Prism Stan

// source: https://github.com/stan-dev/example-models/blob/8a6964135560f54f52695ccd4d2492a8067f0c30/misc/linear-regression/regression_std.stan

// normal mixture, unknown proportion and means, known variance
// p(y|mu,theta) = theta * Normal(y|mu[1],1) + (1-theta) * Normal(y|mu[2],1);

data {
  int<lower=0> N;
  real y[N];
}
parameters {
  real<lower=0,upper=1> theta;
  real mu[2];
}
model {
  theta ~ uniform(0,1); // equivalently, ~ beta(1,1);
  for (k in 1:2)
    mu[k] ~ normal(0,10);
  for (n in 1:N)
    target += log_mix(theta, normal_lpdf(y[n]|mu[1],1.0), normal_lpdf(y[n]|mu[2],1.0));
}