On 16.12.2013 23:57, David Luposchainsky wrote:

On 16.12.2013 23:39, Andreas Abel wrote:

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"Only one data constructor" should be understood hereditarily; the description is not entirely accurate:

-- Only one data constructor: do not add MonadFail constraint do (Only x) <- computation >>> let f (Only x) = more more >>> in computation >>= f

(Only x) should be pat such that every constructor in pat is an "only-one" constructor.

Do you mean in case patterns of nested "Only" types appear, like `Only (Only' x)`? I hadn't thought of that, good point.

Yes; its easy to give a grammar for "only-one" patterns: p ::= x (variable) | C p1 ... pn (C is the only constructor of its data/newtype)

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- The case of one data constructor should emit a warning if the data type is defined via `data`: adding another data constructor can make patterns in unrelated modules refutable.

I don't understand the restriction "is defined via data" since I am not aware of defining constructors outside of data or with something other than the data keyword. Please clarify.

I meant "data and not newtype". If "Only" is a newtype data constructor, the pattern is irrefutable by design, is it not?

Of course, I tend to forget about newtype --- in my mind this is just a data with a single-field single constructor. I never understood why there is a separate "newtype" syntax when the automatic inference of "being a newtype" is so trivial. There may be some obscure reason like relying on a specific internal memory layout of data types in foreign function interfacing or so... Cheers, Andreas -- Andreas Abel <>< Du bist der geliebte Mensch.