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| newtype is not exactly like data. #haskell is frequently being questioned about the difference.
| | #REDIRECT [[Newtype]] |
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| Firstly, as mentioned in most tutorials, newtype can handle only a single data constructor, and a single field in that single data constructor.
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| Secondly, newtype is strict on its field.
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| So how is:
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| <code>newtype NewType = NewType Int</code>
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| different from:
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| <code>data NewData = NewData Int</code>
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| The number of bottoms differ. Since data is lazy, it introduces a new bottom in addition to the bottom that existed in the Int itself.
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| NewData can be _|_ or it can be (NewData _|_). NewType cannot be _|_ but can only be (NewType _|_).
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| So, lets try another comparison:
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| How is:
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| <code>newtype NewType = NewType Int</code>
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| different from:
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| <code>data NewData = NewData !Int</code>
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| Here, the number of bottoms is the same, but they are not quite the same bottoms. NewType's bottom is: (NewType _|_) (the bottom is <b>inside</b> the pattern), whereas NewData's bottom is (_|_) (the bottom is the <b>whole</b> pattern).
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| To demonstrate this difference, we can use the following code;
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| <code>
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| case (NewType undefined) of
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| (NewType _) -> "Happy happy joy joy :-)"
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| case (NewData undefined) of
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| (DataType _) -> "Won't ever get here :-("
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| </code>
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| Since the newtype bottom is inside the pattern, and we catch that with (_) and never use it, we don't actually hit the bottom. But the NewData bottom is on the whole pattern, so even matching the single data constructor that way is a bottom.
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