Difference between `newtype` and `data` with a strictness annotation
up vote
2
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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)
haskell lazy-evaluation newtype
asked Nov 20 at 10:42
yairchu
15.4k75894
add a comment |
up vote
2
down vote
favorite
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)
haskell lazy-evaluation newtype
asked Nov 20 at 10:42
yairchu
15.4k75894
add a comment |
up vote
2
down vote
favorite
up vote
2
down vote
favorite
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)
haskell lazy-evaluation newtype
asked Nov 20 at 10:42
yairchu
15.4k75894
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)
haskell lazy-evaluation newtype
haskell lazy-evaluation newtype
asked Nov 20 at 10:42
yairchu
15.4k75894
asked Nov 20 at 10:42
yairchu
15.4k75894
asked Nov 20 at 10:42
yairchu
15.4k75894
asked Nov 20 at 10:42
yairchu
15.4k75894
asked Nov 20 at 10:42
yairchu
15.4k75894
15.4k75894
add a comment |
add a comment |
1 Answer
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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).
answered Nov 20 at 10:56
chi
72.5k280134
So if no such pattern matching is done, there's absolutely no difference between the two?
– yairchu
Nov 20 at 14:28
4
There should be a slight memory overhead with the strict data that isn't there with the newtype, but I don't remember if GHC can optimize that out.
– DarthFennec
Nov 20 at 17:33
1
@yairchu I think so. There's a small memory overhead with strict data, as DarthFennec points out. Note that "if no such pattern matching" can be a big "if" -- pattern matching is everything for data types. E.g.case expensiveFunction 23 of D d -> ...performs the expensive computation immediately for big data, potentially keeping in memory a long listdfor a long time depending on the.... With a newtype, that list would be bound lazily, and only generated afterwards, when (and if) demanded by the....
– chi
Nov 20 at 19:33
add a comment |
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
5
down vote
accepted
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).
answered Nov 20 at 10:56
chi
72.5k280134
So if no such pattern matching is done, there's absolutely no difference between the two?
– yairchu
Nov 20 at 14:28
4
There should be a slight memory overhead with the strict data that isn't there with the newtype, but I don't remember if GHC can optimize that out.
– DarthFennec
Nov 20 at 17:33
1
@yairchu I think so. There's a small memory overhead with strict data, as DarthFennec points out. Note that "if no such pattern matching" can be a big "if" -- pattern matching is everything for data types. E.g.case expensiveFunction 23 of D d -> ...performs the expensive computation immediately for big data, potentially keeping in memory a long listdfor a long time depending on the.... With a newtype, that list would be bound lazily, and only generated afterwards, when (and if) demanded by the....
– chi
Nov 20 at 19:33
add a comment |
up vote
5
down vote
accepted
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).
answered Nov 20 at 10:56
chi
72.5k280134
So if no such pattern matching is done, there's absolutely no difference between the two?
– yairchu
Nov 20 at 14:28
4
There should be a slight memory overhead with the strict data that isn't there with the newtype, but I don't remember if GHC can optimize that out.
– DarthFennec
Nov 20 at 17:33
1
@yairchu I think so. There's a small memory overhead with strict data, as DarthFennec points out. Note that "if no such pattern matching" can be a big "if" -- pattern matching is everything for data types. E.g.case expensiveFunction 23 of D d -> ...performs the expensive computation immediately for big data, potentially keeping in memory a long listdfor a long time depending on the.... With a newtype, that list would be bound lazily, and only generated afterwards, when (and if) demanded by the....
– chi
Nov 20 at 19:33
add a comment |
up vote
5
down vote
accepted
up vote
5
down vote
accepted
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).
answered Nov 20 at 10:56
chi
72.5k280134
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).
answered Nov 20 at 10:56
chi
72.5k280134
answered Nov 20 at 10:56
chi
72.5k280134
answered Nov 20 at 10:56
chi
72.5k280134
answered Nov 20 at 10:56
chi
72.5k280134
72.5k280134
So if no such pattern matching is done, there's absolutely no difference between the two?
– yairchu
Nov 20 at 14:28
4
There should be a slight memory overhead with the strict data that isn't there with the newtype, but I don't remember if GHC can optimize that out.
– DarthFennec
Nov 20 at 17:33
1
@yairchu I think so. There's a small memory overhead with strict data, as DarthFennec points out. Note that "if no such pattern matching" can be a big "if" -- pattern matching is everything for data types. E.g.case expensiveFunction 23 of D d -> ...performs the expensive computation immediately for big data, potentially keeping in memory a long listdfor a long time depending on the.... With a newtype, that list would be bound lazily, and only generated afterwards, when (and if) demanded by the....
– chi
Nov 20 at 19:33
add a comment |
So if no such pattern matching is done, there's absolutely no difference between the two?
– yairchu
Nov 20 at 14:28
4
There should be a slight memory overhead with the strict data that isn't there with the newtype, but I don't remember if GHC can optimize that out.
– DarthFennec
Nov 20 at 17:33
1
@yairchu I think so. There's a small memory overhead with strict data, as DarthFennec points out. Note that "if no such pattern matching" can be a big "if" -- pattern matching is everything for data types. E.g.case expensiveFunction 23 of D d -> ...performs the expensive computation immediately for big data, potentially keeping in memory a long listdfor a long time depending on the.... With a newtype, that list would be bound lazily, and only generated afterwards, when (and if) demanded by the....
– chi
Nov 20 at 19:33
So if no such pattern matching is done, there's absolutely no difference between the two?
– yairchu
Nov 20 at 14:28
So if no such pattern matching is done, there's absolutely no difference between the two?
– yairchu
Nov 20 at 14:28
4
4
There should be a slight memory overhead with the strict data that isn't there with the newtype, but I don't remember if GHC can optimize that out.
– DarthFennec
Nov 20 at 17:33
There should be a slight memory overhead with the strict data that isn't there with the newtype, but I don't remember if GHC can optimize that out.
– DarthFennec
Nov 20 at 17:33
1
1
@yairchu I think so. There's a small memory overhead with strict data, as DarthFennec points out. Note that "if no such pattern matching" can be a big "if" -- pattern matching is everything for data types. E.g.
case expensiveFunction 23 of D d -> ... performs the expensive computation immediately for big data, potentially keeping in memory a long list d for a long time depending on the .... With a newtype, that list would be bound lazily, and only generated afterwards, when (and if) demanded by the ....– chi
Nov 20 at 19:33
@yairchu I think so. There's a small memory overhead with strict data, as DarthFennec points out. Note that "if no such pattern matching" can be a big "if" -- pattern matching is everything for data types. E.g.
case expensiveFunction 23 of D d -> ... performs the expensive computation immediately for big data, potentially keeping in memory a long list d for a long time depending on the .... With a newtype, that list would be bound lazily, and only generated afterwards, when (and if) demanded by the ....– chi
Nov 20 at 19:33
add a comment |
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