"An environment contains local variable bindings, so no subcomputation will ever need to return its environment. " - That is not true. A subcomputation can possible modify an environment except the language forbids such a case. On 06/30/2011 02:36 PM, Holger Siegel wrote:

Am 29.06.2011 um 23:50 schrieb Philipp Schneider:

...

Hi cafe,

in my program i use a monad of the following type

newtype M a = M (State -> (a, State))

i use the monad in two different ways. The type variable "a" can be a pair as in

interp :: Term -> Environment -> M (Value,Environment)

and it can be just a value as in

type Environment = [(Name, Either Value (M Value))] Simple rule: Never return an environment!

An environment contains local variable bindings, so no subcomputation will ever need to return its environment. I don't know anything about the language your program interprets, but I'm sure that you can rewrite function interp as

interp :: Term -> Environment -> M Value

The structure of the interpreter will become clearer and your problem will vanish.

...

now in any case when i print the monad, i just want to print the value and never the environment.

More specific i want to use somthing like the following

instance (Show a,Show b) => Show (M (a,b)) where show (M f) = let ((v,_), s) = f 0 in "Value: " ++ show v ++ " Count: " ++ show s

instance Show a => Show (M a) where show (M f) = let (v, s) = f 0 in "Value: " ++ show v ++ " Count: " ++ show s

however this gives me the following error message:

Overlapping instances for Show (M (Value, Environment)) arising from a use of `print' Matching instances: instance (Show a, Show b) => Show (M (a, b)) -- Defined at /home/phil/code/haskell/vorlesung/ue09/ue09-3c3.hs:78:10-42 instance Show a => Show (M a) -- Defined at /home/phil/code/haskell/vorlesung/ue09/ue09-3c3.hs:82:10-29 In a stmt of an interactive GHCi command: print it

Any ideas how to fix it? Thanks! Philipp

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On 06/30/2011 08:25 PM, Philipp Schneider wrote:

On 06/30/2011 02:36 PM, Holger Siegel wrote:

...

Am 29.06.2011 um 23:50 schrieb Philipp Schneider:

...

Hi cafe,

in my program i use a monad of the following type

newtype M a = M (State -> (a, State))

i use the monad in two different ways. The type variable "a" can be a pair as in

interp :: Term -> Environment -> M (Value,Environment)

and it can be just a value as in

type Environment = [(Name, Either Value (M Value))] Simple rule: Never return an environment!

An environment contains local variable bindings, so no subcomputation will ever need to return its environment. I don't know anything about the language your program interprets, but I'm sure that you can rewrite function interp as

interp :: Term -> Environment -> M Value

The structure of the interpreter will become clearer and your problem will vanish.

Hello Holger,

I'm giving two lambda interpreters. The first one is a call by value interpreter, the second one a call by name interpreter which are described in Philip Wadler's paper "The essence of functional programming" page 4 and 12. Now my task is to write a lazy lambda interpreter. The exercise is more playful than serious since Wadler's call by value interpreter is, since written in lazy Haskell, already a lazy lambda interpreter. (To get true call by value one would need to force evaluations of the arguments with the seq function.) For both of Wadler's interpreters the type of the interpertation function is: interp :: Term -> Environment -> M Value

Now to simulate lazy interpretation i need to do the following: Decide is the value I need already evaluated or is it still a computation. In the later case I need to evaluate it and save its value in the environment. This is the reason I changed the type of the interpretation function to: interp :: Term -> Environment -> M (Value,Environment)

I appened my full interpreter. If you can find a more elegant way to save the newly interpreted values, you are more than welcome to show my how to do it.

Cheers, Philipp I forgot to add the interpreter.