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A Type Driven Approach to Functional Design

Authors: Michael Feathers
Time:1:00 pm - 1:20 pm

Been thinking about how we design functional programs. It's gained ascendancy in the past 5-10 years, but you don't hear people talk about how you design those programs. This is in contrast to ascendancy of OOP, when everyone had an opinion they were happy to share.

Had this thought that the Haskell type signature was useful for describing how to assemble programs. Wrote some Ruby code, and even in Ruby he was adding comments to describe expectations that looked a lot like Haskell type signatures. For example:

map :: (a -> b) -> [a] -> [b]

Describes the function map that takes a function as a parameter that takes a and reduces to b; it can also take a list of "a" and return a list of "b". There's no clear demarcation between input and return, the return value just happens to be the last thing returned.

region 7 9 "expersexchange"

region :: Int -> Int -> String -> String

So region is a function that takes two indices and a string, and returns another string.

Thinking about a line break algorithm in these terms. Describing the steps in terms of types helped him understand the "shape" of the data.