Ambiguous type variable with HashMap
haskell
Solution
The reason is that ghci uses extended default rules, while the compiler uses (by default) the defaulting rules specified in the language report:
Ambiguities in the class Num are most common, so Haskell provides another way to resolve them—with a default declaration: default (t1 , … , tn) where n ≥ 0, and each ti must be a type for which Num ti holds. In situations where an ambiguous type is discovered, an ambiguous type variable, v, is defaultable if:
- v appears only in constraints of the form C v, where C is a class, and
- at least one of these classes is a numeric class, (that is, Num or a subclass of Num), and
- all of these classes are defined in the Prelude or a standard library
The relevant rule is that per the report defaulting is only done if all involved classes are defined in a standard library, but the `Hashable` constraint on the key involves a class that doesn't meet that requirement.
Thus the compiler rejects it, because it can't resolve the ambiguous type variable arising from the key, while ghci defaults it to `Integer`, since ghci also defaults if other classes are involved.
Problem
I recently starting to play with haskell and I came across a problem while using HashMap that can be illustrated by this toy example: ``` import Data.HashMap as HashMap foo = HashMap.insert 42 53 HashMap.empty ``` Here is the error I get when I load my file in the interpreter or compile it: ``` Prelude List HashMap> :load TypeError.hs [1 of 1] Compiling Main ( TypeError.hs, interpreted ) TypeError.hs:3:22: Ambiguous type variable `k0' in the constraints: (Num k0) arising from the literal `42' at TypeError.hs:3:22-23 (Ord k0) arising from a use of `insert' at TypeError.hs:3:7-20 (Data.Hashable.Hashable k0) arising from a use of `insert' at TypeError.hs:3:7-20 Possible cause: the monomorphism restriction applied to the following: foo :: Map k0 Integer (bound at TypeError.hs:3:1) Probable fix: give these definition(s) an explicit type signature or use -XNoMonomorphismRestriction In the first argument of `insert', namely `42' In the expression: insert 42 53 empty In an equation for `foo': foo = insert 42 53 empty Failed, modules loaded: none. Prelude List HashMap> ``` However if I define the exact same function directly in the interpreter, I get no error: ``` Prelude List HashMap> let foo = HashMap.insert 42 53 HashMap.empty Prelude List HashMap> ``` Does anyone have any clue about this? Thanks.