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# QSP Type system
Internally, there is only two types in the engine : String and Integer. This is
represented with this C struct :
```C
typedef struct
{
union
{
QSPString Str;
int Num;
} Val;
QSP_TINYINT Type;
} QSPVariant;
```
## Variables and assignation
The type of a variable is determined by the first character in the name:
```C
#define QSP_STRCHAR QSP_FMT("$")
// …
#define QSP_VARBASETYPE(name) (*(name).Str == QSP_STRCHAR[0]) /* QSP_TYPE_STRING | QSP_TYPE_NUMBER */
```
But during the assignation, a conversion is automatically done when the types does not match :
```C
INLINE void qspSetVar(QSPString name, QSPVariant *val, QSP_CHAR op)
{
…
if (QSP_VARBASETYPE(name) != QSP_BASETYPE(val->Type))
{
if (!qspConvertVariantTo(val, QSP_VARBASETYPE(name)))
{
qspSetError(QSP_ERR_TYPEMISMATCH);
return;
}
qspSetVarValueByReference(var, index, val);
}
```
This can raise uncontrolled error with some functions like `INPUT` if you
directly the result into a numeric variable.
## Conversion and comparaison between values
Most of the functions provide automatic conversion between them: a string will
be converted in integer if the other operand is an integer:
- math operation
That’s why `'1' + 1` will give the result `2` and not `'11'`
- comparaison
For example, the source code for the comparaison is the following one:
```C
case qspOpEq:
QSP_NUM(tos) = QSP_TOBOOL(qspAutoConvertCompare(args, args + 1) == 0);
break;
```
When string cannot be converted into a number, the number will be converted in
string, and the comparaison will compare thoses two strings. In the ASCII
table, a text will always be greater than a number (`'a' > 9999` will return
`-1` and `'$' > 0` will return `0`)
## Explicit conversion
### From string to num
`isnum(…)` will tell you if a value is an integer or not:
```
isnum('123') -> -1
isnum('abc') -> 0
```
Using `val(…)` you can convert a string into a integer:
```
val('123') -> 123
val('abc') -> 0 ! No error here
```
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