Common Mistakes

Forgetting parentheses with rank

      ⌽⍤1 M           ⍝ WRONG: 1 and M form a strand used as the rank operand
      (⌽⍤1) M          ⍝ RIGHT: parentheses group the derived function

This is the most common mistake when using the rank operator. Always parenthesise (f⍤k).

Confusing reduce and compress

/ has two meanings depending on context:

• Operator (after a function glyph): +/V -- reduce
• Function (with a left argument): 1 0 1/V -- compress (replicate)

      +/1 2 3          ⍝ reduce: 6
      1 0 1/1 2 3      ⍝ compress: 1 3

Shape mismatch in dyadic functions

Dyadic scalar functions require matching shapes (or one argument must be a scalar):

      1 2 3 + 1 2
LENGTH ERROR

      1 2 3 + 10
11 12 13

Expecting nested arrays

MARPLE has flat arrays only. You cannot put an array inside an array:

      ⊂ 1 2 3          ⍝ ERROR: no ⊂ function

Use the rank operator instead of Each (¨):

      (f⍤1) M          ⍝ apply f to each row -- replaces f¨

Index origin confusion

MARPLE defaults to ⎕IO←1. If you are used to 0-origin:

      V ← 10 20 30
      V[0]             ⍝ INDEX ERROR with ⎕IO←1
      V[1]             ⍝ 10

Right-to-left evaluation

APL evaluates right to left. This catches many newcomers:

      2×3+4            ⍝ 14, not 10 -- (3+4) is evaluated first

Confusing assignment with comparison

← is assignment, = is comparison:

      x ← 5            ⍝ assigns 5 to x
      x = 5            ⍝ returns 1 (true)

Name class conflicts

You cannot reuse a name that holds a function to store an array, or vice versa:

      f ← {⍵+1}
      f ← 42           ⍝ CLASS ERROR