Syntax Error after variable declaration by using simplex

Modeling
1

Hello, I am relatively new to Stan and I keep getting in circles with a syntax error within my Stan model which is the following:

For reference here is the model and the error seems to be due to the simplex line, I am using python 3.11.2 and using PyStan 3.9.1

data {
    int<lower=0> N; 
    int<lower=0> S;
    matrix[N, S] entries; 
    vector[N * S] exits; 
}

parameters {
    vector[(S-1) * S] raw_alpha; 
    vector[S] intercept; 
    real<lower=0> sigma; // common standard deviation for exits
}

model {
    // Priors for raw alpha parameters
    raw_alpha ~ normal(0, 1); // prior on the full raw_alpha vector

    // Simplex vector definition
    simplex[S-1] alpha[S];

    // Populating the alpha array using portions of raw_alpha
    for (int s = 1; s <= S; ++s) {
        alpha[s] = softmax(segment(raw_alpha, (s-1)*(S-1) + 1, S-1));
    }

    // Model exits using the transformed parameters
    for (int n = 1; n <= N; ++n) {
        vector[S] exits_mu;
        for (int s = 1; s <= S; ++s) {
            exits_mu[s] = dot_product(entries[n], alpha[s]) + intercept[s];
        }
        
        exits[(n-1)*S + 1 : n*S] ~ normal(exits_mu, sigma);
    }
}

After compiling I get this error

Building: Syntax error:
   -------------------------------------------------
    18:  
    19:      // Define a simplex vector inside the model block
    20:      simplex[S-1] alpha[S];
             ^
    21:  
    22:      // Populate the alpha array using portions of raw_alpha
   -------------------------------------------------

Variable declaration, statement or "}" expected.

Anyone knows how to resolve the issue?
Thanks

2

What are you hoping to declare?

If it is a S-long array of S-1 simplexes, try array[S] simplex[S-1] alpha;