A failing fail in the monad list¶
I’m following the Real World Haskell book. In the chapter about Monads, they create simple example using the list monad compute all pairs of numbers (x, y) that such that x * y == n for a given n.
Their solution is:
multiplyTo :: Int -> [(Int, Int)]
multiplyTo n = do
x <- [1..n]
y <- [x..n]
guarded (x * y == n) $
return (x, y)
guarded :: Bool -> [a] -> [a]
guarded True xs = xs
guarded False _ = []
But I was wondering if I could restate guarded for any monad. Since fail in the list monad is fail _ = [], I though I could do:
guarded :: Monad m => Bool -> m a -> m a
guarded True = id
guarded False = fail "skipped"
However this actually fails in ghci:
*Main> multiplyTo 24
*** Exception: skipped
I let myself sleep, and this morning I figured there’s actually a small/important difference between guarded False _ = [] and guarded False = fail "..."
The type of guarded False is Monad m => m a -> m a. However the type of fail 'skipped' is just Monad m => m a.
Why does guarded False = fail "skipped" type-checks? That’s an open question (as of the time of writing).
If I define guarded as:
guarded :: Monad m => Bool -> m a -> m a
guarded True xs = xs
guarded False _ = fail "skipped"
Or:
guarded :: Monad m => Bool -> m a -> m a
guarded True = id
guarded False = \s -> fail "skipped"
Both work correctly.
Warning
fail is strongly discouraged to use in real world haskell code.