Re: Unexpected ambiguity in a seemingly valid Haskell 2010 program
Apparently not — the code comilers with any of -XNoMonoLocalBinds and -XMonoLocalBinds, but not with -XNoMonomorphismRestriction. * wagne...@seas.upenn.edu wagne...@seas.upenn.edu [2012-11-09 14:07:59-0500] It's possible that the below blog post is related. ~d http://hackage.haskell.org/trac/ghc/blog/LetGeneralisationInGhc7 Quoting Roman Cheplyaka r...@ro-che.info: For this module module Test where import System.Random data RPS = Rock | Paper | Scissors deriving (Show, Enum) instance Random RPS where random g = let (x, g') = randomR (0, 2) g in (toEnum x, g') randomR = undefined ghc (7.4.1 and 7.6.1) reports an error: rand.hs:9:9: No instance for (Random t0) arising from the ambiguity check for g' The type variable `t0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there are several potential instances: instance Random RPS -- Defined at rand.hs:7:10 instance Random Bool -- Defined in `System.Random' instance Random Foreign.C.Types.CChar -- Defined in `System.Random' ...plus 34 others When checking that g' has the inferred type `g' Probable cause: the inferred type is ambiguous In the expression: let (x, g') = randomR (0, 2) g in (toEnum x, g') In an equation for `random': random g = let (x, g') = randomR ... g in (toEnum x, g') Failed, modules loaded: none. There should be no ambiguity since 'toEnum' determines the type of x (Int), and that in turn fixes types of 0 and 2. Interestingly, annotating 0 or 2 with the type makes the problem go away. jhc 0.8.0 compiles this module fine. Roman ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Unexpected ambiguity in a seemingly valid Haskell 2010 program
That's strange. Here, it only fails with both NoMonomorphismRestriction and NoMonoLocalBinds (which makes sense). I've tested on 7.4.1 and 7.6.1. Erik On Sun, Nov 11, 2012 at 3:54 PM, Roman Cheplyaka r...@ro-che.info wrote: Apparently not — the code comilers with any of -XNoMonoLocalBinds and -XMonoLocalBinds, but not with -XNoMonomorphismRestriction. * wagne...@seas.upenn.edu wagne...@seas.upenn.edu [2012-11-09 14:07:59-0500] It's possible that the below blog post is related. ~d http://hackage.haskell.org/trac/ghc/blog/LetGeneralisationInGhc7 Quoting Roman Cheplyaka r...@ro-che.info: For this module module Test where import System.Random data RPS = Rock | Paper | Scissors deriving (Show, Enum) instance Random RPS where random g = let (x, g') = randomR (0, 2) g in (toEnum x, g') randomR = undefined ghc (7.4.1 and 7.6.1) reports an error: rand.hs:9:9: No instance for (Random t0) arising from the ambiguity check for g' The type variable `t0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there are several potential instances: instance Random RPS -- Defined at rand.hs:7:10 instance Random Bool -- Defined in `System.Random' instance Random Foreign.C.Types.CChar -- Defined in `System.Random' ...plus 34 others When checking that g' has the inferred type `g' Probable cause: the inferred type is ambiguous In the expression: let (x, g') = randomR (0, 2) g in (toEnum x, g') In an equation for `random': random g = let (x, g') = randomR ... g in (toEnum x, g') Failed, modules loaded: none. There should be no ambiguity since 'toEnum' determines the type of x (Int), and that in turn fixes types of 0 and 2. Interestingly, annotating 0 or 2 with the type makes the problem go away. jhc 0.8.0 compiles this module fine. Roman ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Unexpected ambiguity in a seemingly valid Haskell 2010 program
Right. What I meant is that with -XMonomorphismRestriction, it compiles with with both -XMonoLocalBinds and -XNoMonoLocalBinds. That means that MonoLocalBinds can not be solely responsible for this behaviour. Anyway, I just noticed that a very similar example (using Read) is described in the Haskell report's section on the monomorphism restriction. Roman * Erik Hesselink hessel...@gmail.com [2012-11-11 16:43:20+0100] That's strange. Here, it only fails with both NoMonomorphismRestriction and NoMonoLocalBinds (which makes sense). I've tested on 7.4.1 and 7.6.1. Erik On Sun, Nov 11, 2012 at 3:54 PM, Roman Cheplyaka r...@ro-che.info wrote: Apparently not — the code comilers with any of -XNoMonoLocalBinds and -XMonoLocalBinds, but not with -XNoMonomorphismRestriction. * wagne...@seas.upenn.edu wagne...@seas.upenn.edu [2012-11-09 14:07:59-0500] It's possible that the below blog post is related. ~d http://hackage.haskell.org/trac/ghc/blog/LetGeneralisationInGhc7 Quoting Roman Cheplyaka r...@ro-che.info: For this module module Test where import System.Random data RPS = Rock | Paper | Scissors deriving (Show, Enum) instance Random RPS where random g = let (x, g') = randomR (0, 2) g in (toEnum x, g') randomR = undefined ghc (7.4.1 and 7.6.1) reports an error: rand.hs:9:9: No instance for (Random t0) arising from the ambiguity check for g' The type variable `t0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there are several potential instances: instance Random RPS -- Defined at rand.hs:7:10 instance Random Bool -- Defined in `System.Random' instance Random Foreign.C.Types.CChar -- Defined in `System.Random' ...plus 34 others When checking that g' has the inferred type `g' Probable cause: the inferred type is ambiguous In the expression: let (x, g') = randomR (0, 2) g in (toEnum x, g') In an equation for `random': random g = let (x, g') = randomR ... g in (toEnum x, g') Failed, modules loaded: none. There should be no ambiguity since 'toEnum' determines the type of x (Int), and that in turn fixes types of 0 and 2. Interestingly, annotating 0 or 2 with the type makes the problem go away. jhc 0.8.0 compiles this module fine. Roman ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Unexpected ambiguity in a seemingly valid Haskell 2010 program
That makes sense: MonomorphismRestriction makes bindings without parameters monomorphic, and MonoLocalBinds makes local bindings monomorphic. So either one will make this binding monomorphic. Only when both are off does it become polymorphic and does the error occur. Erik On Sun, Nov 11, 2012 at 5:37 PM, Roman Cheplyaka r...@ro-che.info wrote: Right. What I meant is that with -XMonomorphismRestriction, it compiles with with both -XMonoLocalBinds and -XNoMonoLocalBinds. That means that MonoLocalBinds can not be solely responsible for this behaviour. Anyway, I just noticed that a very similar example (using Read) is described in the Haskell report's section on the monomorphism restriction. Roman * Erik Hesselink hessel...@gmail.com [2012-11-11 16:43:20+0100] That's strange. Here, it only fails with both NoMonomorphismRestriction and NoMonoLocalBinds (which makes sense). I've tested on 7.4.1 and 7.6.1. Erik On Sun, Nov 11, 2012 at 3:54 PM, Roman Cheplyaka r...@ro-che.info wrote: Apparently not — the code comilers with any of -XNoMonoLocalBinds and -XMonoLocalBinds, but not with -XNoMonomorphismRestriction. * wagne...@seas.upenn.edu wagne...@seas.upenn.edu [2012-11-09 14:07:59-0500] It's possible that the below blog post is related. ~d http://hackage.haskell.org/trac/ghc/blog/LetGeneralisationInGhc7 Quoting Roman Cheplyaka r...@ro-che.info: For this module module Test where import System.Random data RPS = Rock | Paper | Scissors deriving (Show, Enum) instance Random RPS where random g = let (x, g') = randomR (0, 2) g in (toEnum x, g') randomR = undefined ghc (7.4.1 and 7.6.1) reports an error: rand.hs:9:9: No instance for (Random t0) arising from the ambiguity check for g' The type variable `t0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there are several potential instances: instance Random RPS -- Defined at rand.hs:7:10 instance Random Bool -- Defined in `System.Random' instance Random Foreign.C.Types.CChar -- Defined in `System.Random' ...plus 34 others When checking that g' has the inferred type `g' Probable cause: the inferred type is ambiguous In the expression: let (x, g') = randomR (0, 2) g in (toEnum x, g') In an equation for `random': random g = let (x, g') = randomR ... g in (toEnum x, g') Failed, modules loaded: none. There should be no ambiguity since 'toEnum' determines the type of x (Int), and that in turn fixes types of 0 and 2. Interestingly, annotating 0 or 2 with the type makes the problem go away. jhc 0.8.0 compiles this module fine. Roman ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
How to use `trace` while debuging GHC
Hello, While working on GHC sometimes I find it useful to dump the values of intermediate expressions, perhaps in the middle of pure code, using a combination of `trace` and `ppr`. The issue is that `ppr` returns an `SDoc`, and to turn an `SDoc` into a `String`, I need some `DynFlags`. There used to be a value called `tracingDynFlags` that I could use to dump values, but it has disappeared... Did it get moved somewhere, or is there a better way to get the same effect? -Iavor ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: How to use `trace` while debuging GHC
On Sun, Nov 11, 2012 at 05:24:06PM -0800, Iavor Diatchki wrote: There used to be a value called `tracingDynFlags` that I could use to dump values, but it has disappeared... Did it get moved somewhere, or is there a better way to get the same effect? There is now StaticFlags.unsafeGlobalDynFlags. Thanks Ian ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: How to use `trace` while debuging GHC
I've recently used the conveniently-typed (pprTrace :: String - SDoc - a - a) for this purpose. You have to compile with -DDEBUG, but it works great. Richard On Nov 11, 2012, at 8:36 PM, Ian Lynagh i...@well-typed.com wrote: On Sun, Nov 11, 2012 at 05:24:06PM -0800, Iavor Diatchki wrote: There used to be a value called `tracingDynFlags` that I could use to dump values, but it has disappeared... Did it get moved somewhere, or is there a better way to get the same effect? There is now StaticFlags.unsafeGlobalDynFlags. Thanks Ian ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
