Common Misunderstandings

Common Mistakes and Incorrect Beliefs By Haskell Beginners

People going from zero to Haskell are likely gain a misunderstanding or miss a point that isn't stressed enough. Here are some mistakes that have been observed from multiple sources.

Indentation

Perhaps the first trip-up - you might understand that indentation defines where a code block starts and the lack of an equal amount of indentation indicates the previous code block ended. What some miss is that then and else must be indented deeper than the if statement:

if boolean
    then expr1
    else expr2

Or they can be on the same line as the if:

if boolean then expr1 else expr2

If / Then / Else

if-then statements must always include an 'else' portion. It might be best not to think of if-then-else as flow control, as in most imperative languages, but think of it as construction of a value using a well formed expression.

x = b ? y : z;

The above is valid, though not common, C code. It states that if b is true then x = y otherwise x = z. Notice how this makes no sense without z. Similarly, in Haskell an if/then makes no sense without an else.

let x = if b then y   -- compare to x = b ? y

What is x when b is false? One should also recognize that the types returned by the then and else branches must match due to Haskells strong and static type system.

When if is used for sequencing IO it is not uncommon to see an else that returns a null value:

main = do
    startNetwork <- askUser "Network? "
    if startNetwork
        then do iface <- initNetworkInterface
                handlePackets iface
        else return ()

Such uses can be more succinct if they use the when keyword:

main = do
    startNetwork <- askUser "Network? "
    when startNetwork (do
        iface <- initNetworkInterface
        handlePackets iface )

do Notation

If the do notation page ever exists I'll advice you to check it out. Until then, understand that a missing do from the top of a function or code block can result in your compiler giving an error message citing a much later line number. Also, any new blocks (ex: from an if or case) must have their own do, even if the higher level code block already had one.

Sorry this isn't the full picture - for an inverse point of view see do notation considered harmful.