Version 4 (modified by simonpj@…, 9 years ago) (diff)

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## Make pattern bindings monomorphic

### The Proposal

I propose that we make all pattern bindings completely monomorphic (regardless of type signatures). As a result, the expression

```  let (p,q) = (\x -> x, True) in (p True, p 'c')
```

would fail to typecheck, because it requires p to be polymorphic. The program would be rejected even if p had a type signature:

```  let { p :: a -> a
; (p,q) = (\x -> x, True)
} in (p True, p 'c')
```

We need to be precise about what a "pattern binding" is. Definition: a pattern binding has the form pat=rhs, where pat is not simply a variable. Examples:

```  (x,y) = e    -- Pattern binding
[x]   = e    -- Pattern binding
(x)   = e    -- Pattern binding
x@y   = e    -- Pattern binding
~x    = e    -- Pattern binding

x     = e    -- NOT a pattern binding
```

This rule is simple and easy to remember.

Notice that enclosing the variable in parens, thus (x)=e, makes it a pattern binding, and therfore forces it to be monomorphic. So this is a way to get the monomorphic form of binding that John Hughes has, at times, argued for.

Another way to say it is this: only function bindings are generalised, where a function binding has the form

f p1 ... pn = e

where n>=0. The n=0 case is a bare variable.

### Discussion

At the moment you can say

```	let (f,g) = e in ...
```

and get *polymorphic* values f,g. This causes a good deal of trouble in the implementation (think of what the System F translation looks like), and it's an odd thing to do. Why odd? Well here it's fine:

```	data T = C (forall a. a->a)
h x = let C f = e in ...
```

Constructor C has a polymorphic argument, so we can match e against (C f) and get a polymorphic function. But in the first example, (,) only takes monomorphic arguments, so we have to generalise after selection. To put it another way, even if the "..." is definitely strict in f, we cannot transform to

```	case e of (f,g) -> ...
```