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    “Elm requires functions to be total”

    “Requires” in the sense that it’s culturally encouraged? Or did Elm get a totality checker since I last looked?

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      “Requires” in that the compiler rejects functions that don’t handle all constructors of their input types. For example this doesn’t compile:

      f [x] = x
      

      …it leads to the compilation error This pattern does not cover all possible inputs.

      Haskell, on the other hand, would be okay with this (though it would give a warning). And PureScript would compile it but annotate its type as Partial.

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      Why does Elm discourage exposing type constructors? And how do they go about doing so?

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        They discourage it through their design guidelines, preferring “opaque” types, since constructors are the internal representation of a type, so if you expose it as an API then the internal representation and external API are coupled, which can be annoying if you want to change either. http://package.elm-lang.org/help/design-guidelines#keep-tags-and-record-constructors-secret

        The big downside is that you can only pattern match on constructors, so I have mixed feelings…

        But it does encourage things like, for example, having two different “views” of a datatype, as shown in the article, where a non-empty list Cons a can be thought of either as (a, List a) or (a, Maybe (Cons a)) and you don’t need to know or care which one the internal representation is actually using.

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        Why wouldn’t you use the (a, List a) representation?

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          That representation is difficult to recurse on, since if your function expects a non-empty list, then to recurse on the tail you need it to be expressible as a non-empty list.

          But you’re right, you can use a fromList : List a -> Maybe (Cons a) function to achieve the same thing when working with the (a, List a) representation, it’s just slightly more verbose.