In Haskell, I can use the type
a -> Maybe b to model a function that either returns a value of type
b, or returns nothing (it fails).
If I have types
a1, ..., a(n+1) and functions
f1, ..., fn, with
fi :: ai -> Maybe a(i+1) for all
1 <= i <= n, I can chain the functions by using the
>>= operator of the
Maybe monad and write:
f1 x >>= f2 >>= f3 >>=... >>= fn
>>= operator ensures that each function is applied as long as its predecessor has returned a meaningful value. As soon as a function in the chain fails, the whole chain fails (returns
Nothing) and further functions in the chain are not evaluated.
I have a somewhat similar pattern in which I want to try several functions on the same input, and return as soon as one function succeeds. If all functions fail (return
Nothing), the whole computation should fail. More precisely, I have functions
f1, ..., fn :: a -> Maybe b and I define the function
tryFunctions :: [a -> Maybe b] -> a -> Maybe b tryFunctions  _ = Nothing tryFunctions (f : fs) x = case f x of Nothing -> tryFunctions fs x r@(Just _) -> r
In a sense this is dual to the
Maybe monad in that a computation stops at the first success instead of at the first failure.
Of course, I can use the function I have written above but I was wondering if there is a better, well-established and idiomatic way of expressing this pattern in Haskell.
return f1 ?? f2 ?? f3 ?? DefaultValue;
Alternativewhich is the symbol infix operator
<|>and is defined in terms of a Monoid