CODEWITHSUNDEEP
haskellmonads
Tomer
Tomer

Reputation: 1229

Converting a function that output a Monad type to a function that output the raw value type

I have 2 type definitions 

type Mapper a k v = a -> [(k,v)]
type MapperM m a k v = a -> m [(k,v)]

I want to write a function that convert a value from type 

(Ord k, Monad m) => MapperM m a k v

to type 

Ord k => Mapper a k v

basically a function that takes a -> m [(k,v)] as an argument and return a -> [(k,v)] as output.

All my attempts are failing with the Haskell type checks.

Upvotes: 1

Views: 147

Answers (1)

Alistair Wall
Alistair Wall

Reputation: 332

I believe it is mathematically impossible.

Consider the case when m is Maybe: the function could return [(k,v)] from Just [(k,v)], but what would it return from Nothing?

If the monad is IO, extracting the argument from the monad would break the safety that IO provides.

Also consider that your function would have to work for any conceivable monad.

Upvotes: 3

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