map - Type check error in Haskell monad chaining -


i'm new haskell , i'm following example on rwh. i'm having problems following programs on chapter 14:

import qualified data.map m  type personname = string type phonenumber = string type billingaddress = string data mobilecarrier = honest_bobs_phone_network                    | morrisas_marvelous_mobiles                    | petes_plutocratic_phones                      deriving (eq, ord)  findcarrierbillingaddress :: personname                           -> m.map personname phonenumber                           -> m.map phonenumber mobilecarrier                           -> m.map mobilecarrier billingaddress                           -> maybe billingaddress -- work findcarrierbillingaddress person phonemap carriermap addressmap =   phone <- m.lookup person phonemap   carrier <- m.lookup phone carriermap   address <- m.lookup carrier addressmap   return address  -- not work: findcarrierbillingaddress person phonemap carriermap addressmap =     return person >>=     lookup phonemap >>=     lookup carriermap >>=     lookup addressmap   lookup = flip m.lookup

it seems when writing findcarrierbillingaddres in monad chaining format using >>=, not type check:

/home/bruce/programming/haskell/real/ch14/hello.hs:21:9:     couldn't match type `[char]' `mobilecarrier'     expected type: mobilecarrier -> maybe billingaddress       actual type: personname -> maybe billingaddress     in return type of call of `lookup'     in second argument of `(>>=)', namely `lookup addressmap'     in expression:       return person >>= lookup phonemap >>= lookup carriermap       >>= lookup addressmap  /home/bruce/programming/haskell/real/ch14/hello.hs:21:16:     couldn't match type `mobilecarrier' `[char]'     expected type: m.map personname billingaddress       actual type: m.map mobilecarrier billingaddress     in first argument of `lookup', namely `addressmap'     in second argument of `(>>=)', namely `lookup addressmap'     in expression:       return person >>= lookup phonemap >>= lookup carriermap       >>= lookup addressmap failed, modules loaded: none.

the question is.. why second format using >>= not type check?

it's monomorphism restriction acting again. since have pattern binding no type signature inferred type monomorphic , therefore determined first usage.

just change to

lookup m k = flip m.lookup m k

or even

lookup m = flip m.lookup m

you have convince ghc generalize function won't when it's simple pattern binding. adding parameter turns function binding means generalized.

if lost bit there, i've blogged this


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