[curry] Specifying default rules: Why and how?

From: Jeffrey Brown <jeffbrown.the_at_gmail.com>
Date: Sun, 18 Nov 2018 19:50:17 -0500

I posted this question on Stackoverflow[1] yesterday and haven't received
any answers yet. I've duplicated the question below, but an answer posted
to StackOverflow would be more useful to future students, if you're so
inclined.

Thanks!

---

In section 3.5.6 of the Curry tutorial (pdf)
<https://www.informatik.uni-kiel.de/~curry/tutorial/tutorial.pdf>, we are
advised to use default rules to "regain control after a failed search". The
following example is given. (For clarity I have added a type signature and
curried the input.)

lookup :: k -> [(k,v)] -> Maybe v
lookup key (_++[(key,value)]++_ ) = Just value
lookup’default _ _ = Nothing

I can't get that to compile unless I replace the ’ with a '. Once I=
 do, it
behaves like this:

test> test.lookup 1 [(2,3)]
*** No value found!

Question 1: What is the default declaration for?

Why would you need to specify that a particular clause is the default one?
Won't it be arrived at one way or another, once the others fail?
Question 2: How is it written? Should it be written at all?

If instead I drop the string 'default:

lookup :: k -> [(k,v)] -> Maybe v
lookup key (_++[(key,value)]++_ ) = Just value
lookup _ _ = Nothing

it behaves as intended:

test> test.lookup 1 [(2,3)]
Nothing
test>

Has the 'default syntax changed since the tutorial was written? Has it been
removed altogether?
[1]
https://stackoverflow.com/questions/53357361/specifying-default-rules-in-th=
e-curry-language-why-and-how

--
Jeff Brown | Jeffrey Benjamin Brown
Website <https://msu.edu/~brown202/> | Facebook
<https://www.facebook.com/mejeff.younotjeff> | LinkedIn
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Received on Mo Nov 19 2018 - 18:07:09 CET

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