How can I use ghci more wisely?

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yi lu

23 Jul 2013 23 Jul '13

10:30 p.m.

I am wondering how can I ask ghci to show an infinite list wisely. When I type *fst ([1..],[1..10])* The result is what as you may guess *1,2,3,4,...*(continues to show, cut now) How could I may ghci show *[1..]* this wise way not the long long long list itself? Yi

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Kristopher Micinski

24 Jul 24 Jul

12:23 a.m.

Knowing whether a computation will terminate is in general the halting problem, so immediately you're looking at a syntactic restriction. Here the only ones I can think of are artificial at best (i.e., they don't work for examples more than what you've shown here): http://trac.haskell.org/haskell-platform/ticket/180 There was some discussion [1] on putting a limit to what the interpreter prints out. Off the top of my head I suppose a hacky way to do this would be to define a new type deriving show in a way that printed out the list to some bounded depth. Kris [1] http://projects.haskell.org/pipermail/haskell-platform/2011-July/001619.html On Tue, Jul 23, 2013 at 10:30 PM, yi lu wrote:

...

I am wondering how can I ask ghci to show an infinite list wisely. When I type

fst ([1..],[1..10])

The result is what as you may guess

1,2,3,4,...(continues to show, cut now)

How could I may ghci show

[1..]

this wise way not the long long long list itself?

Yi

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Michael Sloan

4:41 a.m.

This portion of haskell-mode (haskell-interactive-mode-eval-pretty) is what the UI for something like this could look like: https://www.youtube.com/watch?v=pu9AGSOySlE This isn't an answer to your question, though, because expanding subparts of the output doesn't drive evaluation. It would be very cool, and quite possible, to have a variant of the Show typeclass that had output with such structured laziness. Another non-answer is to take a look at using vaccum[0] and vaccum-graphviz[1] together, to get an idea of the heap structure of unforced values. I've made a gist demonstrating how to use these to visualize the heap without forcing values[2]. This doesn't show any concrete values (as that would require some serious voodoo), but does show how the heap changes due to thunks being forced. -Michael [0] http://hackage.haskell.org/package/vacuum [1] http://hackage.haskell.org/package/vacuum-graphviz [2] https://gist.github.com/mgsloan/6068915 On Tue, Jul 23, 2013 at 7:30 PM, yi lu wrote:

...

I am wondering how can I ask ghci to show an infinite list wisely. When I type

*fst ([1..],[1..10])*

The result is what as you may guess

*1,2,3,4,...*(continues to show, cut now)

How could I may ghci show

*[1..]*

this wise way not the long long long list itself?

Yi

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Joachim Breitner

7:20 a.m.

Hi, Am Mittwoch, den 24.07.2013, 01:41 -0700 schrieb Michael Sloan:

...

Another non-answer is to take a look at using vaccum[0] and vaccum-graphviz[1] together, to get an idea of the heap structure of unforced values. I've made a gist demonstrating how to use these to visualize the heap without forcing values[2]. This doesn't show any concrete values (as that would require some serious voodoo), but does show how the heap changes due to thunks being forced.

if you want to stay in GHCi with it you can use ghc-heapview instead of vacuum: Prelude> :script /home/jojo/.cabal/share/ghc-heap-view-0.5.1/ghci Prelude> let x = [1..] Prelude> take 20 x [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20] Prelude> :printHeap x Prelude> :printHeap x let x1 = S# 20 in S# 1 : S# 2 : S# 3 : S# 4 : S# 5 : S# 6 : S# 7 : S# 8 : S# 9 : S# 10 : S# 11 : S# 12 : S# 13 : S# 14 : S# 15 : S# 16 : S# 17 : S# 18 : S# 19 : x1 : _thunk x1 (S# 1) For this kind of infinite values you don’t see its finite, but for others you do: Prelude> let inf = let x = "ha" ++ x in x Prelude> take 20 inf "hahahahahahahahahaha" Prelude> :printHeap inf let x1 = C# 'h' : C# 'a' : x1 in x1 Greetings, Joachim -- Joachim “nomeata” Breitner mail@joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata@joachim-breitner.de • GPG-Key: 0x4743206C Debian Developer: nomeata@debian.org

Jun Inoue

7:33 a.m.

The data-pprint package's pprint function might give you a quick fix. For example: Prelude> :m Data.PPrint Prelude Data.PPrint> pprint [1..] [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, …, ……] Prelude Data.PPrint> let long_computation = long_computation Prelude Data.PPrint> pprint [1, long_computation, 3] [1, ⊥₁, 3] ⊥₁: timeout at 0% It's a bit of a hassle to have to type "pprint" all the time though, and it doesn't give you a way to show the data without printing to the terminal in the IO monad. On Wed, Jul 24, 2013 at 4:30 AM, yi lu wrote:

...

I am wondering how can I ask ghci to show an infinite list wisely. When I type

fst ([1..],[1..10])

The result is what as you may guess

1,2,3,4,...(continues to show, cut now)

How could I may ghci show

[1..]

this wise way not the long long long list itself?

Yi

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

-- Jun Inoue

David McBride

4:16 p.m.

You might like to know about this option for ghci -interactive-print I tested it with data-pprint though and it didn't work because it returns an IO Doc instead of IO () (I assume). But if you wrote a function that used that, returned the right type, cabal installed it and put it in your .ghci, you would have your pprinting by default whenever you use ghci. On Wed, Jul 24, 2013 at 7:33 AM, Jun Inoue wrote:

...

The data-pprint package's pprint function might give you a quick fix. For example:

Prelude> :m Data.PPrint Prelude Data.PPrint> pprint [1..] [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, …, ……] Prelude Data.PPrint> let long_computation = long_computation Prelude Data.PPrint> pprint [1, long_computation, 3] [1, ⊥₁, 3] ⊥₁: timeout at 0%

It's a bit of a hassle to have to type "pprint" all the time though, and it doesn't give you a way to show the data without printing to the terminal in the IO monad.

On Wed, Jul 24, 2013 at 4:30 AM, yi lu wrote:

...
I am wondering how can I ask ghci to show an infinite list wisely. When I type

fst ([1..],[1..10])

The result is what as you may guess

1,2,3,4,...(continues to show, cut now)

How could I may ghci show

[1..]

this wise way not the long long long list itself?

Yi

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

-- Jun Inoue

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

Jun Inoue

8:03 p.m.

Thanks for the tip, David, I didn't know about that flag! Looks really handy for playing with EDSLs, which is usually better off displayed through Doc, but the default Show instance is indispensable when I find a bug in the conversion to the Doc. Unfortunately, though, I'd be reluctant to make data-pprint the universal default as it is now. I forgot to mention this in my previous post, but data-pprint doesn't let you customize the output per-datatype. It just works generically over Data.Data instances and the format is fixed to be the same as default Show instances (except for lists, which are special-cased internally). So as annoying as the explicit pprint is, I see it as a necessary evil. Perhaps I can generalize its interface and send a patch. I have some ideas but never got around to trying them. On Wed, Jul 24, 2013 at 10:16 PM, David McBride wrote:

...

You might like to know about this option for ghci -interactive-print

I tested it with data-pprint though and it didn't work because it returns an IO Doc instead of IO () (I assume). But if you wrote a function that used that, returned the right type, cabal installed it and put it in your .ghci, you would have your pprinting by default whenever you use ghci.

On Wed, Jul 24, 2013 at 7:33 AM, Jun Inoue wrote:

...
The data-pprint package's pprint function might give you a quick fix. For example:

Prelude> :m Data.PPrint Prelude Data.PPrint> pprint [1..] [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, …, ……] Prelude Data.PPrint> let long_computation = long_computation Prelude Data.PPrint> pprint [1, long_computation, 3] [1, ⊥₁, 3] ⊥₁: timeout at 0%

It's a bit of a hassle to have to type "pprint" all the time though, and it doesn't give you a way to show the data without printing to the terminal in the IO monad.

On Wed, Jul 24, 2013 at 4:30 AM, yi lu wrote:

...
I am wondering how can I ask ghci to show an infinite list wisely. When I type

fst ([1..],[1..10])

The result is what as you may guess

1,2,3,4,...(continues to show, cut now)

How could I may ghci show

[1..]

this wise way not the long long long list itself?

Yi

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

-- Jun Inoue

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

-- Jun Inoue

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David McBride
Joachim Breitner
Jun Inoue
Kristopher Micinski
Michael Sloan
yi lu