BoundCond

bound_cond/1 — a cond that lets you thread interim variables through its clauses.

Plain cond runs the first truthy clause's body, but each clause is isolated: a value you compute while testing one condition can't be reused by the next. withcan thread state, but it models a single happy path with fallbacks — not several branches of equal priority. bound_cond fills that gap.

import BoundCond

bound_cond do
  in_range?( x, y) -> 
    n

  :bind ->
    last = get_last( x)
    pos = get_pos( y)

    pos > last -> 
      last
      
    true -> 
      pos
end

Clauses are tried top to bottom, exactly like cond. A :bind -> clause never matches — its body runs only when execution falls through to it, and the variables it binds are in scope for every clause below it. You can use as many :bind -> steps as you like, and each sees the ones before it. As with cond, nothing bound inside leaks out of the bound_cond. If no clause matches, bound_cond raises CondClauseError, just like cond.

It expands to nested if/else wrapped in a case (used only to scope the bindings), so there is effectively no runtime overhead — the macro is purely a way to keep the source flat and declarative instead of hand-rolling nested ifs.

Installation

Add bound_cond to your dependencies in mix.exs:

def deps do
  [
    { :bound_cond, "~> 0.1.0"}
  ]
end

Then run mix deps.get. The API reference lives on HexDocs.

How it works

Once a do block contains any ->, Elixir parses it in stab-clause mode, where every non-arrow line is folded into the preceding clause's body. So free-floating bindings between clauses aren't possible; instead, a binding step is written as a clause with the reserved head :bind, and the macro splices that body into the fall-through path of the if/else chain it builds.

Test

mix test

License

Released under the MIT License — see LICENSE.