[m-users.] Adding impurity to Mercury code: Backtracking in IO states

[barthwal]

Hi,



In one of the module, I am trying to add impurity to Mercury. I code looks like below and I get the following error:



ex2.m:023: In clause for predicate `ex2.print'/2:

ex2.m:023: warning: variable `IOState_out' occurs only once in this scope.

ex2.m:025: In clause for `print(in, out)':

ex2.m:025: in argument 2 of call to predicate `io.write_string'/3:

ex2.m:025: mode error: variable `IOState_in' has instantiatedness `ground',

ex2.m:025: expected instantiatedness was `unique'.

ex2.m:034: In clause for `main(di, uo)':

ex2.m:034: in argument 2 of call to predicate `io.write_string'/3:

ex2.m:034: mode error: variable `IOState_temp' has instantiatedness `ground',

ex2.m:034: expected instantiatedness was `unique'.



How do I solve it?



I don't want to use the All solutions predicate or even Foreign language interface!







:- module ex2.

:- interface.

:- import_module io.

:- pred main(io::di, io::uo) is det.

:- implementation.

:- pred text(string::out) is multi.

:- pred print(io::in, io::out) is det.



text("A").

text("B").

text("C").

text("D").

text("E").

text("F").



print(IOState_in, IOState_out) :-

text(X),

io.write_string(X, IOState_in, _),

X = "D".



print(IOState_in, IOState_out) :-

IOState_out = IOState_in.



main(IOState_in, IOState_out) :-

print(IOState_in, IOState_temp),

io.write_string("Hello, World!\n", IOState_temp, IOState_out).
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