i:.''":"\'0:"i/7.in"
f:{+/{y*|/y={,/x.\:(y;z)}[x]/z}[x].'i}  / given a list of operators, how many equations can be solved?
f(+;*)                                  / part 1: add and multiply
f(+;*;{y+x*/(#$y)#10})                  / part 2: add, multiply, and concatenate

Fun day!

For part 2, I used a slightly odd concatenation function. This is because it works on a list x and scalar y, and not just scalar x and y. I found that by going from an explicit (previously, I was using eachright) to an implicit loop like this, I sped up my solution by a factor of ten.

P.S. ngn/k is a beast for being able to brute force this (I'm not doing anything clever) in 100 ms

~/aoc24k/7.html