Trouble with Implicit Parameters

[JoergWMittag]

I was toying around with Magpie's functional programming features and ran into some trouble with Implicit Parameters. I have a repository where I keep a simple implementation of the Church Encoding of lists in the λ-calculus in various languages and wanted to add a Magpie implementation.

Originally, I had written this:

val kons = fn(hd, tl) fn if _ then hd else tl
val virst = fn _ call(true )
val rrest = fn _ call(false)

val lstt  = kons call(1, kons call(2, nothing))

virst call(rrest call(lstt))

Which I thought should work (although I admit I didn't look into it too deeply), but I always get an error:

Uncaught NoVariableError: Could not find a variable named "_" (lambdaconscarcdr.mag (line 3, col 16-17)).

I am also befuddled by the fact that that the error appears on line 3, but not on the equivalent line 2. However, when I fix the error on line 3 by making the parameter explicit, it then reports line 1 as erroneous. When I fix that, the error moves to line 2.

The explicit version works:

val kons = fn(hd, tl) fn(x) if x then hd else tl
val virst = fn(l) l call(true )
val rrest = fn(l) l call(false)

val lstt  = kons call(1, kons call(2, nothing))

virst call(rrest call(lstt))

I am using the latest version of the Java interpreter at the moment, since the documentation says that the C++ VM is still missing some language features. (And also, I was lazy and I already had a Java environment set up anyway.)

[munificent]

The short answer is that I haven't touched Magpie in years, so it's probably a lost cause to try to write some working Magpie code at this point. :)

Could it maybe be related to not having an end for the if? I can't recall if Magpie allows eliding them for a single-line if statement.

[JoergWMittag]

The short answer is that I haven't touched Magpie in years, so it's probably a lost cause to try to write some working Magpie code at this point. :)

Haha, okay, I'll take that answer.

Could it maybe be related to not having an end for the if? I can't recall if Magpie allows eliding them for a single-line if statement.

I don't think so. It works in the explicit version, after all, where all I do is replace the parameters and leave the rest of the code untouched, and there are several instances in the docs of one line ifs without end.

This is just a toy project. Don't sweat it.

I had written that code in Ruby, then wondered what it would look like in Python, then ECMAScript, and pretty soon I had written it in all languages I knew and then started writing it in languages I don't know, which is how I stumbled across such beauties as MISC, GOO, or Magpie.