Why is Integer inferred when the type variable should be ambiguous?

Question

This tiny Haskell program reads two space-separated numbers from console and prints their sum:

main = do
  line <- getLine
  let n1:n2:_ = map read $ words line
  print $ n1 + n2

I'd expect the compilation to fail because of ambiguity: + restricts the type of n1 and n2 to Num typeclass and not to a specific type. However GHC 7.10 successfully compiles it inferring Integer type (replacing + by / changes the inferred type to Double). What is the explanation for that?

I know the answer in the linked question mentions defaulting, but still marking the question as a duplicate of "What is the monomorphism restriction?" is misleading since the monomorphism restriction is not relevant here. — Reid Barton, Nov 03 '15 at 15:45

score 4 · Accepted Answer · answered Nov 03 '15 at 04:13

Haskell has a special case for dealing with the numeric classes (Num, Fractional and a few others) which picks a default type for ambiguous type variables. (See the 2010 report for more details.)

The way it works is by trying a series of numeric types until one satisfies all of the constraints. By default it first tries Integer and then Double, but you can also control this on a per-module basis:

default (Int, Rational)

If you want to turn defaulting off for your module, just don't give it any types:

default ()

Honestly, this is a big hack to make Haskell easier to use as a calculator. But it does make life quite a bit easier since we don't have to explicitly annotate the types of literals in programs like yours, which would also be confusing.

Why is Integer inferred when the type variable should be ambiguous?

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