haskell - Handling nested variable scopes when implementing an interpreter -

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i'm writing interpreter simple programming language , wanted ask on best approach tackle it.

the environment program follows:

type env = [[(var, int)]]

so i've coded lookup , update i'm bit stuck on how deal the scope each begin block. example shown below:

begin [a,b,c]      read     n = 1     while < 0         begin         n = 2 * n         = - 1         end;     write n end

from understanding scope of first begin [a,b,c,i,n] , second begin contain [i, n]

therefore env 

[ [ ("a",0), (b",0), ("c",0), ("i",3), ("n",2) ], [("n",8), ("i",0) ]    ]`

currently lookup function returns first occurrence of variable, i'm having problems 2nd scope (2nd begin).

i'm not quite sure how can make update , lookup function return value associated particular scope.

basically have code working 1 begin statement, having issues 2 or more statements in sample program.


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