Phil's (Robin Milner's ??) example typechecks in ML.
%Incidentally, there has been a lot of work done on Milner's type
%system extended to allow polymorphic recursion; this system is
%sometimes called ML+. One motivating example -- which I first
%heard from Milner -- is:
%
%
Phil Writes:
|However, for the extended type system that allows polymorphism in
|recursion, this is no longer the case -- my thanks to Lennart
|Augustsson for pointing this out. The counter-example (similar
|to one of Mark's) is:
|
|g :: a - Bool
|g x = g [x]
|
|This function
Joe writes:
| Phil Writes:
|
| | The closest one can come is
| |
| |class G a where
| |g :: a - Bool
| |g x = g [x]
| |
| |instance G Int
| |instance G [Int]
| |instance G [[Int]]
| |...
| |
| |which requires an
I haven't tried it in Haskell,
but it should either be illegal or cause the type-checker to enter an
infinite loop. (Hmmm! Maybe someone should try it ...)
Yes, after all, everyone knows that a language is defined by its
implementation... (:-)
-Paul
In the Report 1.2 on page 98, in the definition of rationalToRealFloat, the
line beginning "where y" has unbalanced parentheses.
-Norman
---
Messing about with signature files fills a much-needed gap in the working
day.
| From: [EMAIL PROTECTED]
|
| I'm puzzled by a detail in the Report, which seems to contradict itself.
|
| On page 13 it says:
|
| The special form -e denotes prefix negation, [...] and is simply
| syntax for negate (e), where negate is as defined in the standard
|
Joe writes:
I forgot to mention that I did try it with hbc:
... transcript of working program omitted ...
That's surprising! I'd be interested to know if `maptry' works,
since in that case it makes sense to run the program, as well
as type it. Cheers, -- P
TRANSLATING MONOMORPHISM TO POLYMORPHISM, AND POLYMORPHIC RECURSION
(An addition to Konstantin and Mark's remarks.)
The great thing about Milner's polymorphism is that it came for free:
adding a function declaration does not make a function more efficient.
This is true for the usual
I'm puzzled by a detail in the Report, which seems to contradict itself.
On page 13 it says:
The special form -e denotes prefix negation, [...] and is simply
syntax for negate (e), where negate is as defined in the standard
prelude.
The standard prelude defines negate
