| | 5 | I propose that we '''make all pattern bindings completely monomorphic (regardless of type signatures)'''. As a result, the expression |
| | 6 | {{{ |
| | 7 | let (p,q) = (\x -> x, True) in (p True, p 'c') |
| | 8 | }}} |
| | 9 | would fail to typecheck, because it requires p to be polymorphic. The program would be rejected even if p had a type signature: |
| | 10 | {{{ |
| | 11 | let { p :: a -> a |
| | 12 | ; (p,q) = (\x -> x, True) |
| | 13 | } in (p True, p 'c') |
| | 14 | }}} |
| | 15 | 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: |
| | 16 | {{{ |
| | 17 | (x,y) = e -- Pattern binding |
| | 18 | [x] = e -- Pattern binding |
| | 19 | (x) = e -- Pattern binding |
| | 20 | x@y = e -- Pattern binding |
| | 21 | ~x = e -- Pattern binding |
| | 22 | |
| | 23 | x = e -- NOT a pattern binding |
| | 24 | }}} |
| | 25 | This rule is simple and easy to remember. |
| | 26 | |
| | 27 | 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. |
| | 28 | |
| | 29 | Another way to say it is this: only function bindings are generalised, where a function binding has the form |
| | 30 | f p1 ... pn = e |
| | 31 | |
| | 32 | where n>=0. The n=0 case is a bare variable. |
| | 33 | |
| | 34 | === Discussion === |
