You can take a look at the migrator cheatcodes tests (v0.4.2).
https://github.com/software-mansion/protostar/tree/53cb5dc5cda1a74a1ffb862d84e7035e44a5c56b/tests/integration/migrator

(Migration scripts are intended to be used with proxy pattern.) 

_This answer was originally posted on [StarkNet Discord](https://www.newton.so/view?tags=discord)_

Newton

is there an example of migration scripts somewhere ?

_This question was originally posted on [StarkNet Discord](https://www.newton.so/view?tags=discord)_

Is there an example of migration scripts somewhere [Cairo/StarkNet]?

I have a question about creating elements via a function vs doing them directly. Here's a small gist: [https://gist.github.com/rlkelly/6cc8f4cce06762c3153fc7e3acaf9429](https://gist.github.com/rlkelly/6cc8f4cce06762c3153fc7e3acaf9429)

```
%lang starknet

from starkware.cairo.common.alloc import alloc

struct Foo:
    member val: felt
end

func make_foo(value: felt) -> (foo: Foo):
    let foo = Foo(
        val=value,
    )
    return (foo)
end

func make_foo_array() -> (entry: Foo*):
    let (struct_array : Foo*) = alloc()

    let foo0 = make_foo(0)
    assert struct_array[0] = foo0
    return (struct_array)
end

@external
func test_make_foo{syscall_ptr : felt*, range_check_ptr}():
    let (foo_array) = make_foo_array()
    return ()
end

#   File "starkware/cairo/lang/compiler/preprocessor/preprocessor.py", line 1223, in visit_CodeElementCompoundAssertEq
#  starkware.cairo.lang.compiler.preprocessor.preprocessor_error.PreprocessorError: contracts/tests/test_foo.cairo:22:5: Cannot compare '__main__.Foo' and '(foo : __main__.Foo)'.
```

i'm curious why `__main__.Foo` and `(foo : __main__.Foo)` are typed differently

it appears like `let foo =` and `let (foo) =`  allocate memory differently

Creating Elements via a function

Hello! This is day 13 of the 17 days of the Cairo Challenge. And I have no idea how to solve the [playground](https://www.cairo-lang.org/playground/) exercise “Pedersen Hash”.  Perhaps you can help me?

```
// The following code implements a Pedersen hash chain.
// Given the [Pedersen hash](https://docs.starkware.co/starkex-docs/crypto/pedersen-hash-function) function `H`, which
// takes two field elements and returns one representing their hash,
// a hash chain on inputs x_0, x_1, ..., x_n is the result of
// H(H(...H(H(x_0, x_1), x_2), ...,x_{n-1}), x_n)
// For example, the hash chain of (1, 2, 3, 4) is H(H(H(1, 2), 3), 4).
//
// Fix the function `hash_chain` to correctly implement the
// hash chain. Run your code to test it.
//
// Note: Don't use starkware.cairo.common.hash_chain (it computes the chain differently).

// Use the Pedersen builtin.
%builtins pedersen

from starkware.cairo.common.alloc import alloc
from starkware.cairo.common.cairo_builtins import HashBuiltin
from starkware.cairo.common.hash import hash2

// Computes the Pedersen hash chain on an array of size `length` starting from `data_ptr`.
func hash_chain{hash_ptr: HashBuiltin*}(data_ptr: felt*, length: felt) -> (result: felt) {
    if (length == 2) {
        let (result) = hash2(x=[data_ptr], y=[data_ptr + 1]);
        return (result=result);
    } else {
        // Fix here:
        return (result=123456);
    }
}

func main{pedersen_ptr: HashBuiltin*}() {
    alloc_locals;
    // Allocate a new array.
    let (local data_ptr: felt*) = alloc();
    // Fill the new array with field elements.
    assert [data_ptr] = 314;
    assert [data_ptr + 1] = 159;
    // Run the hash chain.
    let (result) = hash_chain{hash_ptr=pedersen_ptr}(data_ptr=data_ptr, length=2);
    // Check the result.
    assert result = 307958720726328212653290369969069617958360335228383070119367204176047090109;

    // Fill two more cells in the array.
    assert [data_ptr + 2] = 265;
    assert [data_ptr + 3] = 358;
    // Compute the hash chain on all four cells.
    let (result) = hash_chain{hash_ptr=pedersen_ptr}(data_ptr=data_ptr, length=4);
    assert result = 1828757374677754028678056220799392919487521050857166686061558043629802016816;
    return ();
}
```

---

> Answers to this question are a part of the ✨ 17 days of Cairo Lang with Playground & Newton. ✨
> 

*Vote for your favorite answer - the best answer will win a $10 award. A new day – a new reward! During the next 17 days, our goal is to attract more developers to the Cairo language and to systematize the knowledge of Cairo lang. [Read rules](https://www.notion.so/17-days-of-Cairo-Lang-with-Newton-550dfcda6a2d449ca468be293c776578)*

How to use Pedersen Hash in StarkNet / Cairo Language?

I am getting the following full error message based on my code:


Can you please explain what is causing this error? I am curious why the struct is failing while 'expanding'


`struct simple_struct:`


 `member a : felt`


`end`


`﻿`


`func main():`


 `alloc_locals`


 `local test_struct : simple_struct`


 `test_func(test_struct)`


 `return ()`


`end`


`﻿`


`func test_func(s : simple_struct):`


 `return()`


`end`


﻿  



While expanding the reference 'test_struct' in:


 test_func(test_struct)


 ^*********^


Unknown value for memory cell at address 1:2.


 local test_struct : simple_struct


 ^*********^


﻿  



ㅤ
_This question was originally posted on [Triality](https://triality.org/question/1046)_

Cairo Error: 'While expanding the reference 'test_struct' in'

Some questions that could be answered separately.

1. The only way to invoke a contract is using migrations (this applies for both devnet and testnet?)
2. When doing a migration file I receive the “Hint is not whitelisted. This may indicate that this library function cannot be used in StarkNet contracts.” error. For example for this code:

```
%lang starknet

@external
func up() {
    %{
        contract_address = deploy_contract(
             "./build/main.json",
             config={"wait_for_acceptance": True}
        ).contract_address

        invoke(
            contract_address,
            "initialize",
            {"new_authority": 123},
            config={
                "auto_estimate_fee": True,
                "wait_for_acceptance": True,
            }
        )
    %}

    return ();
}
```

_This question was originally posted on [StarkNet Discord](https://www.newton.so/view?tags=discord)_

The only way to invoke a contract is using migrations (this applies for both devnet and testnet?)

I can't find info on what `vm_enter_scope` does within hints in the cairo-lang repo. Heres an example:
https://github.com/starkware-libs/cairo-lang/blob/d61255f32a7011e9014e1204471103c719cfd5cb/src/starkware/cairo/common/patricia.cairo#L117

Can anyone summarize?

_This question was originally posted on [StarkNet Discord](https://www.newton.so/view?tags=discord)_

I can't find info on what `vm_enter_scope` does within hints in the cairo-lang repo

Any nft marketplace has launched on starknet L2?

_This question was originally posted on [StarkNet Discord](https://www.newton.so/view?tags=discord)_

Any NFT marketplace has launched on starknet L2?

is there anything wrong with the testnet? why is it so slow now?

_This question was originally posted on [StarkNet Discord](https://www.newton.so/view?tags=discord)_

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Is there an example of migration scripts somewhere [Cairo/StarkNet]?

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