haskell - How to use replicateM? -

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i started learning code haskell apologies if stupid question. trying redo 8 queen problem making use of [] monad. here code,

import control.monad addqueen :: [int] -> [[int]] addqueen xs =  [x:xs|x<-[1,2..8], not $ x `elem` xs || (any (\(index,q) -> abs (x-q) ==index) $ zip [1..] xs)]

when try to

[[]]>>= replicatem 8 addqueen

it not work yields following error: 

couldn't match expected type `t0 -> t1' actual type `[[a0]]' first argument of ($) takes 1 argument, type `[[a0]]' has none in expression: [[]] >>= replicatem 8 $ addqueen in equation `it': = [[]] >>= replicatem 8 $ addqueen

so how achieve want here?

replicatem wrong choice here:

prelude control.monad> :t replicatem replicatem :: (monad m) => int -> m -> m [a] prelude> let addqueen :: [int] -> [[int]]; addqueen [] = undefined

this means in expression replicatem 8 addqueen, m ~ ([int] -> [[int]]) i.e. m ~ ((->) [int]) , a ~ [[int]]. , type m [a] ~ ([int] -> [[[int]]]). not intended.

(if type error "no instance (monad ((->) [int]))", try loading e.g. control.applicative first, bring in definition instance monad ((->) r). happen if you're using older version of ghc).

try this, instead:

prelude> :m +control.monad prelude control.monad> :t (>=>) (>=>) :: (monad m) => (a -> m b) -> (b -> m c) -> -> m c prelude control.monad> :t foldl1 (>=>) $ replicate 8 addqueen foldl1 (>=>) $ replicate 8 addqueen :: [int] -> [[int]] prelude control.monad> :t [[]] >>= ( foldl1 (>=>) $ replicate 8 addqueen ) [[]] >>= ( foldl1 (>=>) $ replicate 8 addqueen ) :: [[int]]

update: expression, x1 = foldl1 (>=>) $ replicate 8 addqueen, stands on own, in prolog terms corresponding a "goal" of adding 8 queens initial solution. uses above-basic-level operator "fish"1 i.e. left-to-right kleisli composition operator >=>, defined that

(m >>= a) >>= b  ===  m >>= (a >=> b)

i.e. >=> composition operator monadic functions. use x1 feeding empty solution it, [ [] ] >>= x1.

the expression given in comments sassa nf, x2 = foldl (>>=) [[]] $ replicate 8 addqueen, uses basic monadic bind operator >>=, works whole.

1 http://haskellrescue.blogspot.com/2011/03/cooking-delicious-fish.html


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