David Virebayre wrote:

On Fri, Oct 16, 2009 at 8:19 PM, Andrew Coppin wrote:

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Is there any way that you can turn an arbitrary Haskell value into a string?

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I rephrase: There *is* a way to turn arbitrary values into strings. I know there is, because the GHCi debugger *does* it. The question is, does anybody know of an /easy/ way to do this?

Ghci only displays values with a Show instance.

Well, I can live with getting an empty string if no Show instance is available. But I can't figure out how to even do that...

My GHCi can't do that :o I just wrote data A = B | C and loaded the file into GHCi. Typing B gives me: <interactive>:1:0: No instance for (Show A) arising from a use of `print' at <interactive>:1:0 Possible fix: add an instance declaration for (Show A) In a stmt of a 'do' expression: print it The error also gives an idea of what GHCi is doing behind the scenes: it's just calling print, which has a Show constraint. On Fri, Oct 16, 2009 at 2:40 PM, Andrew Coppin wrote:

Jochem Berndsen wrote:

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I rephrase: There *is* a way to turn arbitrary values into strings. I know there is, because the GHCi debugger *does* it. The question is, does anybody know of an /easy/ way to do this?

No. GHCi does not always do this:

Prelude Data.Ratio> let plus1 = (+1) Prelude Data.Ratio> plus1

<interactive>:1:0:    No instance for (Show (a -> a))      arising from a use of `print' at <interactive>:1:0-4    Possible fix: add an instance declaration for (Show (a -> a))    In a stmt of a 'do' expression: print it Prelude Data.Ratio>

The GHCi *debugger* can print out even values for which no Show instance exists. (But yes, it fails to print anything interesting for function types... It works for ADTs that don't have Show though.)

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Anybody know of a way to do this? (As it happens, the values I'm testing with are all Showable anyway, but the type checker doesn't know that...)

What is the problem with adding a function showMyContainer :: (Show a) => Container a -> String ? In this case you can show your container (for debugging purposes), but only if you have Showable elements in your container.

This could plausibly work...

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Whoops, sorry about that then! On Fri, Oct 16, 2009 at 2:59 PM, Ross Mellgren wrote:

Andrew has mentioned the debugger several times, NOT the interactive REPL. That is, using :-commands to inspect values.

-Ross

On Oct 16, 2009, at 2:46 PM, Daniel Peebles wrote:

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My GHCi can't do that :o

I just wrote data A = B | C and loaded the file into GHCi. Typing B gives me:

<interactive>:1:0:   No instance for (Show A)     arising from a use of `print' at <interactive>:1:0   Possible fix: add an instance declaration for (Show A)   In a stmt of a 'do' expression: print it

The error also gives an idea of what GHCi is doing behind the scenes: it's just calling print, which has a Show constraint.

On Fri, Oct 16, 2009 at 2:40 PM, Andrew Coppin wrote:

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Jochem Berndsen wrote:

...

...

I rephrase: There *is* a way to turn arbitrary values into strings. I know there is, because the GHCi debugger *does* it. The question is, does anybody know of an /easy/ way to do this?

No. GHCi does not always do this:

Prelude Data.Ratio> let plus1 = (+1) Prelude Data.Ratio> plus1

<interactive>:1:0:   No instance for (Show (a -> a))     arising from a use of `print' at <interactive>:1:0-4   Possible fix: add an instance declaration for (Show (a -> a))   In a stmt of a 'do' expression: print it Prelude Data.Ratio>

The GHCi *debugger* can print out even values for which no Show instance exists. (But yes, it fails to print anything interesting for function types... It works for ADTs that don't have Show though.)

...

...

Anybody know of a way to do this? (As it happens, the values I'm testing with are all Showable anyway, but the type checker doesn't know that...)

What is the problem with adding a function showMyContainer :: (Show a) => Container a -> String ? In this case you can show your container (for debugging purposes), but only if you have Showable elements in your container.

This could plausibly work...

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See vacuum: http://hackage.haskell.org/package/vacuum

On 10/17/09, Andrew Coppin wrote:

Derek Elkins wrote:

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See vacuum: http://hackage.haskell.org/package/vacuum

Could be useful... Thanks!

As Derek mentioned, vacuum would be perfect for this: ----------------------------------------------------------------------------- import Data.Word import GHC.Vacuum import GHC.Vacuum.ClosureType import qualified Data.IntMap as IM type Info = (ClosureType -- what kind of heap node is this? ,[String] -- [pkg,mod,con] for constructors ,[Int] -- "pointers" refering to other nodes in IntMap ,[Word]) -- literal data in constructors overview :: HNode -> Info overview o = let ptrs = nodePtrs o lits = nodeLits o itab = nodeInfo o ctyp = itabType itab -- only available -- for constructors (pkg,mod,con) = itabName itab names = filter (not . null) [pkg,mod,con] in (ctyp ,names -- [] for non-data ,ptrs ,lits) -- returns an adjacency-list graph info :: a -> [(Int,Info)] info = fmap (\(a,b)->(a,overview b)) . IM.toList . vacuum -- returns an adjacency-list graph infoLazy :: a -> [(Int,Info)] infoLazy = fmap (\(a,b)->(a,overview b)) . IM.toList . vacuumLazy ----------------------------------------------------------------------------- -- example usage data A a = A Int | B a | forall b. C b [A a] val0 = [A 42, B (Left Nothing), C (pi,()) val0] val1 = fmap (\n -> C n []) [0..] {- ghci> mapM_ print (info val0) Loading package vacuum-1.0.0 ... linking ... done. (0,(CONSTR_2_0,["ghc-prim","GHC.Types",":"],[1,2],[])) (1,(CONSTR,["main","Main","A"],[3],[])) (2,(CONSTR_2_0,["ghc-prim","GHC.Types",":"],[4,5],[])) (3,(CONSTR_0_1,["ghc-prim","GHC.Types","I#"],[],[42])) (4,(CONSTR,["main","Main","B"],[6],[])) (5,(CONSTR_2_0,["ghc-prim","GHC.Types",":"],[8,9],[])) (6,(CONSTR_1_0,["base","Data.Either","Left"],[7],[])) (7,(CONSTR_NOCAF_STATIC,["base","Data.Maybe","Nothing"],[],[])) (8,(CONSTR,["main","Main","C"],[10,0],[])) (9,(CONSTR_NOCAF_STATIC,["ghc-prim","GHC.Types","[]"],[],[])) (10,(CONSTR_2_0,["ghc-prim","GHC.Tuple","(,)"],[11,12],[])) (11,(CONSTR_NOCAF_STATIC,["ghc-prim","GHC.Types","D#"],[],[4614256656552045848])) (12,(CONSTR_NOCAF_STATIC,["ghc-prim","GHC.Unit","()"],[],[])) ghci> mapM_ print (infoLazy val1) (0,(AP,[],[],[])) ghci> val1 `seq` () () ghci> mapM_ print (infoLazy val1) (0,(CONSTR_2_0,["ghc-prim","GHC.Types",":"],[1,2],[])) (1,(THUNK_2_0,[],[],[])) (2,(THUNK_2_0,[],[],[])) ghci> length . take 2 $ val1 2 ghci> mapM_ print (infoLazy val1) (0,(CONSTR_2_0,["ghc-prim","GHC.Types",":"],[1,2],[])) (1,(THUNK_2_0,[],[],[])) (2,(CONSTR_2_0,["ghc-prim","GHC.Types",":"],[3,4],[])) (3,(THUNK_2_0,[],[],[])) (4,(THUNK_2_0,[],[],[])) ghci> case val1 of a:b:_ -> a `seq` b `seq` () () ghci> mapM_ print (infoLazy val1) (0,(CONSTR_2_0,["ghc-prim","GHC.Types",":"],[1,2],[])) (1,(CONSTR,["main","Main","C"],[3,4],[])) (2,(CONSTR_2_0,["ghc-prim","GHC.Types",":"],[5,6],[])) (3,(CONSTR_0_1,["integer","GHC.Integer.Internals","S#"],[],[0])) (4,(CONSTR_NOCAF_STATIC,["ghc-prim","GHC.Types","[]"],[],[])) (5,(CONSTR,["main","Main","C"],[7,4],[])) (6,(THUNK_2_0,[],[],[])) (7,(CONSTR_0_1,["integer","GHC.Integer.Internals","S#"],[],[1])) -} ----------------------------------------------------------------------------- Matt