Error messages

Error messages are evolving in ATS2. There are five primary classes of error messages: parsing errors, linker errors, runtime errors, compiler errors, and the most common -- typechecking errors. For error messages known to have been used in the past, especially in ATS1, please see the [old wiki][1].

---

Perhaps the most common error for most people new to ATS will be typechecking errors. For this reason, the typechecking directive $showtype can be immensely helpful. For example, in the following simple example:

val x = 7
val _ = $showtype x

val y = ref<int> 7
val _ = $showtype y
val _ = $showtype !y

we might think !y and x should have the same type. This can be checked by the $showtype output when running the typechecker:

**SHOWTYPE**(/home/myhome/showtype_test.dats: 143(line=12, offs=19) -- 144(line=12, offs=20)): S2Eapp(S2Ecst(g1int_int_t0ype); S2Ecst(int_kind), S2Eintinf(7))
**SHOWTYPE**(/home/myhome/showtype_test.dats: 183(line=15, offs=19) -- 184(line=15, offs=20)): S2Eapp(S2Ecst(ref_vt0ype_type); S2Eapp(S2Ecst(g0int_t0ype); S2Ecst(int_kind)))
**SHOWTYPE**(/home/myhome/showtype_test.dats: 203(line=16, offs=19) -- 205(line=16, offs=21)): S2Eapp(S2Ecst(g0int_t0ype); S2Ecst(int_kind))

More concisely,

type of x:  S2Eapp(S2Ecst(g1int_int_t0ype); S2Ecst(int_kind), S2Eintinf(7))
type of y:  S2Eapp(S2Ecst(ref_vt0ype_type); S2Eapp(S2Ecst(g0int_t0ype); S2Ecst(int_kind)))
type of !y: S2Eapp(S2Ecst(g0int_t0ype); S2Ecst(int_kind))

So we can see that the assumption proved incorrect. Let's try again with a new val, z:

val z = ref<int(7)> 7
val _ = $showtype !z

type of !z: S2Eapp(S2Ecst(g1int_int_t0ype); S2Ecst(int_kind), S2Eintinf(7))
```

which shows that !z is now the same type as x. This reflects that by default, integers have types parameterized by their values in ATS, as was the case for x.

## See also

[[Internal Types|Internal-types]], to understand the `S2Eapp`, `S2Ecst`, etc, which often appears in error messages.

[1]: https://sourceforge.net/p/ats-lang/wiki/Error%20messages/