Re: [Haskell-cafe] Desugaring of infix operators is (always?) the wrong way round
Brian Hulley wrote: Dan Piponi wrote: On 9/25/07, Brian Hulley [EMAIL PROTECTED] wrote: .. I don't understand what you mean. For example, with the prefix definition of a function with multiple clauses, the function name at the start of each clause is already lined up since it must appear at the margin of the current layout block (especially if you follow the simple rule of always following a layout starter token by a newline rather than starting a new multi-line layout block in the middle of a line), whereas with the postfix notation you'd need to manually line up the function names if you wanted the same neat look. Or you could have everything be backwards, and use an editor that right aligns things. It's not so clear to me what the syntax for types should be in a postfix language. Postfix, of course! So you'd write data a Tree = Leaf | a a Tree data a a Tree | Leaf = a Tree A postfix version could be: map :: (a - b) - a List - b List Empty _ map = Empty (h t PushF) f map = (h f) (t f map) PushF (a - b) - [a] - [b] :: map [] = [] _ map x f : xs f map = (x:xs) f map - Sam ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Noob question and sequence of operations (print then do something else)
John Wicket wrote:
On 9/24/07, Sam Hughes [EMAIL PROTECTED] wrote:
John Wicket wrote:
I am still in an imperative way of thinking. In this example here; how
would I call putStrLn and then set the function with a value. Eg:
aa :: String - IO ()
aa instr = do
putStrLn abc
putStrLn abc
return 123
--- The error I am getting.
Couldn't match expected type `()' against inferred type `[Char]'
In the first argument of `return', namely `123'
In the expression: return 123
In the expression:
do putStrLn abc
putStrLn abc
return 123
Your type signature is wrong. If you want an IO action whose return
value is a String, say so:
aa :: String - IO String
Sorry, I was actually trying to use this as an example for something more
complicated I am trying to do. In this example, why would the
inferred type
be IO ()
aa :: String - String
aa instr = do
putStrLn abc
putStrLn abc
return Az
Couldn't match expected type `[t]' against inferred type `IO ()'
In the expression: putStrLn abc
In a 'do' expression: putStrLn abc
In the expression:
do putStrLn abc
putStrLn abc
return Az
Ah. Because the value, putStrLn abc, is a value of type IO (). Your
problem is that you're trying to do an input/output action in a pure
function. You'll need a function that returns a value of type 'IO
String' (or 'IO somethingElse'). Then the do notation is used to
construct actions, by chaining small actions together. Here your
compiler thinks it's trying to construct a list, because do notation can
be used for any monad...
Note that 'return' isn't a keyword that returns from a function, it's a
function that returns a value. For example, in the code
foo :: IO Int
foo = do print 3
return 5
return 5 is of type IO Int. That is, it's an action that 'returns' a
value of type Int (when executed), so to speak. ('print 3' is an action
that 'returns' a value of type (), by the way.)
When you say you have
aa :: String - String,
you're advertising that (aa foo) is a String. You're not advertising
that it's an action that returns a String, because they're different
things. do notation isn't for doing things and returning a value, it's
for combining actions together into one bigger action. If you want to
do things, you need your function to construct an action that does
things, which means you want
aa :: String - IO String
But the function 'aa' doesn't do things, it constructs an action.
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Re: [Haskell-cafe] Noob question and sequence of operations (print then do something else)
John Wicket wrote: I am still in an imperative way of thinking. In this example here; how would I call putStrLn and then set the function with a value. Eg: aa :: String - IO () aa instr = do putStrLn abc putStrLn abc return 123 --- The error I am getting. Couldn't match expected type `()' against inferred type `[Char]' In the first argument of `return', namely `123' In the expression: return 123 In the expression: do putStrLn abc putStrLn abc return 123 Your type signature is wrong. If you want an IO action whose return value is a String, say so: aa :: String - IO String ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How can I stop GHCi from calling show for IO actions?
Ryan Ingram wrote: Prelude let inf = repeat 1 Prelude inf [1,1,(lots of output until I press ctrl-c),Interrupted. (I expect this to happen) Prelude let x = inf (no output here!) Prelude :t x x :: [Integer] Prelude return inf [1,1,(lots of output until I press ctrl-c),Interrupted. (I also expect this to happen) Prelude y - return inf [1,1,(lots of output until I press ctrl-c),Interrupted. (I do not expect this to happen here!) Prelude :t y interactive:1:0: Not in scope: 'y' Is this a bug? Why does y - return exp have different behavior than let y = exp? Is there a way to make GHCi not print the result of an action but still make my variables get bound? That's weird. Prelude (x,y) - return $ (repeat 1, repeat 2) Prelude Just x - return $ Just (repeat 1) [1,1,1,... Prelude (x,_) - return $ (repeat 1, repeat 2) [1,1,1,... Prelude Just (x,y) - return $ Just (repeat 1, repeat 2) Prelude It seems that GHCi outputs the contents of the variable you've created when there's only one of them. - Sam ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] IO within parser
Malte Milatz wrote: Sam Hughes [EMAIL PROTECTED], Sun, 12 Aug 2007 20:12:55 -0400: [A parser like Parsec, with monad transformers:] $ darcs get http://samuelhughes.com/darcs/partran/ Is this related in any way to the following GSoC project? http://code.google.com/soc/2007/haskell/appinfo.html?csaid=B97EF4562EF3B244 No, I just wanted it for my own purposes. - Sam ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] IO within parser
Malte Milatz wrote: If not using unsafePerformIO, which is usually not what we want, the monad m in question must incorporate IO. That is, it could be defined something like (say we want a parser with state): newtype IOParser tok s a = IOParser (s - [tok] - IO (s,a)) You can then define liftIO without “taking apart” the IO value; instead you put liftIO's IO action (IO a) into IOParser's IO action (IO (s,a)). Parsec does not define any such parser, though, so there's nothing for which you may define an instance of MonadIO. Malte I'm making (or have made) such a thing, although not necessarily atop IO, atop any monad that provides certain stream-like functionality. It's kind of done, but not really. $ darcs get http://samuelhughes.com/darcs/partran/ That is virtually completely untested and has not been proven correct either. Also, whatever documentation there is has probably accidentally been copied and pasted from Parsec's (except for the documentation of Stream.hs). Some of the unthinkingly done translations of the files found in Parsec's hierarchy seem to be idiomatically immoral. (Token.hs comes to mind. Something went wrong with functional dependencies and avoiding things like breaking the coverage condition/undecidable instances and it hurts. Maybe somebody could tell me what I'm doing wrong.) Also, some functions and definitions of things might be appropriately moved around, and you'll probably end up having to import more than you'd like to. Text.ParserCombinators.ParTran.Parsec /might/ currently contain a correct simulation of Text.ParserCombinators.Parsec. - Sam ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
