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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
# measures the length of a list
length y [[[rec]]] (+0) ⧗ (List a) → Number
```
- Write several tests including all edge cases
``` bruijn
# measures the length of a list
length y [[[rec]]] (+0) ⧗ (List a) → Number
:test (length empty) ((+0))
:test (length "a") ((+1))
:test (length ({}empty)) ((+1))
:test (length (empty : {}empty)) ((+2))
:test (length ("abc")) ((+3))
```
- 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
# measures the length of a list
length y [[[rec]]] (+0) ⧗ (List a) → Number
rec 0 [[[case-inc]]] case-end
case-inc 5 ++4 1
case-end 1
:test (length empty) ((+0))
:test (length "a") ((+1))
:test (length ({}empty)) ((+1))
:test (length (empty : {}empty)) ((+2))
:test (length ("abc")) ((+3))
```
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