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h n= h is a function of type Int -> IO () mapM_ mapM_ executes a function returning monadic actions on all objects in a list, in order. Then it executes these actions, in order. For this code, it transforms each value in the list into a monadic action that prints the corresponding line ( the function consists of two components putStrLn the second part is printing the result of the first part to stdout . concatenating both components (\k-> the first parts first prints (n-k) spaces and then (n+k-1) asterisks ([k..n]>>" ") create the list of the integers from k to n (That is actually one more entry than necessary, but just results in a leading whitespace per line, while saving 2 bytes). Then create a new list where each element of that first list is replaced with the string " " and concatenate that result into one string ++ concatenate both lists ([2..n+k]>>"* ") create the list of the integers from 2 to n+k (of length n+k-1). Then create a new list where each element of that first list is replaced with the string "* " and concatenate that result into one big string ) ) ([1..n-1]++[n,n-1..1]) the list simply goes from 1 to n and back, supplying the k 

h n= h is a function of type Int -> IO () mapM_ mapM_ executes a function returning monadic actions on all objects in a list, in order. Then it executes these actions, in order. For this code, it transforms each value in the list into a monadic action that prints the corresponding line ( the function consists of two components putStrLn the second part is printing the result of the first part to stdout . concatenating both components (\k-> the first parts first prints (n-k) spaces and then (n+k-1) asterisks ([k..n]>>" ") create the list of the integers from k to n (That is actually one more entry than necessary, but just results in a leading whitespace per line, while saving 2 bytes compared to [1..n-k]). Then create a new list where each element of that first list is replaced with the string " " and concatenate that result into one string ++ concatenate both lists ([2..n+k]>>"* ") create the list of the integers from 2 to n+k (of length n+k-1). Then create a new list where each element of that first list is replaced with the string "* " and concatenate that result into one big string ) ) ([1..n-1]++[n,n-1..1]) the list simply goes from 1 to n and back, supplying the k 

Haskell, 99 97 79 bytes

Haskell, 99 97 79 bytes

Haskell, 99 97 bytes

h n=foldr(>>)(return())$map(putStrLn.(\k->([k..n]>>" ")++([2..n+k]>>"* ")))([1..n-1]++[n,n-1..1]) 

h n= h is a function of type Int -> IO () foldr this fold receives a list of monadic actions, each of which prints one line of  the hexagon. foldr folds these actions into one resulting value, which will be the printing   of the complete hexagon (>>)  to combine two actions, we simply execute   them after one another (return())  we initialize the fold with  a monadic action that does nothing  $map  map executes a function on all objects    in a list, in order. For this code, it transforms each value in the list into a monadic action that prints the corresponding line ( the function consists of two components putStrLn the second part is printing the result of the first part to stdout . concatenating both components (\k-> the first parts first prints (n-k) spaces and then (n+k-1) asterisks ([k..n]>>" ") create the list of the integers from k to n (That is actually one more entry than necessary, but just results in a leading whitespace per line, while saving 2 bytes). Then create a new list where each element of that first list is replaced with the string " " and concatenate that result into one string ++ concatenate both lists ([2..n+k]>>"* ") create the list of the integers from 2 to n+k (of length n+k-1). Then create a new list where each element of that first list is replaced with the string "* " and concatenate that result into one big string ) ) ([1..n-1]++[n,n-1..1]) the list simply goes from 1 to n and back, supplying the k (\k-> ([1..n-k]>>" ") create the list of the integers from 1 to n-k. Then create a new list where each element of that first list is replaced with the string " " and concatenate that result into one string ++ concatenate both lists ([2..n+k]>>"* ") create the list of the integers from 2 to n+k (of length n+k-1). Then create a new list where each element of that first list is replaced with the string "* " and concatenate that result into one big string ) 

Haskell, 99 97 79 bytes

h n=mapM_(putStrLn.(\k->([k..n]>>" ")++([2..n+k]>>"* ")))([1..n-1]++[n,n-1..1]) 

h n= h is a function of type Int -> IO () mapM_ mapM_ executes a function returning monadic actions on all objects in a list, in order. Then it executes these actions, in order. For this code, it transforms each value in the list into a monadic action that prints the corresponding line ( the function consists of two components putStrLn the second part is printing the result of the first part to stdout . concatenating both components (\k-> the first parts first prints (n-k) spaces and then (n+k-1) asterisks ([k..n]>>" ") create the list of the integers from k to n (That is actually one more entry than necessary, but just results in a leading whitespace per line, while saving 2 bytes). Then create a new list where each element of that first list is replaced with the string " " and concatenate that result into one string ++ concatenate both lists ([2..n+k]>>"* ") create the list of the integers from 2 to n+k (of length n+k-1). Then create a new list where each element of that first list is replaced with the string "* " and concatenate that result into one big string ) ) ([1..n-1]++[n,n-1..1]) the list simply goes from 1 to n and back, supplying the k 

Edit: Switched to mapM_. I was not aware that was available without using import