Logic games in APL: Electric charges

Rules

1. The aim is to fill the grid with + and - charges.
2. The numbers in the grid indicate the sum of adjacent charges (+1 for +, -1 for -).

Source code

  mat ← 4 4⍴3 9 ¯1 9 9 9 9 9 0 9 9 0 ¯2 9 0 9

  pmat ← {{,[⍳2]↑(⊂⊂⎕IO,1+⍵)⌷¨⍒¨↓∘.=⍨⍳1+1↓⍴⍵}⍣⍵⍉⍪⍬}

  f ← {
    y←⍺ ⋄ x←⍵
    n ← ≢near ← ({1↓↓⍵⌿⍨∧/⍵∊⍳≢mat}y+⍤1⊢↑,∘.,⍨0 1 ¯1)~⍸0≠x+9≠mat
    s ← y⌷mat-{+/,⍵}⌺3 3⊢x
    ((0=n)∧0≠s)∨(≠/2|n s)∨n<|s: ⍬
    sum ← {1 ¯1/⍨⍵⊃⍨⍸s=-/¨⍵}(⌽,¨⊢)0,⍳n
    {⍵@near⊢x}¨↓∪sum[pmat n]
  }

  solver ← {⊃⊃{⊃,/⍺∘f¨⍵}/(⍸9≠⍵),(⊂∘⊂0⍴⍨⍴)⍵⊣mat←⍵}