- Glasgow-haskell-users

Type checker's expected and inferred types (reformatted)
by C Rodrigues 25 Oct '09

25 Oct '09

(Some formatting was apparently lost en route. Trying again with extra newlines.) I came across a type error that misled me for quite a while, because the expected and inferred types were backwards (from my point of view). A simplified example is below. Can someone explain how GHC's type checker creates the error message? In this example, fun1 and fun2 are basically the same. The type error is because they try to run an IO () together with a Maybe (). > import Control.Monad > > foo :: Maybe () > foo = return () > > bar :: IO () > bar = return () > > fun1 = let fooThen m = foo>> m > in fooThen (bar>> undefined) > > fun2 = let fooThen m = foo>> m > in fooThen (do {bar; undefined}) With ghc 6.10.4, both functions attribute the error message to `bar'. However, the expected and inferred monads are swapped. fun1 produces the error message: Couldn't match expected type `Maybe a' against inferred type `IO ()' In the first argument of `(>>=)', namely `bar' fun2 produces the error message: Couldn't match expected type `IO ()' against inferred type `Maybe ()' In a stmt of a 'do' expression: bar It's confusing because 'bar' is inferred to have type Maybe (), even though it's explicitly declared to be an IO (). _________________________________________________________________ Windows 7: Simplify your PC. Learn more. http://www.microsoft.com/Windows/windows-7/default.aspx?ocid=PID24727::T:WL…

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Type checker's expected and inferred types (reformatted)
by C Rodrigues 24 Oct '09

24 Oct '09

> Which message do you prefer? I couldn't tell which it was. I prefer fun1. In my understanding, the 'inferred' type is gleaned by looking at theexpression itself, while the 'expected' type is implied by the context. _________________________________________________________________ Windows 7: It works the way you want. Learn more. http://www.microsoft.com/Windows/windows-7/default.aspx?ocid=PID24727::T:WL…

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Type checker's expected and inferred types
by C Rodrigues 24 Oct '09

24 Oct '09

I came across a type error that misled me for quite a while, because the expected and inferred types were backwards (from my point of view). A simplified example is below. Can someone explain how GHC's type checker creates the error message? In this example, fun1 and fun2 are basically the same. The type error is because they try to run an IO () together with a Maybe (). > import Control.Monad>> foo :: Maybe ()> foo = return ()>> bar :: IO ()> bar = return ()>> fun1 = let fooThen m = foo >> m> in fooThen (bar >> undefined)>> fun2 = let fooThen m = foo >> m> in fooThen (do {bar; undefined}) With ghc 6.10.4, both functions attribute the error message to `bar'. However, the expected and inferred monads are swapped.fun1 produces the error message:Couldn't match expected type `Maybe a' against inferred type `IO ()'In the first argument of `(>>=)', namely `bar'fun2 produces the error message:Couldn't match expected type `IO ()' against inferred type `Maybe ()'In a stmt of a 'do' expression: bar It's confusing because 'bar' is inferred to have type Maybe (), even though it's explicitly declared to be an IO (). _________________________________________________________________ New Windows 7: Find the right PC for you. Learn more. http://www.microsoft.com/windows/pc-scout/default.aspx?CBID=wl&ocid=PID2472…

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how can I get a listing of everything that's done in a program
by Ralph Crawford 22 Oct '09

22 Oct '09

Hi. I'm working with GHC 6.10.4 on Solaris. I'm new at Haskell and have inherited a rather large and complex program to maintain and modify. I'd like to get a listing of everything - every single step - that's done in a run of the program. Something similar to the output of the :step command, but I'd like the entire run (which would involve tens of thousands of steps). Is there a way to just do this - have it run and print everything it does to the screen? I've tried doing the following, after starting a "script" to capture all of the screen output... ghci -fglasgow-exts parsequery.hs :break 36 main which takes me to this prompt... Loading package mtl-1.1.0.2 ... linking ... done. Stopped at lesearch.hs:36:13-39 _result :: IO SearchState = _ input :: [String] = _ st :: SearchState = _ [lesearch.hs:36:13-39] *Main> This takes me to the call before the one that I want to output each step of so that I can then manually, with hardcopy, read it and get an idea of what happened. So I start a step through the next call. [lesearch.hs:36:13-39] *Main> :step process_query_loop st input Stopped at lesearch.hs:(46,0)-(69,39) _result :: IO SearchState = _ ... [lesearch.hs:(46,0)-(69,39)] *Main> So far so good. This is what I want to see - a listing like this for every (interpreted of course) line of haskell code that runs, all the way to the end. Since this is a very large program, at this point I started pasting this to the terminal - 4 steps at a time... :step :step :step :step This gave me the listing I wanted. But after a certain point, it inevitably fails with a core dump, and what I capture from the screen is garbled up to that point anyway. I'm hoping there's a simpler way to do this. Thanks for taking the time to read this. Ralph

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IP issues in writing a ghci adaption?
by Christopher Svanefalk 21 Oct '09

21 Oct '09

Hey everyone, If one wants to write a ghci-like application for the C++ language (and/or other languages as well FTM), which is not a derivative work of the ghci source, are there any intellectual property issues that need to be addressed? For example, would this tool be allowed to use similar syntax and commands as those in ghci (:r, :l etc), have a similar name (gcci?), carry some specific license, etc, or is the field clear? The reason I am asking is of course that I am currently working on something like this (although it will probably differ from ghci in terms of design, implementation, features etc) so I am thankful for any answers! kind regards, Christopher Svanefalk, Student (comp.sc.), Univeristy of Gothenburg, Sweden.

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The template-haskell@haskell.org mailing list
by Simon Peyton-Jones 21 Oct '09

21 Oct '09

Question: is the template-haskell(a)haskell.org mailing list still useful? * It has 250 subscribers compared to 850 for ghc-users. * It has low traffic * template-haskell is the only GHC-specific mailing list associated with a particular language feature. Eg type functions, ghci, etc do not have their own list * The traffic on ghc-users is not dauntingly high. I suggest we abolish it in favour of ghc-users. Any dissenters? (I'd emit a warning, so that people could check they are subscribed to ghc-users.) Simon

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RE: [Template-haskell] How to extract name and type of exported functions in modules
by Simon Peyton-Jones 21 Oct '09

21 Oct '09

[ccing ghc-users] | I'm trying to extract the names and types of exported functions in a module. Quite a reasonable request. But it's not conveniently possible at the moment. Here's what you can do in the Q monad: class (Monad m, Functor m) => Quasi m where -- Fresh names qNewName :: String -> m Name -- Error reporting and recovery qReport :: Bool -> String -> m () -- Report an error (True) or warning (False) -- ...but carry on; use 'fail' to stop qRecover :: m a -> m a -> m a -- Recover from the monadic 'fail' -- The first arg is the error handler -- Inspect the type-checker's environment qReify :: Name -> m Info qLocation :: m Loc -- Input/output (dangerous) qRunIO :: IO a -> m a But you want one more function: qReifyModule :: String -> m ModuleInfo where ModuleInfo is something like: data ModuleInfo = MI { mi_exports :: [Name] , ... instances,rules, etc } Then you could use qReify to get info about a particular thing. This would not be hard in principle, the details need thought. For example, if module exports T( C1, C2), where C1, C2 are constructors of T which also has constructors C3, C4, what should appear in mi_exports? Should mi_exports reflect the structure of the export list (see the AvailInfo type in GHC). What is the info for an instance? Do we need a way to ask for all the instances of a class (or rules for a function) regardless of what module those instances come from? etc Does this ring bells for anyone else? Would someone like to write the spec? Are there other reify-related things that we need? Simon | -----Original Message----- | From: template-haskell-bounces(a)haskell.org [mailto:template-haskell- | bounces(a)haskell.org] On Behalf Of Oscar Finnsson | Sent: 16 October 2009 18:47 | To: template-haskell(a)haskell.org | Subject: [Template-haskell] How to extract name and type of exported functions in | modules | | Hi, | | I'm trying to extract the names and types of exported functions in a module. | | At the moment I've managed to get a list of all functions in a module | but I can't seem to figure out how to get their types. | | Lets say I got the module | | module Foo where | | foo1 :: String -> String | foo1 value = value | | foo2 = "hej" | | and then in anothor module... | | module Bar where | | bar = $(getAllFunctions "<some-path>/Foo.hs") | | At the moment I got getAllFunctions returning ["foo1","foo2"], but I | would really like to get it to return [("foo1",AppT (ConT "String") | (ConT "String")), ("foo2",ConT "String")] | | Using "parseModuleWithMode" from Language.Haskell.Exts I can get hold | of the names and the type signature of foo1 (since it's specified in | the source code) but I cannot get hold of the type signature of foo2 | (since it's not specified in the source code). | | Is there another way to get the names/signatures of exported functions | from a module other than using parseModuleWithMode so the developer | writing the Foo-module isn't forced to explicitly supply type | signatures of the exported functions? | | If I try "reify" to get information about the functions I get the error message: | "foo1 is not in scope at a reify" | | This seems to be a known bug about reify (reported here | http://hackage.haskell.org/trac/ghc/ticket/2339) My problem is that I | cannot use the workaround since I don't know the name of the | functions. | | Another disadvantage with this approach is that "getAllFunctions" must | have access to the source code of the module and that I must supply | the path the the module. If possible I would like to have code such as | | bar = $(getAllFunctions "Foo") | | instead of "<come-path>/Foo.hs". | | Regards, | Oscar Finnsson | _______________________________________________ | template-haskell mailing list | template-haskell(a)haskell.org | http://www.haskell.org/mailman/listinfo/template-haskell

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6.12.1rc1: non-Latin filenames in GHCi
by Chryssochoidis Christos 20 Oct '09

20 Oct '09

I've noticed that when I load in GHCi a file that has a non-latin name, although it gets loaded, its name appears garbled in the message after the loading: > Prelude> :load πρόχειρον.hs > [1 of 1] Compiling Main ( ÏÏÎ > ¿ÌÏÎµÎ¹ÏÎ¿Î½.hs, interpreted ) > Ok, modules loaded: Main. > *Main> Is this expected, or something is wrong with my setup? It's a rather insignificant issue, but I thought to ask about it. I'm using GHC 6.12.1rc1 on Mac OS X 10.6.1. Thanks very much, Christos Chryssochoidis

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Broken link in GHC page
by Marco Túlio Gontijo e Silva 20 Oct '09

20 Oct '09

Hi. The link to the mailing list on the GHC page is not correct. It should link to http://www.haskell.org/ghc/docs/latest/html/users_guide/mailing-lists-GHC.h… instead of http://www.haskell.org/ghc/docs/latest/html/users_guide/introduction-GHC.ht… . Greetings. -- marcot http://marcot.iaaeee.org/

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TAG IN 6.8
by Aswad, Mustafa 20 Oct '09

20 Oct '09

Hi I am try update GUM But I have problem with Tagging and Untagging closures any can direct me to a document to understand the tagging procedure. best washes -- Heriot-Watt University is a Scottish charity registered under charity number SC000278.

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