# GHC/Type system

### From HaskellWiki

Type system extensions in GHC

GHC comes with a rather large collection of type-system extensions (beyond Haskell 98). They are all documented in the user manual, but this page is a place to record observations, notes, and suggestions on them.

## Contents |

## 1 Type signatures and ambiguity

It's quite common for people to write a function definition without a type signature, load it into GHCi, use `:t` to see what type it has, and then cut-and-paste that type into the source code as a type signature. Usually this works fine, but alas not always. Perhaps this is a deficiency in GHC, but here's one way it can happen:

class C a b where foo :: a -> b konst :: a -> Bool konst x = True f :: (C a b) => a -> Bool f x = konst (foo x)

If you compile this code, you'll get this error:

Foo1.hs:12:13: Could not deduce (C a b1) from the context (C a b) arising from use of `foo' at Foo1.hs:12:13-17 Possible fix: add (C a b1) to the type signature(s) for `f' In the first argument of `konst', namely `(foo x)' In the expression: konst (foo x) In the definition of `f': f x = konst (foo x)

What's going on? GHC knows, from the type signature that `x::a`. Then applying `foo` means GHC must pick a return type for `foo`, say `b1`, and generates the type constraint `(C a b1)`. The function `konst` just discards its argument, *so nothing further is known abouut b1*.

Now GHC finished typechecking the right hand side of `f`, so next it checks that the constraints *needed* in the RHS, namely `(C a b1)`, can be satisfied from the constraints *provided* by the type signature, namely `(C a b)`. Alas there is nothing to tell GHC that `b` and `b1` should be identified together; hence the complaint. (Probably you meant to put a functional dependency in the class declaration, thus

class C a b | a->b where ...

but you didn't.)

The surprise is that if you comment out the type signature for `f`, the module will load fine into GHCi! Furthermore `:t` will report a type for `f` that is exactly the same as the type signature that was rejected!

`x`, say

`x::a`. Then applying

`foo`means GHC must pick a return type for

`foo`, say

`b`, and generates the type constraint

`(C a b)`. The function

`konst`just discards its argument, so nothing further is known abouut

`b`. Finally, GHC gathers up all the constraints arising from the right hand side, namely

`(C a b)`, and puts them into the inferred type of

`f`. So GHC ends up saying that

This is probably a very stupid type. Suppose you called `f` thus: `(f 'a')`. Then you'd get a constraint `(C Char b)` where nothing is known about `b`. If the instances of `C` constrain both type parameters, you'd be in trouble:

instance C Char Bool where ...

The call gives a `(C Char b)` constraint, with absolutely no way to fix `b` to be `Bool`, or indeed anything else. We're back to very much the same situation as before; it's just that the error is deferred until we call `f`, rather than when we define it.

(However, notice that the call `(f 'a')` would be OK if there was an instance like:

instance C Char w where ...

Now the constraint `(C Char b)` matches the instance declaration, even though we know nothing about `b`.)

This behaviour isn't ideal. It really only arises in programs that are ambiguous anyway (that is, they could never really work), but it is undoubtedly confusing. But I don't know an easy way to improve it. Yet, anyway.

## 2 Overlapping instances

Here an interesting message about the interaction of existential types and overlapping instances.

## 3 Indexed data types and indexed newtypes

Indexed data types (including associated data types) are a very recent addition to GHC's type system extensions that is not yet included in the user manual. To use the extension, you need to obtain a version of GHC from its source repository.

## 4 Stand-alone deriving clauses

Bjorn Bringert has recently implemented "stand-alone deriving" declarations.