Parse distribution expression — monty_dsl_parse

Parse an expression as if it were a call to one of monty's distribution functions (e.g., Normal, Poisson). This will fill in any defaults, disambiguate where multiple parameterisations of the distribution are available, and provide links through to the C++ API. This function is designed for use from other packages that use monty, and is unlikely to be useful to most users.

Usage

monty_dsl_parse_distribution(expr, name = NULL)

Arguments

expr: An expression
name: Name for the expression, used in constructing messages that you can use in errors.

Value

A list; the contents of this are subject to change. However you can (to a degree) rely on the following elements:

name: The name of the distribution (e.g., Normal). This will be the same as the name of the function called in expr
variant: The name of the distribution variant, if more than one is supported.
args: The arguments that you provided, in position-matched order
cpp: The names of the C++ entrypoint to use. This is a list with elements density and sample for the log-density and sampling functions, and NULL where these do not yet exist.

Currently we also include:

density: A function to compute the log-density. This will likely change once we support creation of differentiable models because we will want to do something with the arguments provided!
sample: A function to sample from the distribution, given (as a first argument) a rng object (see monty_rng)

Examples

# A successful match
monty_dsl_parse_distribution(quote(Normal(0, 1)))
#> $success
#> [1] TRUE
#> 
#> $value
#> $value$name
#> [1] "Normal"
#> 
#> $value$variant
#> NULL
#> 
#> $value$args
#> $value$args[[1]]
#> [1] 0
#> 
#> $value$args[[2]]
#> [1] 1
#> 
#> 
#> $value$density
#> function (x, mean, sd) 
#> dnorm(x, mean, sd, log = TRUE)
#> <bytecode: 0x55cac6caf878>
#> <environment: namespace:monty>
#> 
#> $value$domain
#> [1] -Inf  Inf
#> 
#> $value$sample
#> function (rng, mean, sd) 
#> rng$normal(1, mean, sd)
#> <bytecode: 0x55cac6cabe70>
#> <environment: namespace:monty>
#> 
#> $value$expr
#> $value$expr$density
#> -(x - mean)^2/(2 * sd^2) - log(2 * pi)/2 - log(sd)
#> 
#> $value$expr$mean
#> mean
#> 
#> 
#> $value$cpp
#> $value$cpp$density
#> [1] "normal"
#> 
#> $value$cpp$sample
#> [1] "normal"
#> 
#> 
#> 

# An unsuccessful match
monty_dsl_parse_distribution(quote(Normal()))
#> $success
#> [1] FALSE
#> 
#> $error
#>                                                                       x 
#>        "Invalid call to 'Normal()'" "Failed to match given arguments: " 
#>                                   i                                   * 
#>                "Call should match:"                          "mean, sd" 
#>