newtype pattern matching

[David Feuer]

Marcin 'Qrczak' Kowalczyk  qrczak@knm.org.pl  said:

The side effect of using data with a strict argument 
instead of
newtype is that

    f' (C2 x) (C2 y) = C2 (f x y)

unexpectedly becomes strict in both arguments, and we 
would have
to write

    f' x y = C2 (f (case x of C2 x' -> x')
                   (case y of C2 y' -> y'))

in the general case where we want to preserve strictness 
and don't
want to analyze it ourselves. Newtypes allow to use the 
convenient
syntax of pattern matching on the lhs.
---------

Surely you could just write
f' ~(C2 x) ~(C2 y) = C2 (f x y)

which makes the laziness explicit.  Since you have to 
modify the pattern anyway when changing from a type to a 
newtype/datatyp., it would seem that the added documentary 
value of the explicit lazy patterns is worth the extra 
work....

David Feuer

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