feature request: unicode in source

Introduction

Some languages now support Unicode (mostly UTF8) for writing source code. It would be great if one could also use Unicode in Stan source. (Note that comments in UTF8, or any superset that embeds ASCII, are already supported in the sense the parser just ignores them.)

Broadly, there are two possible levels of support:

1. in variable and function names (eg ϕ), and
2. in operators (eg ≤), which provide synonyms for existing ones (eg <=)

Example

This is how the 8 schools example would look like in unicode:

data {
  int<lower=0> J;             // number of schools
  real y[J];                  // estimated treatment effect (school j)
  real<lower=0> σ[J];         // std err of effect estimate (school j)
}
parameters {
  real μ;
  real θ[J];
  real<lower=0> τ;
}
model {
  θ ~ normal(μ, τ); 
  y ~ normal(θ, σ);
}

Possible benefits

1. more compact source code
2. better mapping to equations in papers

Possible downsides

1. editor/entry support
2. font support
3. possibly corrupted files

The first two are mitigated by the fact that ASCII is a subset of UTF8, so using the feature is optional.

UTF8 support in various languages which have interfaces for Stan

Editor support

Emacs

See this list for various UTF8 implementations using autocomplete, company-mode, and quail.

See also

• discussion on stan-dev
• detailed description for Julia

Start by reviewing issue #1406, the linked closed pull request #1407, and the eight_schools.stan example referenced in the issue. Determine the parser entry points involved, then define the supported Unicode identifier and operator cases while preserving existing ASCII Stan syntax; completion should be demonstrated with representative Stan source examples.