haskell - Why is the return value of head implicitly converted to Maybe? -

i've started learn haskell , practice i've decided create function takes tree, elements have position , size, , returns element located @ position. code looks this:

import data.list (dropwhile)  data node = node (int,int) (int,int) [node] deriving(eq, show)  findnode :: (int,int) -> node -> maybe node findnode loc node@(node (x, y) (width, height) []) = if loc >= (x, y) && loc <= (x+width, y+height) node else nothing findnode loc node@(node (x, y) (width, height) children) =      if loc >= (x, y) && loc <= (x+width, y+height)         if not $ null nodes             head nodes         else             node     else nothing     nodes = dropwhile (==nothing) $ map (findnode loc) children 

this code compiles , runs far can tell, i'm curious 1 thing, why head nodes acceptable instead of just $ head nodes?.

that's because nodes has type of [maybe node]. can infact verify annotating explicitly , compiling again:

findnode loc node@(node (x, y) (width, height) children) =    if loc >= (x, y) && loc <= (x+width, y+height)       if not $ null nodes     head nodes     else node   else nothing       nodes = dropwhile (==nothing) $ map (findnode loc) children :: [maybe node] 

so how does, nodes have [maybe node] type ?

the type of map (findnode loc) children [maybe node]. (general type of map map :: (a -> b) -> [a] -> [b] , in case type of findnode loc node -> maybe node , hence resultant type [maybe node] ). , type of dropwhile (a -> bool) -> [a] -> [a]. in case a maybe node , hence nodes has type of [maybe node].