Erlang list matching -


i'm working through book thinking in erlang. in "figure 10: example of case" has following example:

many(x) -> case x of     [] ->         none;     [ _one ] ->         one;     [ _one, _two ] ->         two;     [ _one, _two , _three | _tail ] ->         many end.

it says :

if wondering why line 9 not match against [ _one, _two | _tail ], review list matching rules list tails @ end of previous section.

but if match against [ _one, _two | _tail ] still works expected. there error in book or getting wrong ?

i think might not error.

the semantics of

[_one, _two, _three | _tail]

is list of 3 elements or more.

the semantics of

[_one, _two | _tail]

is list of 2 elements or more.

since third pattern [ _one, _two ] indicates case "a list of 2 elements", using [_one, _two | _tail] little bit redundant.

there reason "everything's working expected". if place fourth pattern before third one, gives:

many(x) ->   case x of     [] ->       none;     [_one] ->       one;     [_one, _two | _tail] ->  %% switched       many;     [_one, _two] ->          %% switched       2   end.

then won't work expected. mod:many([a,b]) yield many instead of expected two. because when "case" expression evaluated, x matched in turn against patterns. , sequential order guaranteed. many returned because [a,b] matches [_one, _two | _tail] first, _tail being [](an empty list).

so, though [ _one, _two | _tail ] work in case, using [ _one, _two , _three | _tail ] considered practice in case switch patterns afterwards.


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