[bcc haskell, cc haskell-cafe]
On 9/5/07, Tomi Owens
Hi there. I'm a teacher of Maths and am working my way through the Euler Project problems for fun. I have mostly been using Basic, but have read up about Haskell and think it looks like a sensible way to solve many of the problems.
OK, so I've downloaded GHCi and am trying to teach myself.
So far I have done this:
___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 6.6.1, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help.
Loading package base ... linking ... done. Prelude> let f (a,b) = a * floor (100000/b) Prelude> f(2,5) 40000
Here you can find out type ghci has inferred for this function.
:t f f :: (RealFrac b, Integral b1) => (b1, b) -> b1
This function works just as I want it to.
Now I try creating a list:
Prelude> [(a2+b2,a)| a <- [1..4] , b<- [1..4], a2+b2<20, b<=a] [(2,1),(5,2),(8,2),(10,3),(13,3),(18,3),(17,4)]
Let's assign this to an intermediate variable so we can query it's type: Prelude> let lst = [(a ^ 2 + b ^ 2, a) | a <- [1..4], b <- [1..4], a^2 + b^2 < 20, b <= a] Prelude> lst [(2,1),(5,2),(8,2),(10,3),(13,3),(18,3),(17,4)] Prelude> :t lst lst :: [(Integer, Integer)] aha; here's the source of the type mismatch: Prelude> :t floor floor :: (RealFrac a, Integral b) => a -> b Floor has to take a RealFrac. According to hoogle[1], we can use various floating-point approximations (Float, Double, CFloat, etc) or we can use the exact Rational type. [1] http://haskell.org/hoogle/?q=RealFrac You can get your types to match by declaring your list to be of type [(Rational, Rational)] either by explicitly typing one of the untyped variables or the entire expression: Prelude> let lst = [(a ^ 2 + b ^ 2, a) | (a::Rational) <- [1..4], b <- [1..4], a^2 + b^2 < 20, b <= a] Prelude> :t lst lst :: [(Rational, Rational)] Prelude> let lst :: [(Rational, Rational)] = [(a ^ 2 + b ^ 2, a) | a <- [1..4], b <- [1..4], a^2 + b^2 < 20, b <= a] Prelude> :t lst lst :: [(Rational, Rational)] and this works
So now I try to apply the function to the list:
Prelude> map (f) [(a2+b2,a)| a <- [1..4] , b<- [1..4], a2+b2<20, b<=a]
and I get this result:
<interactive>:1:5: Ambiguous type variable `t' in the constraints: `Integral t' arising from use of `f' at <interactive>:1:5 `RealFrac t' arising from use of `f' at <interactive>:1:5 Probable fix: add a type signature that fixes these type variable(s)
I'm sorry, but I don't quite get how to set the type signature and how it
will apply to my function...
Thanks,
Hope this helps Tomi
--------------------------------------------------------------------------------
Department for Education, Sport and Culture E Mail This message is for the named person's use only. It may contain confidential, proprietary or legally privileged information. No confidentiality or privilege is waived or lost by any mistransmission. If you receive this message in error, please immediately delete it and all copies of it from your system, destroy any hard copies of it and notify the sender. You must not, directly or indirectly, use, disclose, distribute, print, or copy any part of this message if you are not the intended recipient. The Department for Education, Sport and Culture and any of its establishments each reserve the right to monitor all e-mail communications through its networks.
Any views expressed in this message are those of the individual sender, except where the message states otherwise and the sender is authorised to state them to be the views of any such entity.
The Department for Education, Sport and Culture shall not be liable to the recipient or any third party for any loss or damage, however it appears, from this e-mail or its content. This includes loss or damage caused by viruses. It is the responsibility of the recipient to ensure that the opening of this message and its attachments shall not adversely affect systems or data.
--------------------------------------------------------------------------------
_______________________________________________ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
-- Scott Williams
I think you want something like this
{-# OPTIONS -fglasgow-exts #-}
f :: (Integer, Float) -> Integer
f (a,b) = a * floor (100000/b)
lst :: [(Integer, Integer)]
lst = [(a ^ 2 + b ^ 2, a) | a <- [1..4], b <- [1..4], a^2 + b^2 < 20, b <=
a]
lst3 = map (f) ( map ( intTupToFloatTup ) lst )
intTupToFloatTup :: (Integer, Integer) -> (Integer, Float)
intTupToFloatTup (int1, int2) = (int1, fromInteger int2)
load the whole thing into ghci with ghci proggie.hs
when I have this type of problem, my usual approach is to put the code
into a text file, load that in ghci, derive type sigs on the functions
that work, and then see if I can figure out
the mismatch.
you could probably get a fast answer to this kind of question on the
#haskell irc channel as well.
hope this helps,
thomas.
"Scott Williams"
:t f f :: (RealFrac b, Integral b1) => (b1, b) -> b1
This function works just as I want it to. Now I try creating a list: Prelude> [(a2+b2,a)| a <- [1..4] , b<- [1..4], a2+b2<20, b<=a] [(2,1),(5,2),(8,2),(10,3),(13,3),(18,3),(17,4)] Let's assign this to an intermediate variable so we can query it's type: Prelude> let lst = [(a ^ 2 + b ^ 2, a) | a <- [1..4], b <- [1..4], a^2 + b^2 < 20, b <= a] Prelude> lst [(2,1),(5,2),(8,2),(10,3),(13,3),(18,3),(17,4)] Prelude> :t lst lst :: [(Integer, Integer)] aha; here's the source of the type mismatch: Prelude> :t floor floor :: (RealFrac a, Integral b) => a -> b Floor has to take a RealFrac. According to hoogle[1], we can use various floating-point approximations (Float, Double, CFloat, etc) or we can use the exact Rational type. [1] http://haskell.org/hoogle/?q=RealFrac You can get your types to match by declaring your list to be of type [(Rational, Rational)] either by explicitly typing one of the untyped variables or the entire expression: Prelude> let lst = [(a ^ 2 + b ^ 2, a) | (a::Rational) <- [1..4], b <- [1..4], a^2 + b^2 < 20, b <= a] Prelude> :t lst lst :: [(Rational, Rational)] Prelude> let lst :: [(Rational, Rational)] = [(a ^ 2 + b ^ 2, a) | a <- [1..4], b <- [1..4], a^2 + b^2 < 20, b <= a] Prelude> :t lst lst :: [(Rational, Rational)] and this works So now I try to apply the function to the list: Prelude> map (f) [(a2+b2,a)| a <- [1..4] , b<- [1..4], a2+b2<20, b<=a] and I get this result: <interactive>:1:5: Ambiguous type variable `t' in the constraints: `Integral t' arising from use of `f' at <interactive>:1:5 `RealFrac t' arising from use of `f' at <interactive>:1:5 Probable fix: add a type signature that fixes these type variable(s) I'm sorry, but I don't quite get how to set the type signature and how it will apply to my function... Thanks, Hope this helps Tomi -------------------------------------------------------------------------------- Department for Education, Sport and Culture E Mail This message is for the named person's use only. It may contain confidential, proprietary or legally privileged information. No confidentiality or privilege is waived or lost by any mistransmission. If you receive this message in error, please immediately delete it and all copies of it from your system, destroy any hard copies of it and notify the sender. You must not, directly or indirectly, use, disclose, distribute, print, or copy any part of this message if you are not the intended recipient. The Department for Education, Sport and Culture and any of its establishments each reserve the right to monitor all e-mail communications through its networks. Any views expressed in this message are those of the individual sender, except where the message states otherwise and the sender is authorised to state them to be the views of any such entity. The Department for Education, Sport and Culture shall not be liable to the recipient or any third party for any loss or damage, however it appears, from this e-mail or its content. This includes loss or damage caused by viruses. It is the responsibility of the recipient to ensure that the opening of this message and its attachments shall not adversely affect systems or data. -------------------------------------------------------------------------------- _______________________________________________ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell -- Scott Williams _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.
On Wed, 5 Sep 2007, Thomas Hartman wrote:
I think you want something like this
{-# OPTIONS -fglasgow-exts #-}
Why glasgow-exts? You may also want to read and extend the discussion "generic number type vs. distinct numeric types" http://www.haskell.org/haskellwiki/Generic_number_type
I think at some intermediate stage that flag helped me compile, but in the
final version it's unnecessary.
my bad.
Thomas Hartman
:t f f :: (RealFrac b, Integral b1) => (b1, b) -> b1
This function works just as I want it to. Now I try creating a list: Prelude> [(a2+b2,a)| a <- [1..4] , b<- [1..4], a2+b2<20, b<=a] [(2,1),(5,2),(8,2),(10,3),(13,3),(18,3),(17,4)] Let's assign this to an intermediate variable so we can query it's type: Prelude> let lst = [(a ^ 2 + b ^ 2, a) | a <- [1..4], b <- [1..4], a^2 + b^2 < 20, b <= a] Prelude> lst [(2,1),(5,2),(8,2),(10,3),(13,3),(18,3),(17,4)] Prelude> :t lst lst :: [(Integer, Integer)] aha; here's the source of the type mismatch: Prelude> :t floor floor :: (RealFrac a, Integral b) => a -> b Floor has to take a RealFrac. According to hoogle[1], we can use various floating-point approximations (Float, Double, CFloat, etc) or we can use the exact Rational type. [1] http://haskell.org/hoogle/?q=RealFrac You can get your types to match by declaring your list to be of type [(Rational, Rational)] either by explicitly typing one of the untyped variables or the entire expression: Prelude> let lst = [(a ^ 2 + b ^ 2, a) | (a::Rational) <- [1..4], b <- [1..4], a^2 + b^2 < 20, b <= a] Prelude> :t lst lst :: [(Rational, Rational)] Prelude> let lst :: [(Rational, Rational)] = [(a ^ 2 + b ^ 2, a) | a <- [1..4], b <- [1..4], a^2 + b^2 < 20, b <= a] Prelude> :t lst lst :: [(Rational, Rational)] and this works So now I try to apply the function to the list: Prelude> map (f) [(a2+b2,a)| a <- [1..4] , b<- [1..4], a2+b2<20, b<=a] and I get this result: <interactive>:1:5: Ambiguous type variable `t' in the constraints: `Integral t' arising from use of `f' at <interactive>:1:5 `RealFrac t' arising from use of `f' at <interactive>:1:5 Probable fix: add a type signature that fixes these type variable(s) I'm sorry, but I don't quite get how to set the type signature and how it will apply to my function... Thanks, Hope this helps Tomi -------------------------------------------------------------------------------- Department for Education, Sport and Culture E Mail This message is for the named person's use only. It may contain confidential, proprietary or legally privileged information. No confidentiality or privilege is waived or lost by any mistransmission. If you receive this message in error, please immediately delete it and all copies of it from your system, destroy any hard copies of it and notify the sender. You must not, directly or indirectly, use, disclose, distribute, print, or copy any part of this message if you are not the intended recipient. The Department for Education, Sport and Culture and any of its establishments each reserve the right to monitor all e-mail communications through its networks. Any views expressed in this message are those of the individual sender, except where the message states otherwise and the sender is authorised to state them to be the views of any such entity. The Department for Education, Sport and Culture shall not be liable to the recipient or any third party for any loss or damage, however it appears, from this e-mail or its content. This includes loss or damage caused by viruses. It is the responsibility of the recipient to ensure that the opening of this message and its attachments shall not adversely affect systems or data. -------------------------------------------------------------------------------- _______________________________________________ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell -- Scott Williams _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.
participants (3)
-
Henning Thielemann -
Scott Williams -
Thomas Hartman