- Beginners - Haskell.org

lambda
by John Moore 28 Dec '10

28 Dec '10

Hi, Can someone explain how this lambda is suppose to work, I cannot figure out what it does? data Lambda = Val Double | Add Lambda Lambda | Subtract Lambda Lambda | Multiply Lambda Lambda | Divide Lambda Lambda | Var String | Let String Lambda Lambda deriving Show demo1 = (Add(Multiply(Divide(Subtract(Val 25)(Val 5))(Val 10))(Val 7))(Val 30)) --let x = 1+1 in 3 - x test1 = Let "x" (Add (Val 1) (Val 1)) (Add(Var "x")(Var "x")) test6 = Let "y" (Add (Val 7)(Val 6)) (Subtract (Val 6)(Var "y")) -- 4 * (let x = 1+1 in 3 + x) test2 = Multiply (Val 4) test1 -- x * (let x = 1+1 in 3 + x) test3 = Let "y" (Add (Val 4)(Val 7))(Multiply test1 (Var "y")) test5 = Add (test1) test6 type Dict =[(String,Lambda)]-- dictionary which takes a string and an expression emptyDict :: Dict emptyDict = []-- empty dictionary null set addEntry :: String->Lambda ->Dict -> Dict addEntry= \n e d -> (n,e): d -- add an entry we take a string,expression and a dictionary :gives back dictionary lookupEntry :: String -> Dict -> Maybe Lambda lookupEntry n [] = Nothing lookupEntry n (x:xs) = if (n == k)-- if the entry is equal to then (Just v) else lookupEntry n xs -- find the value of n in the rest of the list where (k,v) = x -- evalStep :: Dict -> Lambda -> Lambda evalStep d (Val x) = (Val x)-- we have a dictionary and a value return the value evalStep d(Add x y)-- dictionary and a add expression of x y = case x of (Val a) -> case y of (Val b) -> Val (a+b) left -> Add x (evalStep d y) right -> Add (evalStep d x)y evalStep d(Subtract x y) = case x of (Val a) -> case y of (Val b) -> Val (a-b) left -> Subtract x (evalStep d y) right -> Subtract (evalStep d x)y evalStep d(Multiply x y) = case x of (Val a) -> case y of (Val b) -> Val (a*b) left -> Multiply x (evalStep d y) right -> Multiply (evalStep d x)y evalStep d (Divide x y) = case x of (Val a) -> case y of (Val b) -> Val (a/b) left -> Divide x (evalStep d y) right -> Divide (evalStep d x)y evalStep d (Let n e1 e2) = case e1 of (Val a) -> case e2 of (Val b)-> Val b left -> evalStep (addEntry n e1 d)e2 right -> evalStep (addEntry n e1 d) e2 evalStep d (Var x) = case lookup x d of Just e -> e Nothing -> error "Error in expression -- no definition for variable!" evaluate :: Dict-> [Lambda] -> Lambda -> IO() evaluate d(x:xs) e = do putStrLn (show e) putStrLn "Do another step (y/n) or rollback (r)? :" c <- getLine case c of "y" -> let e'= (evalStep d e)in evaluate d (e:x:xs) e'-- build up history "r" -> case (x:xs) of (x:xs)-> evaluate d xs x []-> do { putStrLn "Empty" ;evaluate d(x:xs) e } "n" -> putStrLn $ "Ok you said no :" ++ c ev :: Dict-> [Lambda] -> Lambda -> IO() ev = \d (x:xs) e ->do putStrLn (show e) putStrLn "Do another step (y/n) or rollback (r)? :" c <- getLine case c of "y" -> let e'= (evalStep d e)in evaluate d (e:x:xs) e'-- build up history "r" -> case (x:xs) of (x:xs)-> evaluate d xs x []-> do { putStrLn "Empty" ;evaluate d(x:xs) e } "n" -> putStrLn $ "Ok you said no :" ++ c isFree :: String -> Lambda -> Bool isFree n1 (Let n2 e1 e2) |n1 == n2 = False |otherwise = True subst :: String -> Lambda -> Lambda->Lambda subst n e1 e2 |isFree n e2 = subst' n e1 e2 |otherwise = e2 subst' :: String -> Lambda -> Lambda->Lambda subst' n0 e1 (Var n1) |n0 == n1 = e1 |otherwise = Var n1 subst' s v (Add e1 e2) = Add(subst' s v e1)(subst' s v e2) subst' s v (Multiply e1 e2) = Multiply(subst' s v e1)(subst' s v e2) subst' s v (Divide e1 e2) = Divide(subst' s v e1)(subst' s v e2) subst' s v (Subtract e1 e2) = Subtract(subst' s v e1)(subst' s v e2) subst' s v (Let n e1 e2) = Let n (subst' s v e1)(subst' s v e2) John

2 1

Infix operator and precedence
by Patrick LeBoutillier 24 Dec '10

24 Dec '10

Hi, I'm just messing around with infix operators, and I'm trying to implement some kind of ternary if operator: (??) :: Bool -> (Bool -> a) -> a (??) = flip ($) (?:) :: a -> a -> (Bool -> a) t ?: f = \b -> if b then t else f test b = b ?? ("IF" ?: "THEN") I would like to get rid of the parentheses around the ("IF" ?: "THEN") part, but I'm not familiar with the use of theinfixr/infixl functions. I'm not sure which one to use in this case and how exactly to use it. Can anyone shed some light? Thanks a lot, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

3 2

how to input a Let expression
by John Moore 22 Dec '10

22 Dec '10

Hi, I'm missing something basic here, I wrote this program a while back. But I cant remember how to work it. Would someone please explain how I enter a value for a variable x,y etc and get this to work. It goes by entering evaluate [][test1]test1 but I cant remember how to write in The Let part and not have it keep ending in -no definotion of a variable. John

2 1

How to think in Haskell
by Jun HU 21 Dec '10

21 Dec '10

Dear everyone, My first language is C, and I've strong intention in learning a pure functional programming language. My very first question is how to think in the functional programming way, anyone has some ideas. Really appreciate.... Regards, Jun

7 8

Forum vs mailing list
by Amy de Buitléir 21 Dec '10

21 Dec '10

On 21 December 2010 12:42, David Jameson <dhjdhj(a)gmail.com> wrote: > P.S. By the way, is is not possible to have a forum instead of a mailing list --- these mailing lists are so 19th century! If you use an face like GMANE, then you'll get the best of both worlds, plus a bit more. Youll be able to post messages using a forum-like interface. And you don't have to be subscribed to the mailing list to post through GMANE, so if you don't want your inbox filling up, simply unsubscribe. You'll still be able to read all the other posts, but you'll have to go to the GMANE page to do so. Look here for more information: http://dir.gmane.org/gmane.comp.lang.haskell.beginners

2 1

Uncalled for apology.
by Sean Charles 21 Dec '10

21 Dec '10

I felt compelled to PM Paul about his email. In support. If this list is to be vetted and thought-controlled by self-imposed politically correct Ministry of Thought operatives then I am leaving today. It's a shame that the mindset required to understand Haskell is still open to petty thinking and the desire to impose its thoughts upon others. Freedom of speech, thought and expression. Well, that's ruined my Christmas I think. What is it about human beings that makes them unable to accept what somebody else has to say without wanting to pontificate ? Sigh, and it was such a nice list too... :(

6 5

Standard Haskell Module Files
by Jane Ren 21 Dec '10

21 Dec '10

Hi, Could someone tell me where I can get the standard Haskell module files such as source code for the Data module? Thanks

2 1

Equivalent of inheritance in Haskell
by C K Kashyap 21 Dec '10

21 Dec '10

Hi, In oops, one can do reuse code by extending a parent class. What is the equivalent in Haskell? Regards, Kashyap

6 10

haskell / prolog
by dan portin 21 Dec '10

21 Dec '10

I recently discovered that class and instance declarations in Haskell are Horn clauses. So I encoded the arithmetic operations from the first chapter of *Art of Prolog* into Haskell, in the standar fac(0,s(0)). fac(s(N),F) :- fac(N,X), mult(s(N),X,F). class Fac x y | x -> y instance Fac Z (S Z) instance (Fac n x, Mult (S n) x f) => Fac (S n) f I don't understand, however, to make Haskell instantiate values for variables in a proof. I.e., I don't understand what the Haskell equivalent of ?- fac(X,Y) would be for some value of X.

1 1

Include external libs on cabal project
by Marco De Oliveira 20 Dec '10

20 Dec '10

Hi, I try to include an external lib (and C files that I made) on my cabal file and when I launch "configure" I have errors. I have added on my file: extra-libraries: "winscard" include-dirs: "C:\Program Files\Microsoft SDKs\Windows\v7.1\Include" extra-lib-dirs: "C:\Program Files\Microsoft SDKs\Windows\v7.1\Lib" and it fails because of the space on the path. (see logs) Is the syntax correct? What I have to change so that cabal can understand the space on directory path? Thanks for help Cabal version: 1.8.0.6 log after "runhaskell setup.lhs configure -v3" ("C:\\Program Files\\Haskell Platform\\2010.2.0.0\\mingw\\bin\\gcc.exe",["C:\\Us ers\\E6400\\AppData\\Local\\Temp\\8104.c","-o","C:\\Users\\E6400\\AppData\\Local \\Temp\\8104","-D__GLASGOW_HASKELL__=612","-Idist\\build\\autogen","-ISmartCard/ libpcsc/inc",*"-I\"C:\\Program","-IFiles\\Microsoft","-ISDKs\\Windows\\v7.1\\Incl ude\"*","-I.","-IC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib\\base-4.2.0 .2\\include","-IC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib/include","- IC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib/include","-lwinscard",*"-L\ "C:\\Program","-LFiles\\Microsoft","-LSDKs\\Windows\\v7.1\\Lib\"*","-LC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib\\base-4.2.0.2","-LC:\\Program Files\\H askell Platform\\2010.2.0.0\\lib\\integer-gmp-0.2.0.1","-LC:\\Program Files\\Has kell Platform\\2010.2.0.0\\lib\\ghc-prim-0.2.0.0","-LC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib","-LC:\\Program Files\\Haskell Platform\\2010.2.0.0\\l ib/gcc-lib","-LC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib"]) C:\Program Files\Haskell Platform\2010.2.0.0\mingw\bin\gcc.exe returned ExitFailure 1 with error message: C:\Program Files\Haskell Platform\2010.2.0.0\mingw\bin\ld.exe: cannot find -lwinscard collect2: ld returned 1 exit status ("C:\\Program Files\\Haskell Platform\\2010.2.0.0\\mingw\\bin\\gcc.exe",["C:\\Us ers\\E6400\\AppData\\Local\\Temp\\8104.c","-o","C:\\Users\\E6400\\AppData\\Local \\Temp\\8104","-lwinscard","*-L\"C:\\Program","-LFiles\\Microsoft","-LSDKs\\Windo ws\\v7.1\\Lib\"",*"-LC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib\\base-4 .2.0.2","-LC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib\\integer-gmp-0.2 .0.1","-LC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib\\ghc-prim-0.2.0.0" ,"-LC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib","-LC:\\Program Files\\ Haskell Platform\\2010.2.0.0\\lib/gcc-lib","-LC:\\Program Files\\Haskell Platfor m\\2010.2.0.0\\lib"]) C:\Program Files\Haskell Platform\2010.2.0.0\mingw\bin\gcc.exe returned ExitFailure 1 with error message: C:\Program Files\Haskell Platform\2010.2.0.0\mingw\bin\ld.exe: cannot find -lwinscard collect2: ld returned 1 exit status ("C:\\Program Files\\Haskell Platform\\2010.2.0.0\\mingw\\bin\\gcc.exe",["C:\\Us ers\\E6400\\AppData\\Local\\Temp\\8104.c","-o","C:\\Users\\E6400\\AppData\\Local \\Temp\\8104","-lwinscard","-L\"C:\\Program","-LFiles\\Microsoft","-LSDKs\\Windo ws\\v7.1\\Lib\"","-LC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib\\base-4 .2.0.2","-LC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib\\integer-gmp-0.2 .0.1","-LC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib\\ghc-prim-0.2.0.0" ,"-LC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib","-LC:\\Program Files\\ Haskell Platform\\2010.2.0.0\\lib/gcc-lib","-LC:\\Program Files\\Haskell Platfor m\\2010.2.0.0\\lib"]) C:\Program Files\Haskell Platform\2010.2.0.0\mingw\bin\gcc.exe returned ExitFailure 1 with error message: C:\Program Files\Haskell Platform\2010.2.0.0\mingw\bin\ld.exe: cannot find -lwinscard collect2: ld returned 1 exit status ("C:\\Program Files\\Haskell Platform\\2010.2.0.0\\mingw\\bin\\gcc.exe",["C:\\Us ers\\E6400\\AppData\\Local\\Temp\\8104.c","-o","C:\\Users\\E6400\\AppData\\Local \\Temp\\8104","-c","-D__GLASGOW_HASKELL__=612","-Idist\\build\\autogen","-ISmart Card/libpcsc/inc",*"-I\"C:\\Program","-IFiles\\Microsoft","-ISDKs\\Windows\\v7.1\ \Include\*"","-I.","-IC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib\\base- 4.2.0.2\\include","-IC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib/includ e","-IC:\\Program Files\\Haskell Platform\\2010.2.0.0\\lib/include"]) Setup.lhs: Missing dependency on a foreign library: * Missing C library: winscard This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is.

1 0