[Haskell-cafe] do vs. pattern matching

Alexander Solla alex.solla at gmail.com
Sat Aug 4 16:05:22 CEST 2012

On Fri, Aug 3, 2012 at 11:34 PM, Matthew <wonderzombie at gmail.com> wrote:

> I'm a somewhat experienced coder but I am relatively new to Haskell.
> I've got a question about whether a usage of do notation is idiomatic,
> or whether it's better to use pattern matching.
> I've got two functions which take an input and return Maybe SomeType.
> If either returns Nothing, I also want to return Nothing. If they both
> return something, then I'll return something unrelated.
> With do notation, I can write something like this:
>         do
>           foo <- callFoo x
>           bar <- callBar x
>           return (baz)
> Alternatively, there's a straightforward pattern match. After binding
> foo, bar in a couple of where clauses:
>         case (foo,bar) of (Just x, Just y) -> baz
>                           _                -> Nothing
> So which approach is more idiomatic, do you think?

The short answer is to write a "one liner" using (>>=) and (>>), unless you
need to bind more than one value to a variable.  In that case, you should
use an applicative interface, if available and otherwise possible, and
finally do-notation.
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