Distributions¶
Distribution objects represent probability distributions, they have two principle uses:
Samples can be generated from a distribution by passing a distribution object to the sample operator.
The logarithm of the probability (or density) that a distribution assigns to a value can be computed using
dist.score(val)
. For example:Bernoulli({p: .1}).score(true); // returns Math.log(.1)
Several primitive distributions are built into the language. Further distributions are created by performing marginal inference.
Primitives¶

Bernoulli
({p: ...})¶  p: success probability (real [0, 1])
Distribution over
{true, false}

Beta
({a: ..., b: ...})¶  a: shape (real (0, Infinity))
 b: shape (real (0, Infinity))
Distribution over
[0, 1]

Binomial
({p: ..., n: ...})¶  p: success probability (real [0, 1])
 n: number of trials (int (>=1))
Distribution over the number of successes for
n
independentBernoulli({p: p})
trials.

Categorical
({ps: ..., vs: ...})¶  ps: probabilities (can be unnormalized) (vector or real array [0, Infinity))
 vs: support (any array)
Distribution over elements of
vs
withP(vs[i])
proportional tops[i]
.ps
may be omitted, in which case a uniform distribution overvs
is returned.

Cauchy
({location: ..., scale: ...})¶  location: (real)
 scale: (real (0, Infinity))
Distribution over
[Infinity, Infinity]

Delta
({v: ...})¶  v: support element (any)
Discrete distribution that assigns probability one to the single element in its support. This is only useful in special circumstances as sampling from
Delta({v: val})
can be replaced withval
itself. Furthermore, aDelta
distribution parameterized by a random choice should not be used with MCMC based inference, as doing so produces incorrect results.

DiagCovGaussian
({mu: ..., sigma: ...})¶  mu: mean (tensor)
 sigma: standard deviations (tensor (0, Infinity))
A distribution over tensors in which each element is independent and Gaussian distributed, with its own mean and standard deviation. i.e. A multivariate Gaussian distribution with diagonal covariance matrix. The distribution is over tensors that have the same shape as the parameters
mu
andsigma
, which in turn must have the same shape as each other.

Dirichlet
({alpha: ...})¶  alpha: concentration (vector (0, Infinity))
Distribution over probability vectors. If
alpha
has lengthd
then the distribution is over probability vectors of lengthd
.

Discrete
({ps: ...})¶  ps: probabilities (can be unnormalized) (vector or real array [0, Infinity))
Distribution over
{0,1,...,ps.length1}
with P(i) proportional tops[i]

Exponential
({a: ...})¶  a: rate (real (0, Infinity))
Distribution over
[0, Infinity]

Gamma
({shape: ..., scale: ...})¶  shape: (real (0, Infinity))
 scale: (real (0, Infinity))
Distribution over positive reals.

Gaussian
({mu: ..., sigma: ...})¶  mu: mean (real)
 sigma: standard deviation (real (0, Infinity))
Distribution over reals.

Laplace
({location: ..., scale: ...})¶  location: (real)
 scale: (real (0, Infinity))
Distribution over
[Infinity, Infinity]

LogisticNormal
({mu: ..., sigma: ...})¶  mu: mean (vector)
 sigma: standard deviations (vector (0, Infinity))
A distribution over probability vectors obtained by transforming a random variable drawn from
DiagCovGaussian({mu: mu, sigma: sigma})
. Ifmu
andsigma
have lengthd
then the distribution is over probability vectors of lengthd+1
.

LogitNormal
({mu: ..., sigma: ..., a: ..., b: ...})¶  mu: location (real)
 sigma: scale (real (0, Infinity))
 a: lower bound (real)
 b: upper bound (>a) (real)
A distribution over
(a,b)
obtained by scaling and shifting a standard logitnormal.

Multinomial
({ps: ..., n: ...})¶  ps: probabilities (real array with elements that sum to one)
 n: number of trials (int (>=1))
Distribution over counts for
n
independentDiscrete({ps: ps})
trials.

MultivariateBernoulli
({ps: ...})¶  ps: probabilities (vector [0, 1])
Distribution over a vector of independent Bernoulli variables. Each element of the vector takes on a value in
{0, 1}
. Note that this differs fromBernoulli
which has support{true, false}
.

MultivariateGaussian
({mu: ..., cov: ...})¶  mu: mean (vector)
 cov: covariance (positive definite matrix)
Multivariate Gaussian distribution with full covariance matrix. If
mu
has length d andcov
is ad
byd
matrix, then the distribution is over vectors of lengthd
.

Poisson
({mu: ...})¶  mu: mean (real (0, Infinity))
Distribution over integers.

RandomInteger
({n: ...})¶  n: number of possible values (int (>=1))
Uniform distribution over
{0,1,...,n1}

TensorGaussian
({mu: ..., sigma: ..., dims: ...})¶  mu: mean (real)
 sigma: standard deviation (real (0, Infinity))
 dims: dimension of tensor (int (>=1) array)
Distribution over a tensor of independent Gaussian variables.

TensorLaplace
({location: ..., scale: ..., dims: ...})¶  location: (real)
 scale: (real (0, Infinity))
 dims: dimension of tensor (int (>=1) array)
Distribution over a tensor of independent Laplace variables.

Uniform
({a: ..., b: ...})¶  a: lower bound (real)
 b: upper bound (>a) (real)
Continuous uniform distribution over
[a, b]