Am 16.09.2016 um 10:54 schrieb Guillaume Bouchard:
Hello,
Observe this example :
Prelude> let x = f 30 in (x,x) (832040,832040) (3.70 secs, 1,649,318,104 bytes)
Prelude> let x = (f 30) :: Int in (x,x) (832040,832040) (1.77 secs, 854,498,640 bytes)
If we force the result type of f 30 to Int, the result is shared as expected.
This is related to the monomorphism restriction https://wiki.haskell.org/Monomorphism_restriction
We can observe the type of (x, x) :
Prelude> let y = (let x = f 30 in (x, x)) Prelude> :t y y :: (Num t1, Num t) => (t, t1)
Both value does not have the same type. Actually `f` is a polymorphic function which can compute the result for any type t as long as t is a `Num`. So we can compute it for `Double`, `Float`, `BigMatriceOf100GigaBytesOfInfinitePrecisionRationalNumbers`. The compiler will only know the final type when needed, later. Actually, the (x, x) tuple is already computed (but `x` are unevaluated) when, by displaying it, you decide to fix `x` as `Integer` for both (the default `Num`).
The setting is a bit different for compiled code, because in this case, the compiler choose the type earlier. For example, in a file `Foo.hs`
a = 10 y = (a, a) :: (Float, Integer)
BlorkBlork.hs:2:9: error: • Couldn't match expected type ‘Integer’ with actual type ‘Float’ • In the expression: a In the expression: (a, a) :: (Float, Integer) In an equation for ‘z’: z = (a, a) :: (Float, Integer) Failed, modules loaded: none.
Because the compiler takes the first encountered type (Float) as the type of a.
However you can keep the polymorphism with a type signature :
a :: Num t => t a = 10
y = (a, a) :: (Float, Integer)
This is one of the rare case where adding a type signature adds polymorphism compared to the infered type.
Will works.
Hope this help.
Wow, this reply was much more than I expected. You are my heroe. I actually started off with the definition let fibs = 0:1:zipWith (+) fibs (tail fibs) and wondered why looking at a certain cell with for example fibs !! 20000 always took some amount of time to evaluation, as I expected the second time it should be already there. Now I can explain (and do something about it, add a type signature). Thanks that starts to become a good day