# [Haskell-cafe] Generalizing nested list comprehensions

Ishaaq Chandy ishaaq at gmail.com
Fri Feb 26 09:52:15 EST 2010

```Hi all,
If this question sounds a bit noob-ish, that would be because I am one - so

I have functions that look something like these:

f1 :: [a] -> [b]
f1 xs = [foo [x1, x2] |
x1 <- xs,
x2 <- bar x1,
baz x1 /= baz x2]

f2 :: [a] -> [b]
f2 xs = [foo [x1, x2, x3] |
x1 <- xs,
x2 <- bar x1,
x3 <- bar x2,
baz x1 /= baz x2,
baz x1 /= baz x3,
baz x2 /= baz x3]

f3 :: [a] -> [b]
f3 xs = [foo [x1, x2, x3, x4] |
x1 <- xs,
x2 <- bar x1,
x3 <- bar x2,
x4 <- bar x3,
baz x1 /= baz x2,
baz x1 /= baz x3,
baz x1 /= baz x4,
baz x2 /= baz x3,
baz x1 /= baz x4]

Note that for the purposes of this discussion it does not matter what foo,
bar and baz do.

Now what I want to do is write a generalized function fn based on the
pattern set by f1, f2 and f3 such that:
fn :: Int -> [a] -> [b]
and:
(fn 1 xs) == f1 xs
(fn 2 xs) == f2 xs
(fn 3 xs) == f3 xs
(fn 25 xs) == f25 xs

- obviously if I were to implement f25 as nested list comprehensions it
would be ridiculously tedious!

Any ideas how I can implement fn?

Thanks,
Ishaaq

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