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`.
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
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