# Re: [isabelle] Subtyping

```(sorry for multiple copies)
Hi there,

```
I would refrain from going into the "implementation" level of the sum type (Abs_sum, Inl_rep) and just use pattern matching.
```
```
The type A + B represents a type that incorporates values of type A /and/ values of type B. Since we are in a strongly typed setting we need a new type for this, which is essentially an algebraic datatype definable by (also the internal construction is different, I think)
```
datatype 'a + 'b = Inl 'a | Inr 'b

```
here the Inl and Inr are the left and right injections. To extract values you can just use pattern matching.
```
fun is_Inl where
is_Inl (Inl _) = True"
| is_Inl (Inr _) = False"

Hope this helps.

cheers

chris

On 08/24/2011 05:32 AM, John Munroe wrote:
```
```
On 24/08/11 11:03, John Munroe wrote:
```
```Hi,

I'm trying to define a function 'f' such that when the input is of
type 'A' it gives true, but when it's of type 'B' it gives false:
Would I need to make A and B subtypes of some parent type, say, 'P',
and that 'f' is of type "P =>  bool"? I can't seem to find much
documentation on subtyping.
```
```
It sounds like you might want to use a sum type, so that the type "'p"
very list, starting here: http://tinyurl.com/3jzx3ny

Then you could have something like "f p = \<exists>  a. p = Inl a" (but I
haven't tested this).
```
```
Thanks for that. I'm not certain if I properly understand "f p =
\<exists>  a. p = Inl a". So, for my example, is the type of f "A + B
=>  bool"? This is what I have:

typedecl A
typedecl B

axiomatization
f :: "A + B =>  bool"
where
ax1 : "f p = (EX a. p = Inl a)"

consts a :: "A + 'b"

lemma "f a = True"
??

I can't seem to find out what Inl actually does. It's defined in
Sum_Type to be "Inl = Abs_sum \<circ>  Inl_Rep", but I can't seem to
find where Abs_sum is defined.

Thanks

John

```
```
Tim
<><
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.10 (GNU/Linux)
Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/

iEYEARECAAYFAk5UZHMACgkQ/cBxZIxl6rmwnACfdWxXr+HPTl/8EOuAg0Y6j1Ai
378An1HvQucFMwMdkCUv4a+PtWuAXzbO
=Z7Ie
-----END PGP SIGNATURE-----

```
```

```
```

```

This archive was generated by a fusion of Pipermail (Mailman edition) and MHonArc.