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newtype vs data in haskll
Prelude> data D = D Int
Prelude> let f (D i) = "ok"
Prelude> f undefined
"*** Exception: Prelude.undefined
Prelude> newtype D = D Int
Prelude> let f (D i) = "ok"
Prelude> f undefined
"ok"
@mattfenwick

Just ran in to the same issue and got the same result as you ... took forever to figure out what the issue was. Thanks for posting this.

I found this link from this StackOverflow comment.

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