I really liked doing this one. I had no idea how to approach this idea at first. I couldn't think of any simple approach to it at all. I decided to look at the APLX Scrapbook and see if I could glean any inspiration from there. Lo, and behold, what did I find, but a snippet of code for raveling the diagonal of a matrix! I did not really understand what the grade up and grade down functions did until I went through this problem. After playing around with the scrapbook entry for a while, it made sense to me, and I could see how to apply it to this problem.

I am fairly certain that after figuring out the diagonals, I neglected to properly think about the rest of the code, but here it is anyways.

Problem #11:

∇R←PEELEVEN;B;DA;DB;G;S;⎕IO
⎕IO←1

⍝ What is the greatest product of four adjacent numbers in any direction
⍝ (up, down, left, right, or diagonally) in the following 20×20 grid?

G←'08 02 22 97 38 15 00 40 00 75 04 05 07 78 52 12 50 77 91 08 '
G←G,'49 49 99 40 17 81 18 57 60 87 17 40 98 43 69 48 04 56 62 00 '
G←G,'81 49 31 73 55 79 14 29 93 71 40 67 53 88 30 03 49 13 36 65 '
G←G,'52 70 95 23 04 60 11 42 69 24 68 56 01 32 56 71 37 02 36 91 '
G←G,'22 31 16 71 51 67 63 89 41 92 36 54 22 40 40 28 66 33 13 80 '
G←G,'24 47 32 60 99 03 45 02 44 75 33 53 78 36 84 20 35 17 12 50 '
G←G,'32 98 81 28 64 23 67 10 26 38 40 67 59 54 70 66 18 38 64 70 '
G←G,'67 26 20 68 02 62 12 20 95 63 94 39 63 08 40 91 66 49 94 21 '
G←G,'24 55 58 05 66 73 99 26 97 17 78 78 96 83 14 88 34 89 63 72 '
G←G,'21 36 23 09 75 00 76 44 20 45 35 14 00 61 33 97 34 31 33 95 '
G←G,'78 17 53 28 22 75 31 67 15 94 03 80 04 62 16 14 09 53 56 92 '
G←G,'16 39 05 42 96 35 31 47 55 58 88 24 00 17 54 24 36 29 85 57 '
G←G,'86 56 00 48 35 71 89 07 05 44 44 37 44 60 21 58 51 54 17 58 '
G←G,'19 80 81 68 05 94 47 69 28 73 92 13 86 52 17 77 04 89 55 40 '
G←G,'04 52 08 83 97 35 99 16 07 97 57 32 16 26 26 79 33 27 98 66 '
G←G,'88 36 68 87 57 62 20 72 03 46 33 67 46 55 12 32 63 93 53 69 '
G←G,'04 42 16 73 38 25 39 11 24 94 72 18 08 46 29 32 40 62 76 36 '
G←G,'20 69 36 41 72 30 23 88 34 62 99 69 82 67 59 85 74 04 36 16 '
G←G,'20 73 35 29 78 31 90 01 74 31 49 71 48 86 81 16 23 57 05 54 '
G←G,'01 70 54 71 83 51 54 69 16 92 33 48 61 43 52 01 89 19 67 48'
G←20 20⍴⎕FI G

⍝ Diagonal selector
S←6↓394↑⍋+⌿(⍴G)⊤(⍳⍴,G)-⎕IO

⍝ Edge partition
B←1+0,+2>/S

⍝ Grab the diagonals
DA←⊃B⊂(,G)[S]
DB←⊃B⊂(,⊖G)[S]

⍝ Best of the products of each way to grab numbers
R←⌈/,4×/DA⍪DB⍪G⍪⍉G
∇