a newbie's question about something related to runST
Hi, First I give the following primitive operations newVar :: a - ST s (MutVar s a) readVar :: MutVar s a - ST s a writeVar :: MutVar s a -a - ST s () Next consider the function f :: MutVar s a - Mut Var s a f v = runST (newVar v `thenST` \w - readVar w) 1. What is the type given to newVar v by the typechecker? Friendly Jan Brosius
How to debug Haskell programs?
This question may sound a bit strange. I'll try to explain. I have written a piece of Haskell and found that I did not understand things. I tried to look what I got wrong and searched for a possibility to give me output. Now obvious is that Haskell has not debugger so I was thinking helping myself with adding some simple output stuff but that did not seem to work either. So guess I have sum:: [Int] - Int sum [] = 0 sum (x:xs) = Now how can I do the following print the actual sum in every step and how to see e.g what x or xs is on this step? I tried it with simple ouput but did not get it right. Any suggestions? Friedrich
Re: a newbie's question about something related to runST
On 26-Apr-2000, Jan Brosius [EMAIL PROTECTED] wrote: Hi, First I give the following primitive operations newVar :: a - ST s (MutVar s a) readVar :: MutVar s a - ST s a writeVar :: MutVar s a - a - ST s () Next consider the function f :: MutVar s a - Mut Var s a f v = runST (newVar v `thenST` \w - readVar w) 1. What is the type given to newVar v by the typechecker? Let's see... `v' has type `MutVar s a', and after renaming apart `newVar' has type `forall a2,s2 . a2 - ST s2 (MutVar s2 a2)', so, substituting `Mutvar s a' for `a2', we see that `newVar v' has type `forall s2 . ST s2 (MutVar s2 (MutVar s a))'. -- Fergus Henderson [EMAIL PROTECTED] | "I have always known that the pursuit WWW: http://www.cs.mu.oz.au/~fjh | of excellence is a lethal habit" PGP: finger [EMAIL PROTECTED]| -- the last words of T. S. Garp.
Re: How to debug Haskell programs?
On 26 Apr 2000, Friedrich Dominicus wrote: This question may sound a bit strange. I'll try to explain. I have written a piece of Haskell and found that I did not understand things. I tried to look what I got wrong and searched for a possibility to give me output. Now obvious is that Haskell has not debugger so I was thinking helping myself with adding some simple output stuff but that did not seem to work either. So guess I have sum:: [Int] - Int sum [] = 0 sum (x:xs) = Now how can I do the following print the actual sum in every step and how to see e.g what x or xs is on this step? I tried it with simple ouput but did not get it right. Any suggestions? Friedrich import IOExts sum:: [Int] - Int sum [] = 0 sum (x:xs) = trace (show (x:xs) ++ "\n") Note, this is an unpure extension to haskell that should be used for debugging purposes only. It can sometimes be very hard to interpret the output since it will be produced in the order the expressions are evaluated. Anyway, it is sometimes useful. /Lars L
basAlgPropos
Fergus Henderson [EMAIL PROTECTED] writes on my letter on Basic algebra proposal These features are not mandatory in proposal. The jury may decide to rewrite the proposal removing the usage of these features. [..]. I think the "jury" is unlikely to rewrite the proposal; more likely, they will simply reject the proposal. It is up to the proponent(s) of the proposal to rewrite it. It needs clarification. The Proposal says that it is draft. I thought that some committee, - which I called "jury" (maybe, wrongly), - has to consider it. If the committee decides that it is too hard to revise the proposal to something satisfactory, it tells the author that the proposal is rejected. Otherwise, it recommends the author in what way to revise the proposal, and after the revision, considers it again. This was my naive idea of considering the library standard. The committee may be official or in-formal, it may consist of a sidle person or of 1000, I do not know. But I had to refer to it in a letter, somewhat call it. I have no idea of how such committee may appear. Neither do I care much of what with this proposal will happen. -- Sergey Mechveliani [EMAIL PROTECTED]
openfile :: String - String
The Gofer prelude had a function openfile :: String - String which mapped file path names to strings containing the named files' contents. The Hugs98 Prelude doesn't seem to have anything like that function. Instead, it has things like readFile :: String - IO String Why would I want openfile instead of readFile? The problem in which the need for openfile arose is this: I want to load a binary search tree and a list with words read from a file, and then perform, interactively, several tests comparing the cost of searching the tree with that of searching the list. In addition to performing the tests interactively, I want to separate the cost of searching the list and the tree from the cost of constructing them. In order for the list and the tree to be used in several successive command-line evaluations without being reconstructed each time, they must be named globally. This is no problem with openfile, but readFile forces their names to be local to an IO command. Can anyone suggest a solution? Thanks, --Ham -- Hamilton Richards Jr.Department of Computer Sciences Senior Lecturer Mail Code C0500 512-471-9525 The University of Texas at Austin SHC 434 Austin, Texas 78712-1188 [EMAIL PROTECTED] --
RE: openfile :: String - String
You could try this: --OpenFile.hs module OpenFile where import IOExts openfile :: String - String openfile = unsafePerformIO . readFile -- end of file -Original Message- From: Hamilton Richards [mailto:[EMAIL PROTECTED]] Sent: 26 April 2000 14:04 To: [EMAIL PROTECTED] Subject: openfile :: String - String The Gofer prelude had a function openfile :: String - String which mapped file path names to strings containing the named files' contents. The Hugs98 Prelude doesn't seem to have anything like that function. Instead, it has things like readFile :: String - IO String Why would I want openfile instead of readFile? The problem in which the need for openfile arose is this: I want to load a binary search tree and a list with words read from a file, and then perform, interactively, several tests comparing the cost of searching the tree with that of searching the list. In addition to performing the tests interactively, I want to separate the cost of searching the list and the tree from the cost of constructing them. In order for the list and the tree to be used in several successive command-line evaluations without being reconstructed each time, they must be named globally. This is no problem with openfile, but readFile forces their names to be local to an IO command. Can anyone suggest a solution? Thanks, --Ham -- Hamilton Richards Jr.Department of Computer Sciences Senior Lecturer Mail Code C0500 512-471-9525 The University of Texas at Austin SHC 434 Austin, Texas 78712-1188 [EMAIL PROTECTED] --
Re: openfile :: String - String
On Wed, 26 Apr 2000, Hamilton Richards wrote: The Gofer prelude had a function openfile :: String - String which mapped file path names to strings containing the named files' contents. The Hugs98 Prelude doesn't seem to have anything like that function. Instead, it has things like readFile :: String - IO String Why would I want openfile instead of readFile? The problem in which the need for openfile arose is this: I want to load a binary search tree and a list with words read from a file, and then perform, interactively, several tests comparing the cost of searching the tree with that of searching the list. In addition to performing the tests interactively, I want to separate the cost of searching the list and the tree from the cost of constructing them. In order for the list and the tree to be used in several successive command-line evaluations without being reconstructed each time, they must be named globally. This is no problem with openfile, but readFile forces their names to be local to an IO command. Why is this a problem? After all, the names ARE dependent on IO. Can anyone suggest a solution? I do not understand the problem. Is it something like this you want? parseTree :: String - Tree parseList :: String - List buildTree:: IO Tree buildTree = do f - readFile "thetree" return (parseTree f) buildList:: IO List buildList = do f - readFile "thelist" return (parseList f) test :: IO() test = do tree - buildTree list - buildList dowhateverYouwant_interactive_or_not tree list dowhateverYouwant_interactive_or_not :: Tree - List - IO() /Lars L
Re: a newbie's question about something related to runST
On 26-Apr-2000, Jan Brosius [EMAIL PROTECTED] wrote: Hi, First I give the following primitive operations newVar :: a - ST s (MutVar s a) readVar :: MutVar s a - ST s a writeVar :: MutVar s a - a - ST s () Next consider the function f :: MutVar s a - Mut Var s a f v = runST (newVar v `thenST` \w - readVar w) 1. What is the type given to newVar v by the typechecker? Let's see... `v' has type `MutVar s a', and after renaming apart `newVar' has type `forall a2,s2 . a2 - ST s2 (MutVar s2 a2)', so, substituting `Mutvar s a' for `a2', we see that `newVar v' has type `forall s2 . ST s2 (MutVar s2 (MutVar s a))'. -- Ok, Next recall runST :: forall s. (ST s a) - a and let runST1 give the type signature runST1 :: ST s a - a consider v = runST1 ( newVar True) Then newVar True gets the type ST s (MutVar s a) ( I prefer the more sloppy notation without a forall as it gives less clutter). In runST1 :: ST s a - a Let us substitute a by say MutVar s a we then get after renaming the s in runST1 by s2 ST s2 ( MutVar s a ) - MutVar s a ) Have we finished ? One would say no : one needs a substitution for s2 , If we don't "cheat" we must now substitute s2 by s and we get ST s (MutVar s) - MutVar s a In principle this is not the type of runST. Because technically the "domain" of runST is of the form ST s a where a is never of the form T(s) Finally consider runST :: forall s . ( ST s a) - a What to do now ? let us first substitute a by MutVar s a as shown above. After renaming there remains forall s2 . ( ST s2 (MutVar s a)) - MutVar s a What is now the meaning of the forall s2 above? If it means that s2 can be substituted then we must substitute it by s But that is not what one wants : one wants s2 to remain non free. Where I wanted to come is this: give runST the type signature runST :: exists s . ( ST s a ) - a and technically we have also a bounded variable s. What is the reason for choosing forall against exists ? I think there is no reason for it. But I can be wrong. Friendly Jan Brosius
Re: openfile :: String - String
Wy not load the list as program? E.g. list1 = ["word1" ,"word2" ,"word3" ] list2 = words "word1 word2 word3" list3 = words "\ \ word1\ \ word2\ \ word3\ \" John Atwood - Lars Lundgren wrote: On Wed, 26 Apr 2000, Hamilton Richards wrote: The Gofer prelude had a function openfile :: String - String which mapped file path names to strings containing the named files' contents. The Hugs98 Prelude doesn't seem to have anything like that function. Instead, it has things like readFile :: String - IO String Why would I want openfile instead of readFile? The problem in which the need for openfile arose is this: I want to load a binary search tree and a list with words read from a file, and then perform, interactively, several tests comparing the cost of searching the tree with that of searching the list. In addition to performing the tests interactively, I want to separate the cost of searching the list and the tree from the cost of constructing them. In order for the list and the tree to be used in several successive command-line evaluations without being reconstructed each time, they must be named globally. This is no problem with openfile, but readFile forces their names to be local to an IO command. Why is this a problem? After all, the names ARE dependent on IO. Can anyone suggest a solution? I do not understand the problem. Is it something like this you want? parseTree :: String - Tree parseList :: String - List buildTree:: IO Tree buildTree = do f - readFile "thetree" return (parseTree f) buildList:: IO List buildList = do f - readFile "thelist" return (parseList f) test :: IO() test = do tree - buildTree list - buildList dowhateverYouwant_interactive_or_not tree list dowhateverYouwant_interactive_or_not :: Tree - List - IO() /Lars L
Re: openfile :: String - String
At 3:18 PM +0200 4/26/00, Lars Lundgren wrote: [...] Is it something like this you want? [...] test :: IO() test = do tree - buildTree list - buildList dowhateverYouwant_interactive_or_not tree list dowhateverYouwant_interactive_or_not :: Tree - List - IO() Sorry, I didn't make my problem clear enough. The reason your suggestion doesn't solve it is that `tree` and `list` are local to `test`, so they can't be used in several separate **command-line** evaluations without being reconstructed each time. My students don't yet know about interactive I/O; even if they did, I know of no way to get separate reduction counts for, say, tree searches and list searches within dowhateverYouwant_interactive_or_not. But thanks anyway for taking the time to answer my ill-posed question. Cheers, --Ham -- Hamilton Richards Jr.Department of Computer Sciences Senior Lecturer Mail Code C0500 512-471-9525 The University of Texas at Austin SHC 434 Austin, Texas 78712-1188 [EMAIL PROTECTED] --
Re: openfile :: String - String
Angus is right on the track. I would only modify it slightly: content_xxx :: String content_xxx = (unsafePerformIO . readFile) "xxx" From Hugs perspective content_xxx is a constant. Your may easily demonstrate it this way: :!echo blah xxx content_xxx == "blah" :!echo Hello xxx content_xxx == "blah" Jan
Re: a newbie's question about something related to runST
On 26-Apr-2000, Jan Brosius [EMAIL PROTECTED] wrote: runST :: forall s. (ST s a) - a It would be clearer to write that as runST :: (forall s. ST s a) - a rather than relying on the relative precedences of `.' and `-'. give runST the type signature runST :: exists s . ( ST s a ) - a and technically we have also a bounded variable s. What is the reason for choosing forall against exists ? `forall' means that the variable can be instantiated by the caller, but not by the callee. `exists' means the opposite, that the variable can be instantiated by the callee, but not the caller. If a function has a universally quantified type such as `forall s . some type involving s', then the function definition must be polymorphic in `s', but the caller can call it with `s' bound to some particular instance. For `exists', it is the other way around, the caller must be polymorphic in `s' but the function definition could bind it to some particular instance. If some Haskell implementation supported `exists' in that position, and runST were declared with `exists' rather than `forall', then the following example would be well-typed: data MyStore = MyStore side_effect :: STRef MyStore Int - Int - () side_effect ref new_val = runST arg where arg :: exists s . ST s () arg = clobber_it clobber_it :: ST MyStore () clobber_it = writeSTRef ref new_val Note here how the definition `arg = clobber_it' binds the existentially quantified variable `s' to `MyStore'. This allows the side effect to escape the ST monad. You could then go ahead and use this nasty side_effect: main = print (runST nasty) nasty :: ST MyStore Int nasty = do x - newSTRef 1 dummy - newSTRef (side_effect x 42) readSTRef x This might print either 1 or 42, depending on whether side effect in the construction of the unused `dummy' variable was optimized out. Ouch! -- Fergus Henderson [EMAIL PROTECTED] | "I have always known that the pursuit WWW: http://www.cs.mu.oz.au/~fjh | of excellence is a lethal habit" PGP: finger [EMAIL PROTECTED]| -- the last words of T. S. Garp.
