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# Test driven development (TDD)
The suggested technique for bruijn development is the TDD method. When
creating functions, we suggest the following procedure:
- Write a comment, a type signature, and the head of the function
``` bruijn
# returns the item at index in a list, starting from 0
index y [[[rec]]] ⧗ (List a) → Number → a
```
- Write several tests including edge cases
``` bruijn
# returns the item at index in a list, starting from 0
index y [[[rec]]] ⧗ (List a) → Number → a
:test (empty !! (+0)) (empty)
:test ({}(+1) !! (+0)) ((+1))
:test (((+1) : ((+2) : {}(+3))) !! (+0)) ((+1))
:test (((+1) : ((+2) : {}(+3))) !! (+2)) ((+3))
:test (((+1) : ((+2) : {}(+3))) !! (-1)) (empty)
:test (((+1) : ((+2) : {}(+3))) !! (+3)) (empty)
```
- Finish the implementation until all tests pass (e.g. using the
[`:watch`{.bruijn} repl command](REPL.md#watch))
- Refactor and clean up the definition
``` bruijn
# returns the item at index in a list, starting from 0
index y [[[rec]]] ⧗ (List a) → Number → a
rec 0 [[[case-index]]] case-end
case-index =?4 2 (5 --4 1)
case-end empty
:test (empty !! (+0)) (empty)
:test ({}(+1) !! (+0)) ((+1))
:test (((+1) : ((+2) : {}(+3))) !! (+0)) ((+1))
:test (((+1) : ((+2) : {}(+3))) !! (+2)) ((+3))
:test (((+1) : ((+2) : {}(+3))) !! (-1)) (empty)
:test (((+1) : ((+2) : {}(+3))) !! (+3)) (empty)
```
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