Specifying default rules in the Curry language: Why and how?

Question

In section 3.5.6 of the Curry 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?

Answer 1

This is the code you are looking for. Yours was missing the preprocessor directive to allow default rules. And using the wrong quote character.

-- Use default rules
{-# OPTIONS_CYMAKE -F --pgmF=currypp --optF=defaultrules #-}

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

test_positive = lookup 2 [(2,3)] == Just 3
test_negative = lookup 1 [(2,3)] == Nothing

A default rule serves various purposes. Regaining control after a failed search is a particularly useful one, since you cannot check with equality whether an expression is a failure.

Answer 2

If you delete the option "-F", then the preprocessor is not invoked which explains the behavior.

The permission error is due to the fact that not all possible intermediate representations of a Curry program are precompiled in the Ubuntu package. Unfortunately, the "default rule translator" of CurryPP requires one of these intermediate representations.

The Ubuntu/Debian package is intended only for using the kernel of Curry. For extensions and more advanced tools, I recommend to install PAKCS manually, e.g., the current release from https://www.informatik.uni-kiel.de/~pakcs/download.html If you already have Ubuntu, a simple make should be sufficient.