Philipp N. wrote:
> i'm trying to wrap functions (a -> b -> ... -> z) of any arity to functions
> of type ([String] -> y), where list of strings replaces the typed arguments.
> the problem is, that you cannot distinguish type (x->y) from z, so these
> instances are overlapping.
to which apfelmus replied
> Exactly. What you want to do is most likely impossible,
In fact, that distinction is possible. The following article
How to write an instance for not-a-function
http://okmij.org/ftp/Haskell/typecast.html#is-function-type
specifically describes a method of writing an instance which is
selected only when the type in question is NOT a function. The method
is quite general and has been extensively used (for example, to
implement deep monadic join).
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