Hi,
Christopher Howard wrote:
instance Category ...
The Category class is rather restricted:
Restriction 1:
You cannot choose what the objects of the category are. Instead, the
objects are always "all Haskell types". You cannot choose anything at
all about the objects.
Restriction 2:
You cannot freely choose what the morphisms of the category are.
Instead, the morphisms are always Haskell values. (To some degree, you
can choose *which* values you want to use).
These restrictions disallow many categories. For example, the category
where the objects are natural numbers and there is a morphism from m to
n if m is greater than or equal to n cannot be expressed directly:
Natural numbers are not Haskell types; and "is bigger than or equal to"
is not a Haskell value.
Tillmann
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