learning-haskell/22-reader/22-reader.md

22 Reader

22.2 Short Exercise: Warming up

src/warmingup.hs

22.5 Excercise: Ask

src/ask.hs

22.6 Exercise: Reading Comprehension

src/readingcomp.hs

22.7 Exercise: Reader Monad

src/readingcomp.hs

22.9 You can change what comes below, but not above

Trying to understand the following quote:

You can swap in a different type or value of r for function that you call, but not for functions that call you.

Perhaps a better way of saying this would be:

You can can choose the input for a function you are calling, but cannot change the input from within the function.

The input being the context of a Reader. This makes sense, consider:

f :: a -> b

When we call f, we can choose any input value or type to use. However, within f, the value and type are fixed. Immutable. The same applies of course for Reader a b as this is just a newtype for f.

f :: Reader Integer String
f = do
    r <- ask
    -- we cannot change r, but we can do operations on them
    return $ show (r * r)

-- However, we can change the input for f when we call it:
g :: Reader Integer String
g = do
    r <- ask
    -- either via:
    -- return $ runReader f (r + 1)
    -- or:
    withReader (+1) f

22.11 Chapter exercises

A warmup stretch

src/ReaderPractise.hs

Rewriting Shawty

No idea if this is what was requested, but it works:

shawty-prime/app/Main.hs