CODEWITHSUNDEEP
haskelllazy-evaluationnewtype
yairchu
yairchu

Reputation: 24794

Difference between `newtype` and `data` with a strictness annotation

How does this code

data D = D { _d :: ![P] } -- Note the strictness annotation!

Compare to this

newtype D = D { _d :: [P] }

An answer to a related question says:

the main difference between data and newtype is that with data is that data constructors are lazy while newtype is strict

How does this difference work when the data version has a strictness annotation?

(the question is based on real code the I've stumbled on)

Upvotes: 5

Views: 457

Answers (1)

chi
chi

Reputation: 116174

For instance,

case undefined of
   D d -> "hello"

will error out for data types (strict or not strict), but will evaluate to "hello" for newtypes.

This is because, at runtime, applying a newtype constructor, or pattern matching on it corresponds to no operation. Not even forcing the value we case upon.

By contrast, pattern matching on a data constructor always forces the value we case upon.

I think this is the only runtime difference between strict data and newtype. There are some static differences, such as some GHC extensions which only affect newtype, Coercible, etc., but at runtime the two types are isomorphic (but pattern matching operates differently, as shown above).

Upvotes: 7

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