This is the dreaded monomorphism restriction, which is turned on by default
in modules but turned off by default in GHCi. Because your function is not
"syntactically" a function (i.e., there are no arguments to the left of the
= sign), and has no type signature, the type checker insists on it having a
monomorphic type. Unless there's a use of it in the module forcing it to a
particular type, the defaulting rules come into play, and a variable with a
`Num` constraint defaults, by default, to `Integer`.
On Thu, Oct 21, 2021, 2:41 PM Matthias Güdemann
Hi cafe,
today while explaining lambda functions I came across this case which I don't really understand.
This is what I get using ghci
λ> :t (\a b -> a + b) (\a b -> a + b) :: Num a => a -> a -> a
but when loading a .hs file with just this function (without signature)
add = \a b -> a + b
I get
λ> :t add add :: Integer -> Integer -> Integer
but I can explicitly define add to have this type:
add :: Num a => a -> a -> a add = \a b -> a + b
Also ghc infers this polymorphic type for example for
add0 a = \b -> a + b
Why is the type of add without a signature inferred to Integer and not Num a?
Best regards Matthias _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.