The String Calculator Kata in Haskell

Finally got around to recording this one.  Here's the String Calculator Kata in Haskell using a bit of REPL-based TDD through ghci.

The final refactoring felt like a bit of a stretch.  I'm still not sure how far to go when point-free style seems to be just in reach.  In this case, using a pair for the delimiter and the calculation string allowed me to avoid passing the input string as a parameter in the middle of a train of function compositions.  I wonder whether there is a name for this technique?

 

String Calculator Kata in Haskell from mfeathers on Vimeo.

Comments

[Dan Burton]

If you hoogle the type signature for converting a function with 2 parameters into a function with one 2-tuple parameter, you find "uncurry".

(a -> b -> c) -> ((a,b) -> c)

http://haskell.org/hoogle/?hoogle=(a+-%3E+b+-%3E+c)+-%3E+((a,+b)+-%3E+c)

let add x y = x + y
:t add
add :: (Num a) => a -> a -> a
:t uncurry add
uncurry add :: (Num a) => (a,a) -> a

Tupling up extra data in the return type is a little different, but sounds a lot like the Reader monad or State monad.

[sclv]

Two other functions of note -- Data.List.isPrefixOf, and Control.Arrow.&&&. The latter lets you write `(delimiter &&& body) text` which helps the pointfreeness.

[Jeremy]

For anyone curious: the wisely-chosen music is by Recoil. :)

[Jiggaboo]

Great music. I don't understand anything in Haskell :)

[Karep]

I watch it from time to time just to listen to the great music.