LHS of assignment must denote a mutable variable - dafny

Question

datatype CACHE_STATE = I| S| E
datatype MSG_CMD = Empty| ReqS| ReqE| Inv| InvAck| GntS| GntE
type NODE=nat
type DATA=nat
type boolean=bool
class class_0 {
var
Data : DATA,
Cmd : MSG_CMD
}
class class_1 {
var
Data : DATA,
State : CACHE_STATE
}
method n_SendGntEinv__5_0(Cache_State:array<CACHE_STATE>, Chan2_Cmd:array<MSG_CMD>, Chan2_Data:array<DATA>, CurCmd:MSG_CMD, CurPtr:NODE, ExGntd:boolean, MemData:DATA, ShrSet:array<boolean>,
i:nat,N0:nat,p__Inv0:nat,p__Inv2:nat)
requires 0<= i < N0
requires Chan2_Cmd.Length==N0
requires Chan2_Data.Length==N0
requires ShrSet.Length==N0
requires p__Inv0!=p__Inv2&&p__Inv2<N0&& p__Inv0<N0
requires i==p__Inv2
//1
//guard condition
requires ((Chan2_Cmd[i] == Empty) && (CurCmd == ReqE) && (CurPtr == i) && (ExGntd == false) && (forall j |0<= j<N0 :: (ShrSet[j] == false) ))
ensures (!((Cache_State[p__Inv0] == E) && (Chan2_Cmd[p__Inv2] == GntS)))
modifies Chan2_Cmd
modifies Chan2_Data
modifies ShrSet
{
Chan2_Cmd[i] := GntE;
Chan2_Data[i] := MemData;
ShrSet[i] := true;
ExGntd := true;
CurCmd := Empty;
}
I try to compile this code, however tip mentions me
LHS of assignment must denote a mutable variable
and I don't know how to solve the problem. Does the CurCMD and the EXGntd need to be array value, or there has other solution to this problem?

Answer 1

In-parameters are immutable. The error message points out that you're trying to assign to them.
You can introduce (mutable) local variables, if you want. In your example, that would look like:
var ExGntd', CurCmd' := ExGntd, CurCmd;
...
ExGntd' := true;
CurCmd' := Empty;