[Haskell-cafe] overloading show function

Philipp Schneider philipp.schneider5 at gmx.net
Thu Jun 30 20:25:35 CEST 2011

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.


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