Common Pitfalls and How to Avoid Them

Pitfall 1: Spacing Around <<

; Wrong - tokenizer reads this as ≤
(set b=<<1,2,3>>)

; Correct - space before <<
(set b (binary (1 2 3)))

In LFE, this particular pitfall is less common due to the explicit binary form, but it’s worth noting if you’re translating Erlang examples.

Pitfall 2: Non-byte-aligned Binary Segments

; This will fail - 7 bits is not byte-aligned
(binary ((x (size 7) binary) rest binary) data)

; This works - 7 bits as an integer or bitstring
(binary ((x (size 7)) rest binary) data)

Remember: segments typed as binary must be byte-aligned (size divisible by 8).

Pitfall 3: Using Arithmetic in Patterns

; This won't work - can't use arithmetic directly
(binary ((value (size (+ n 1)))) data)

; This works - compute size first
(let ((field-size (+ n 1)))
  (binary ((value (size field-size))) data))

Pattern matching expects literal values or bound variables, not arithmetic expressions. Do your math before the pattern, not during it.