raganwald/problem.md

## problem.md

      
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              problem.md
            
          
    This is the problem I'm having. I've written a Combinatory Logic interpreter in the oscin.es library. It works with combinators (defined as upper-case letters) and unknowns (lower-case letters). You can write things like this:
interpret('Txy')

And get this:
'yx'

I write this as:
interpret('Txy')
  //=> 'yx'

Because I've written this in JavaScript:
function T (a) { return function _T (b) {
  return b(a)
}}

And there's some magic baked into the unknowns such that y(x) evaluates to yx.
It breaks the moment I write something recursive. For example:
interpret('Mx')
  //=> 'xx'
interpret('MM')
  //=> RangeError: Maximum call stack size exceeded

I want it to return 'MM'.
And much worse:
interpret('Uxy')
  //=> y(xxy)
interpret('UUx')
  //=> RangeError: Maximum call stack size exceeded

I very much want it to return 'x(UUx)' so I can investigate fixed points. But I don't want to change the behaviour of things like:
interpret('BCTxyz')
  //=> 'zxy'