Re: [Haskell-cafe] Newbie question on Haskell type
Thanks all of your for your time and your interesting examples. Now I
can see that my problem is parsing a String. I am new in Haskell, so, I
start to study parsing and how to create a parser from beginning.
I start with an example from the book as follows:
%The parser item fails if the input is empty and consumes the first character otherwise.
\begin{code}
newtype Parser a = Parser(String - [(a, String)])
item::Parser Char
item = Parser(\cs - case cs of
- []
(c:cs) - [(c,cs)])
parse :: Parser a - String - [(a, String)]
parse p cs = p cs
\end{code}
and I compile, the error displays. I do not know how to fix it. Please help me.
$ghci parser.lhs
parser.lhs:10:13:
Couldn't match `Parser a' against `t - t1'
Expected type: Parser a
Inferred type: t - t1
Probable cause: `p' is applied to too many arguments in the call (p cs)
In the definition of `parse': parse p cs = p cs
Failed, modules loaded: none.
On 10/14/05, Ralf Hinze [EMAIL PROTECTED] wrote:
Hi Huong,attached you find a small program for parsing values of various (data)types. It uses a generalized algebraic data type for representing typesand a universal data type for representing values. The parser itself is
rather simple-minded: it builds on Haskell's ReadS type.I don't know whether this is what you are after, but it was fun writing.There are many opportunities for improvement: one could use a decent
combinator library for parsing; a type of dynamic values instead of auniversal type etc.Here are some example calls:Main parseAny 4711[(ValInt 4711,)]Main parseAny \4711\
[(ValString 4711,)]Main parseAny [4711, 0][(ValList [ValInt 4711,ValInt 0],)]Main parseAny [4711, 'a'][(ValList [ValInt 4711,ValChar 'a'],)]
Main parseAny [\hello world\][(ValList [ValString hello world],)]Note that parseAny even parses heterogenous lists.Cheers, Ralf
--- {-# OPTIONS -fglasgow-exts #-} data Type :: * - * where Char:: Type Char Int :: Type Int List:: Type a - Type [a] Value :: Type Value
string :: Type String string=List Char parse :: Type t - ReadS t parse (Char) =reads parse (Int)=reads parse (List Char)=reads parse (List a) =parseList (parse (a))
parse (Value)=parseAny data Value =ValCharChar |ValInt Int |ValStringString |ValList[Value] deriving (Show) parseAny
= ValChar $ parse Char + ValInt$ parse Int + ValString $ parse string + ValList $ parse (List Value)Helper functions.
parseList parsea =readParen False (\ s - [ xs | ([, t) - lex s, xs - parsel t ]) where parsels= [ ([], t) | (], t) - lex s ]++[
(x : xs, u) | (x, t) - parsea s,(xs,u)
- parsel' t ]
parsel' s= [ ([], t) |
(], t) - lex s ]++[
(x : xs, v) | (,, t) - lex s,(x,
u) - parsea t,(xs,v)
- parsel' u] infix8$ infixr 6+ (f $ p) s=[ (f a, t) | (a, t) - p s ] (p + q) s=p s ++ q s
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Re: [Haskell-cafe] Newbie question on Haskell type
Huong Nguyen [EMAIL PROTECTED] writes:
newtype Parser a = Parser(String - [(a, String)])
[...]
parse :: Parser a - String - [(a, String)]
parse p cs = p cs
\end{code}
Try this instead:
parse (Parser p) cs = p cs
(You forgot to deconstruct! :) )
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Re: [Haskell-cafe] Newbie question on Haskell type
Right, forgot about seq there, but the point still holds that there are a very limited number of functions of that type, and in particular, the functions can't decide what to do based on the type parameter 'a'. - Cale On 14 Oct 2005 05:49:27 -, [EMAIL PROTECTED] [EMAIL PROTECTED] wrote: --- Cale Gibbard [EMAIL PROTECTED] wrote: As an example of this sort of thing, I know that there are only 4 values of type a - Bool (without the class context). They are the constant functions (\x - True), (\x - False), and two kinds of failure (\x - _|_), and _|_, where _|_ is pronounced bottom and represents something along the lines of nontermination (aborting the program also counts). Not exactly. There are also (\x - seq x True) and (\x - seq x False), neither of which is equivalent to any of the four functions above. Regards, Janis Voigtlaender. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question on Haskell type
On Fri, Oct 14, 2005 at 03:17:12AM -0400, Cale Gibbard wrote: Right, forgot about seq there, but the point still holds that there are a very limited number of functions of that type, and in particular, the functions can't decide what to do based on the type parameter 'a'. actually, without 'seq' _|_ and \_ - _|_ are indistinguishable. so you only have 3 functions without seq, and 6 with it. John -- John Meacham - ⑆repetae.net⑆john⑈ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question on Haskell type
Cale Gibbard wrote: As an example of this sort of thing, I know that there are only 4 values of type a - Bool (without the class context). They are the constant functions (\x - True), (\x - False), and two kinds of failure (\x - _|_), and _|_, where _|_ is pronounced bottom and represents something along the lines of nontermination (aborting the program also counts). Don't forget (\x - x `seq` True) and (\x - x `seq` False). -- Ben ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question on Haskell type
On 10/13/05, Huong Nguyen [EMAIL PROTECTED] wrote: Hi all, I want to write a small functionto test whether an input is a String or not. For example, isString::(Show a) =a -Bool This function will return True if the input is a string and return False if not Any of you have idea about that? Thanks in advance I simply can not think of a reason for why you would want to do that. The system already knows whether a values if of type String at *compile-time* so there shouldn't really be any reason to test it at run-time. I think you'll get more insight if you just tell us why you think you need it and then we could probably show you the idiomatic haskell way to achieve what you need. /S -- Sebastian Sylvan +46(0)736-818655 UIN: 44640862 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question on Haskell type
So this is essentially a parsing problem. You want a user to be able input a string and have it interpreted as an appropriate data value. I think you may want to look at the Parsec library (http://www.cs.uu.nl/~daan/parsec.html). I don't think the direction you are heading will get the results you want. As to typeable, the basic types are mostly all members of Typeable. You can find a pretty good list here: http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data.Typeable.html#t%3ATypeable Additionally, GHC can create Typeable instances automaticly for user defined datatypes; just add a deriving Typeable clause. data SomeType = C1 | C2 deriving (Show,Eq,Typeable) The restriction is that all types which appear in constructors must also be in Typeable. I believe the DrIFT preprocessor can also create Typeable instances if you are not using GHC. Huong Nguyen wrote: Hello, Thanks for your solution. My main purpose is that I want to input a value and check whether this value is belong to some specific types or not. These types can be some popular types (such as: String, Char, Int, etc) or some more complex data structures defined by user. Thus, at first, I try with type String (even with that simple type, I still face difficulty ;-)) I want to ask you which types can be used with Data.Typeable. I read for over 15 minutes but it is still not clear with me. For some other complex data types defined by user, what I should do to use Data.Typeable ? Thank you very much. On 10/13/05, *robert dockins* [EMAIL PROTECTED] mailto:[EMAIL PROTECTED] wrote: In GHC you can do this: import Data.Typeable isString :: (Typeable a) = a - Bool isString x = typeOf x == typeOf (undefined::String) Why do you want this? It's not the kind of operation one does very often in Haskell. Huong Nguyen wrote: Hi all, I want to write a small functionto test whether an input is a String or not. For example, isString::(Show a) =a -Bool This function will return True if the input is a string and return False if not Any of you have idea about that? Thanks in advance ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org mailto:Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question on Haskell type
Hi Huong,
attached you find a small program for parsing values of various (data)
types. It uses a generalized algebraic data type for representing types
and a universal data type for representing values. The parser itself is
rather simple-minded: it builds on Haskell's ReadS type.
I don't know whether this is what you are after, but it was fun writing.
There are many opportunities for improvement: one could use a decent
combinator library for parsing; a type of dynamic values instead of a
universal type etc.
Here are some example calls:
Main parseAny 4711
[(ValInt 4711,)]
Main parseAny \4711\
[(ValString 4711,)]
Main parseAny [4711, 0]
[(ValList [ValInt 4711,ValInt 0],)]
Main parseAny [4711, 'a']
[(ValList [ValInt 4711,ValChar 'a'],)]
Main parseAny [\hello world\]
[(ValList [ValString hello world],)]
Note that parseAny even parses heterogenous lists.
Cheers, Ralf
---
{-# OPTIONS -fglasgow-exts #-}
data Type :: * - * where
Char:: Type Char
Int :: Type Int
List:: Type a - Type [a]
Value :: Type Value
string :: Type String
string = List Char
parse :: Type t - ReadS t
parse (Char) = reads
parse (Int)= reads
parse (List Char) = reads
parse (List a) = parseList (parse (a))
parse (Value) = parseAny
data Value
= ValCharChar
| ValInt Int
| ValString String
| ValList[Value]
deriving (Show)
parseAny
= ValChar $ parse Char
+ ValInt$ parse Int
+ ValString $ parse string
+ ValList $ parse (List Value)
Helper functions.
parseList parsea
= readParen False (\ s - [ xs | ([, t) - lex s, xs - parsel t ])
where parsel s = [ ([], t) | (], t) - lex s ]
++ [ (x : xs, u) | (x, t) - parsea s,
(xs, u) - parsel' t ]
parsel' s = [ ([], t) | (], t) - lex s ]
++ [ (x : xs, v) | (,, t) - lex s,
(x, u) - parsea t,
(xs, v) - parsel' u]
infix 8 $
infixr 6 +
(f $ p) s = [ (f a, t) | (a, t) - p s ]
(p + q) s = p s ++ q s
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Re: [Haskell-cafe] Newbie question on Haskell type
isString::(Show a) =a -Bool This function will return True if the input is a string and return False if not This is not particularly nicely - you certainly can't write it as simple as the 'isString' function, and it will probably require type classes etc, quite possibly with haskell extensions. Since haskell is a statically typed langauge the idea is that you know if something is a string before you run the program, not only at runtime. Why is it you want this? Perhaps what you are hoping to accomplish could be done some other way, without requiring this isString function. Of course, if you want a slightly hacky version: isString x = not (null a) head a == '\' last a == '\' where a = show x is probably good enough :) Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question on Haskell type
In GHC you can do this: import Data.Typeable isString :: (Typeable a) = a - Bool isString x = typeOf x == typeOf (undefined::String) Why do you want this? It's not the kind of operation one does very often in Haskell. Huong Nguyen wrote: Hi all, I want to write a small functionto test whether an input is a String or not. For example, isString::(Show a) =a -Bool This function will return True if the input is a string and return False if not Any of you have idea about that? Thanks in advance ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question on Haskell type
--- Cale Gibbard [EMAIL PROTECTED] wrote: As an example of this sort of thing, I know that there are only 4 values of type a - Bool (without the class context). They are the constant functions (\x - True), (\x - False), and two kinds of failure (\x - _|_), and _|_, where _|_ is pronounced bottom and represents something along the lines of nontermination (aborting the program also counts). Not exactly. There are also (\x - seq x True) and (\x - seq x False), neither of which is equivalent to any of the four functions above. Regards, Janis Voigtlaender. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
