comienzanCon :: [Int] -> Int -> [Int]

comienzanCon [123,51,11,711,52] 1 == [123,11]
comienzanCon [123,51,11,711,52] 5 == [51,52]
comienzanCon [123,51,11,711,52] 6 == []

import Data.Char (intToDigit)

-- 1ª definición (por comprensión)
comienzanCon :: [Int]-> Int -> [Int]
comienzanCon xs d = [x | x <- xs, head (show x) == d']
    where d' = head (show d)

-- 2ª definición (por recursión)
comienzanCon2 :: [Int]-> Int -> [Int]
comienzanCon2 xs d = aux xs
    where aux [] = []
          aux (x:xs) | head (show x) == d' = x : aux xs
                     | otherwise           = aux xs
          d' = head (show d)

-- 3ª definición (por filtrado)
comienzanCon3 :: [Int]-> Int -> [Int]
comienzanCon3 xs d = filter (\x -> head (show x) == d') xs
    where d' = head (show d)

-- 4ª definición (por plegado)
comienzanCon4 :: [Int]-> Int -> [Int]
comienzanCon4 xs d = foldr f [] xs
    where f x ys | head (show x) == d' = x : ys
                 | otherwise           = ys
          d' = head (show d)

-- En las definiciones anteriores se puede usa intToDigit en lugar de
-- read.

comienzanCon' :: [Int]-> Int -> [Int]
comienzanCon' xs d = [x | x <- xs, head (show x) == intToDigit d]

comienzanCon2' :: [Int]-> Int -> [Int]
comienzanCon2' xs d = aux xs
    where aux [] = []
          aux (x:xs) | head (show x) == intToDigit d = x : aux xs
                     | otherwise                     = aux xs

comienzanCon3' :: [Int]-> Int -> [Int]
comienzanCon3' xs d = filter ((==intToDigit d) . head . show) xs

comienzanCon4' :: [Int]-> Int -> [Int]
comienzanCon4' xs d = foldr f [] xs
    where f x ys | head (show x) == intToDigit d = x : ys
                 | otherwise                     = ys