Writing an M4 recursive macro for dotgraph notation

Question

I'm trying to write an M4 macro to produce dotgraph notation but I'm struggling with the recursion.

I would like to have a macro which takes the first parameter, and adds the dotgraph relation for all the remaining parameters. For example

coachExisting(a, b, c, d)

would produce

a -> b; a -> c; a -> d;

This was my attempt at it:

define(coachExisting, `ifelse(eval($#==2), 1, `ifdef(first, first, $1) -> $2;',
ifelse(eval($#>2), 1,
`ifdef(first, ,define(first, $1)) first -> $2; $0(shift($*))')
)')

However this only seems to work if I use numbers

coachExisting(1, 2, 3, 4)
1 -> 2;  1 -> 3; 1 -> 4;

When I try with letters, it seems to lose the first definition.

coachExisting(a, b, c, d)
b -> b;  b -> c; c -> d;

Answer 1

You are playing fast and loose with quoting. Please read the m4 manual carefully for guidance about quoting and recursive macro evaluation. On the first expansion, you get define(first, $1) => define(first, a). But on the second execution, it expands as define(first, $1) => define(first, b) => define(a, b) (because the first is expanded as a macro). You need to quote the first everywhere you can, so define(`first', `$1')', for starters.

A much cleaner way to implement your macro is to avoid a temporary macro to begin with. I'd recommend something like:

define(`coachExisting', `ifelse(eval($#==2), 1,
    `$1 -> $2',
    `$1 -> $2; $0(`$1', shift(shift($@)))')')

(be careful though -- I haven't designed this macro to handle the cases with 0 or 1 argument).