes of course, it is not a very useful thing by itself but it is still interesting - e.g. even the mov instruction is Turing complete.
– theindigamer
Nov 20 '18 at 4:38
4
@theindigamer - x86'smov
instruction is Turing-complete. (Because of addressing modes that let you use lookup tables, and the same mnemonic is used for load, store, and mov-immediate to register.) On many other ISAs that have amov
instruction (e.g. ARM) it's just a reg-reg move and isn't turing-complete. (Although on ARM it can do shifts/rotates.) Also, you actually need ajmp
to create a loop around your block ofmov
instructions, unless you're in 16-bit mode where the instruction pointer can wrap around in a 64k code segment to loop implicitly.
– Peter Cordes
Nov 20 '18 at 8:10
6
@SpaceBison Of course there is :-) github.com/Battelle/movfuscator
– Daniel Näslund
Nov 20 '18 at 13:04
2
@PeterCordes: Ah; in the old days there ware MOV architectures that had memory mapped ALUs and IP was just another register so JMP was another MOV.
– Joshua
Nov 20 '18 at 17:07
1
@Joshua: fun fact: 32-bit ARM does expose PC as one of the 16 general-purpose integer registers.push {r4, lr}
/pop {r4,pc}
is common in functions that need to save/restore one call-preserved register and keep the stack aligned: they also save the link reg and pop it back into the program counter to return. (32-bit ARM's store/load-multiple instructions use a bitfield to indicate which registers to store/load.) And yeah, you can use PC as the destination of amov
or any instruction. I didn't know that was common in the past, though. But I had heard of transport-triggered ISAs.
– Peter Cordes
Nov 20 '18 at 17:18
|
show 1 more comment
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ESR's The Art of Unix Programming claims it is:
We'll examine troff in more detail in Chapter 18; for now, it's
sufficient to note that it is a good example of an imperative
minilanguage that borders on being a full-fledged interpreter (it has
conditionals and recursion but not loops; it is accidentally
Turing-complete).
("Accidentally" as opposed to m4
, which is said to be "deliberately Turing-complete".)
add a comment |
ESR's The Art of Unix Programming claims it is:
We'll examine troff in more detail in Chapter 18; for now, it's
sufficient to note that it is a good example of an imperative
minilanguage that borders on being a full-fledged interpreter (it has
conditionals and recursion but not loops; it is accidentally
Turing-complete).
("Accidentally" as opposed to m4
, which is said to be "deliberately Turing-complete".)
add a comment |
ESR's The Art of Unix Programming claims it is:
We'll examine troff in more detail in Chapter 18; for now, it's
sufficient to note that it is a good example of an imperative
minilanguage that borders on being a full-fledged interpreter (it has
conditionals and recursion but not loops; it is accidentally
Turing-complete).
("Accidentally" as opposed to m4
, which is said to be "deliberately Turing-complete".)
ESR's The Art of Unix Programming claims it is:
We'll examine troff in more detail in Chapter 18; for now, it's
sufficient to note that it is a good example of an imperative
minilanguage that borders on being a full-fledged interpreter (it has
conditionals and recursion but not loops; it is accidentally
Turing-complete).
("Accidentally" as opposed to m4
, which is said to be "deliberately Turing-complete".)
edited Nov 20 '18 at 4:35
answered Nov 20 '18 at 4:29
murumuru
1
1
add a comment |
add a comment |
Yes, troff is Turing-complete. It supports arbitrary recursion and conditional branching, which is sufficient. It also has registers and various other ways to store data, which gives you another path in again.
Turing completeness doesn't imply that highly complex programs are practical - just that they're theoretically possible, somehow, at some level of remove - and nor does its absence imply that they aren't, so neither troff's being Turing-complete nor the absence of complex programs don't suggest anything much one way or the other about that.
Turing completeness is not, generally, a property that means anything useful for you the user. All it means is that you can simulate a Turing machine with it, not that you'd want to, and not that the output that you'd get from it is anything like what you'd expect to read. The input or output might just be a number, or even the number of times something appears, rather than something useful, and the sorts of machine you end up simulating and their programs are often barely comprehensible to start with.
Many languages and systems are incidentally Turing-complete but not reasonably applicable for any actual programming in that subset (for example, Conway's Game of Life or CSS), and some languages that are useful for real programming are not Turing-complete (for example, Agda). The defining characteristics really are that you can
- keep going forever
- remember as much data as you want
- choose what, if anything, to do next
Often those properties - particularly non-termination - are actually undesirable, possibly including for troff. Outside of theoretical computer science and language design, Turing completeness is not a terribly interesting property virtually of the time, despite being catchy.
Yes of course, it is not a very useful thing by itself but it is still interesting - e.g. even the mov instruction is Turing complete.
– theindigamer
Nov 20 '18 at 4:38
4
@theindigamer - x86'smov
instruction is Turing-complete. (Because of addressing modes that let you use lookup tables, and the same mnemonic is used for load, store, and mov-immediate to register.) On many other ISAs that have amov
instruction (e.g. ARM) it's just a reg-reg move and isn't turing-complete. (Although on ARM it can do shifts/rotates.) Also, you actually need ajmp
to create a loop around your block ofmov
instructions, unless you're in 16-bit mode where the instruction pointer can wrap around in a 64k code segment to loop implicitly.
– Peter Cordes
Nov 20 '18 at 8:10
6
@SpaceBison Of course there is :-) github.com/Battelle/movfuscator
– Daniel Näslund
Nov 20 '18 at 13:04
2
@PeterCordes: Ah; in the old days there ware MOV architectures that had memory mapped ALUs and IP was just another register so JMP was another MOV.
– Joshua
Nov 20 '18 at 17:07
1
@Joshua: fun fact: 32-bit ARM does expose PC as one of the 16 general-purpose integer registers.push {r4, lr}
/pop {r4,pc}
is common in functions that need to save/restore one call-preserved register and keep the stack aligned: they also save the link reg and pop it back into the program counter to return. (32-bit ARM's store/load-multiple instructions use a bitfield to indicate which registers to store/load.) And yeah, you can use PC as the destination of amov
or any instruction. I didn't know that was common in the past, though. But I had heard of transport-triggered ISAs.
– Peter Cordes
Nov 20 '18 at 17:18
|
show 1 more comment
Yes, troff is Turing-complete. It supports arbitrary recursion and conditional branching, which is sufficient. It also has registers and various other ways to store data, which gives you another path in again.
Turing completeness doesn't imply that highly complex programs are practical - just that they're theoretically possible, somehow, at some level of remove - and nor does its absence imply that they aren't, so neither troff's being Turing-complete nor the absence of complex programs don't suggest anything much one way or the other about that.
Turing completeness is not, generally, a property that means anything useful for you the user. All it means is that you can simulate a Turing machine with it, not that you'd want to, and not that the output that you'd get from it is anything like what you'd expect to read. The input or output might just be a number, or even the number of times something appears, rather than something useful, and the sorts of machine you end up simulating and their programs are often barely comprehensible to start with.
Many languages and systems are incidentally Turing-complete but not reasonably applicable for any actual programming in that subset (for example, Conway's Game of Life or CSS), and some languages that are useful for real programming are not Turing-complete (for example, Agda). The defining characteristics really are that you can
- keep going forever
- remember as much data as you want
- choose what, if anything, to do next
Often those properties - particularly non-termination - are actually undesirable, possibly including for troff. Outside of theoretical computer science and language design, Turing completeness is not a terribly interesting property virtually of the time, despite being catchy.
Yes of course, it is not a very useful thing by itself but it is still interesting - e.g. even the mov instruction is Turing complete.
– theindigamer
Nov 20 '18 at 4:38
4
@theindigamer - x86'smov
instruction is Turing-complete. (Because of addressing modes that let you use lookup tables, and the same mnemonic is used for load, store, and mov-immediate to register.) On many other ISAs that have amov
instruction (e.g. ARM) it's just a reg-reg move and isn't turing-complete. (Although on ARM it can do shifts/rotates.) Also, you actually need ajmp
to create a loop around your block ofmov
instructions, unless you're in 16-bit mode where the instruction pointer can wrap around in a 64k code segment to loop implicitly.
– Peter Cordes
Nov 20 '18 at 8:10
6
@SpaceBison Of course there is :-) github.com/Battelle/movfuscator
– Daniel Näslund
Nov 20 '18 at 13:04
2
@PeterCordes: Ah; in the old days there ware MOV architectures that had memory mapped ALUs and IP was just another register so JMP was another MOV.
– Joshua
Nov 20 '18 at 17:07
1
@Joshua: fun fact: 32-bit ARM does expose PC as one of the 16 general-purpose integer registers.push {r4, lr}
/pop {r4,pc}
is common in functions that need to save/restore one call-preserved register and keep the stack aligned: they also save the link reg and pop it back into the program counter to return. (32-bit ARM's store/load-multiple instructions use a bitfield to indicate which registers to store/load.) And yeah, you can use PC as the destination of amov
or any instruction. I didn't know that was common in the past, though. But I had heard of transport-triggered ISAs.
– Peter Cordes
Nov 20 '18 at 17:18
|
show 1 more comment
Yes, troff is Turing-complete. It supports arbitrary recursion and conditional branching, which is sufficient. It also has registers and various other ways to store data, which gives you another path in again.
Turing completeness doesn't imply that highly complex programs are practical - just that they're theoretically possible, somehow, at some level of remove - and nor does its absence imply that they aren't, so neither troff's being Turing-complete nor the absence of complex programs don't suggest anything much one way or the other about that.
Turing completeness is not, generally, a property that means anything useful for you the user. All it means is that you can simulate a Turing machine with it, not that you'd want to, and not that the output that you'd get from it is anything like what you'd expect to read. The input or output might just be a number, or even the number of times something appears, rather than something useful, and the sorts of machine you end up simulating and their programs are often barely comprehensible to start with.
Many languages and systems are incidentally Turing-complete but not reasonably applicable for any actual programming in that subset (for example, Conway's Game of Life or CSS), and some languages that are useful for real programming are not Turing-complete (for example, Agda). The defining characteristics really are that you can
- keep going forever
- remember as much data as you want
- choose what, if anything, to do next
Often those properties - particularly non-termination - are actually undesirable, possibly including for troff. Outside of theoretical computer science and language design, Turing completeness is not a terribly interesting property virtually of the time, despite being catchy.
Yes, troff is Turing-complete. It supports arbitrary recursion and conditional branching, which is sufficient. It also has registers and various other ways to store data, which gives you another path in again.
Turing completeness doesn't imply that highly complex programs are practical - just that they're theoretically possible, somehow, at some level of remove - and nor does its absence imply that they aren't, so neither troff's being Turing-complete nor the absence of complex programs don't suggest anything much one way or the other about that.
Turing completeness is not, generally, a property that means anything useful for you the user. All it means is that you can simulate a Turing machine with it, not that you'd want to, and not that the output that you'd get from it is anything like what you'd expect to read. The input or output might just be a number, or even the number of times something appears, rather than something useful, and the sorts of machine you end up simulating and their programs are often barely comprehensible to start with.
Many languages and systems are incidentally Turing-complete but not reasonably applicable for any actual programming in that subset (for example, Conway's Game of Life or CSS), and some languages that are useful for real programming are not Turing-complete (for example, Agda). The defining characteristics really are that you can
- keep going forever
- remember as much data as you want
- choose what, if anything, to do next
Often those properties - particularly non-termination - are actually undesirable, possibly including for troff. Outside of theoretical computer science and language design, Turing completeness is not a terribly interesting property virtually of the time, despite being catchy.
edited Nov 20 '18 at 5:13
answered Nov 20 '18 at 4:35
Michael HomerMichael Homer
48.7k8130168
48.7k8130168
Yes of course, it is not a very useful thing by itself but it is still interesting - e.g. even the mov instruction is Turing complete.
– theindigamer
Nov 20 '18 at 4:38
4
@theindigamer - x86'smov
instruction is Turing-complete. (Because of addressing modes that let you use lookup tables, and the same mnemonic is used for load, store, and mov-immediate to register.) On many other ISAs that have amov
instruction (e.g. ARM) it's just a reg-reg move and isn't turing-complete. (Although on ARM it can do shifts/rotates.) Also, you actually need ajmp
to create a loop around your block ofmov
instructions, unless you're in 16-bit mode where the instruction pointer can wrap around in a 64k code segment to loop implicitly.
– Peter Cordes
Nov 20 '18 at 8:10
6
@SpaceBison Of course there is :-) github.com/Battelle/movfuscator
– Daniel Näslund
Nov 20 '18 at 13:04
2
@PeterCordes: Ah; in the old days there ware MOV architectures that had memory mapped ALUs and IP was just another register so JMP was another MOV.
– Joshua
Nov 20 '18 at 17:07
1
@Joshua: fun fact: 32-bit ARM does expose PC as one of the 16 general-purpose integer registers.push {r4, lr}
/pop {r4,pc}
is common in functions that need to save/restore one call-preserved register and keep the stack aligned: they also save the link reg and pop it back into the program counter to return. (32-bit ARM's store/load-multiple instructions use a bitfield to indicate which registers to store/load.) And yeah, you can use PC as the destination of amov
or any instruction. I didn't know that was common in the past, though. But I had heard of transport-triggered ISAs.
– Peter Cordes
Nov 20 '18 at 17:18
|
show 1 more comment
Yes of course, it is not a very useful thing by itself but it is still interesting - e.g. even the mov instruction is Turing complete.
– theindigamer
Nov 20 '18 at 4:38
4
@theindigamer - x86'smov
instruction is Turing-complete. (Because of addressing modes that let you use lookup tables, and the same mnemonic is used for load, store, and mov-immediate to register.) On many other ISAs that have amov
instruction (e.g. ARM) it's just a reg-reg move and isn't turing-complete. (Although on ARM it can do shifts/rotates.) Also, you actually need ajmp
to create a loop around your block ofmov
instructions, unless you're in 16-bit mode where the instruction pointer can wrap around in a 64k code segment to loop implicitly.
– Peter Cordes
Nov 20 '18 at 8:10
6
@SpaceBison Of course there is :-) github.com/Battelle/movfuscator
– Daniel Näslund
Nov 20 '18 at 13:04
2
@PeterCordes: Ah; in the old days there ware MOV architectures that had memory mapped ALUs and IP was just another register so JMP was another MOV.
– Joshua
Nov 20 '18 at 17:07
1
@Joshua: fun fact: 32-bit ARM does expose PC as one of the 16 general-purpose integer registers.push {r4, lr}
/pop {r4,pc}
is common in functions that need to save/restore one call-preserved register and keep the stack aligned: they also save the link reg and pop it back into the program counter to return. (32-bit ARM's store/load-multiple instructions use a bitfield to indicate which registers to store/load.) And yeah, you can use PC as the destination of amov
or any instruction. I didn't know that was common in the past, though. But I had heard of transport-triggered ISAs.
– Peter Cordes
Nov 20 '18 at 17:18
Yes of course, it is not a very useful thing by itself but it is still interesting - e.g. even the mov instruction is Turing complete.
– theindigamer
Nov 20 '18 at 4:38
Yes of course, it is not a very useful thing by itself but it is still interesting - e.g. even the mov instruction is Turing complete.
– theindigamer
Nov 20 '18 at 4:38
4
4
@theindigamer - x86's
mov
instruction is Turing-complete. (Because of addressing modes that let you use lookup tables, and the same mnemonic is used for load, store, and mov-immediate to register.) On many other ISAs that have a mov
instruction (e.g. ARM) it's just a reg-reg move and isn't turing-complete. (Although on ARM it can do shifts/rotates.) Also, you actually need a jmp
to create a loop around your block of mov
instructions, unless you're in 16-bit mode where the instruction pointer can wrap around in a 64k code segment to loop implicitly.– Peter Cordes
Nov 20 '18 at 8:10
@theindigamer - x86's
mov
instruction is Turing-complete. (Because of addressing modes that let you use lookup tables, and the same mnemonic is used for load, store, and mov-immediate to register.) On many other ISAs that have a mov
instruction (e.g. ARM) it's just a reg-reg move and isn't turing-complete. (Although on ARM it can do shifts/rotates.) Also, you actually need a jmp
to create a loop around your block of mov
instructions, unless you're in 16-bit mode where the instruction pointer can wrap around in a 64k code segment to loop implicitly.– Peter Cordes
Nov 20 '18 at 8:10
6
6
@SpaceBison Of course there is :-) github.com/Battelle/movfuscator
– Daniel Näslund
Nov 20 '18 at 13:04
@SpaceBison Of course there is :-) github.com/Battelle/movfuscator
– Daniel Näslund
Nov 20 '18 at 13:04
2
2
@PeterCordes: Ah; in the old days there ware MOV architectures that had memory mapped ALUs and IP was just another register so JMP was another MOV.
– Joshua
Nov 20 '18 at 17:07
@PeterCordes: Ah; in the old days there ware MOV architectures that had memory mapped ALUs and IP was just another register so JMP was another MOV.
– Joshua
Nov 20 '18 at 17:07
1
1
@Joshua: fun fact: 32-bit ARM does expose PC as one of the 16 general-purpose integer registers.
push {r4, lr}
/ pop {r4,pc}
is common in functions that need to save/restore one call-preserved register and keep the stack aligned: they also save the link reg and pop it back into the program counter to return. (32-bit ARM's store/load-multiple instructions use a bitfield to indicate which registers to store/load.) And yeah, you can use PC as the destination of a mov
or any instruction. I didn't know that was common in the past, though. But I had heard of transport-triggered ISAs.– Peter Cordes
Nov 20 '18 at 17:18
@Joshua: fun fact: 32-bit ARM does expose PC as one of the 16 general-purpose integer registers.
push {r4, lr}
/ pop {r4,pc}
is common in functions that need to save/restore one call-preserved register and keep the stack aligned: they also save the link reg and pop it back into the program counter to return. (32-bit ARM's store/load-multiple instructions use a bitfield to indicate which registers to store/load.) And yeah, you can use PC as the destination of a mov
or any instruction. I didn't know that was common in the past, though. But I had heard of transport-triggered ISAs.– Peter Cordes
Nov 20 '18 at 17:18
|
show 1 more comment
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