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Common Misunderstandings

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1 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.

1.1 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 (or on the same line as the if).

1.2 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:

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 ()

1.3 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 must 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.

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

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