[Haskell-cafe] Resources on how to implement (Haskell 98) kind-checking?

David Feuer david.feuer at gmail.com
Wed Oct 13 12:58:43 UTC 2021

Haskell 2010 defines virtually the same language as Haskell 98. The
differences are too trivial to worry about. One spot you may wish to follow
GHC (and I think Hugs, at least) rather than the Report:

On Tue, Oct 12, 2021, 3:37 PM Benjamin Redelings <
benjamin.redelings at gmail.com> wrote:

> Hi,
> 1. I'm looking for resources that describe how to implement kind Haskell
> 98 checking.  Does anyone have any suggestions?
> * I've looked at the PolyKinds paper, but it doesn't cover type classes.
> * I've looked at the source code to GHC, but it is hard to follow for a
> variety of reasons.  It isn't laid out like an algorithm description,
> and the complexity to handle options like PolyKinds and DataKinds makes
> the code harder to follow.
> 2. One question that came up is how to handle type variables that are
> present in class methods, but are not type class parameters. If there
> are multiple types/classes in a single recursive group, the kind of such
> type variables might not be fully resolved until a later type-or-class
> is processed.  Is there a recommended approach?
> I can see two ways to proceed:
> i) First determine the kinds of all the data types, classes, and type
> synonyms.  Then perform a second pass over each type or class to
> determine the kinds of type variables (in class methods) that are not
> type class parameters.
> ii) Alternatively, record the kind of each type variable as it is
> encountered -- even though such kinds may contain unification kind
> variables.  After visiting all types-or-classes in the recursive group,
> replace any kind variables with their definition, or with a * if there
> is no definition.
> I've currently implement approach i), which requires doing kind
> inference on class methods twice.  Is this the recommended approach?
> 3. Also, is Haskell 98 kind checking the same as Haskell 2010 kind
> checking?
> -BenRI
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