Add "suffix" operand

[gidonkatten]

Problem

The TEAL opcode extract has the syntax:

extract S L where S: start position, L: length
A range of bytes from A starting at S up to but not including S+L. 
If L is 0, then extract to the end of the string. If S or S+L is larger than the array length, the program fails

The behavior of L=0 isn't immediately obvious.

Solution

Introduce a new op named suffix with same behavior as https://pyteal.readthedocs.io/en/stable/api.html#pyteal.Suffix.

Instead of writting op.extract(..., 7, 0), you would write op.suffix(..., 7)

Proposal

The new op.suffix(..., 7) would compile to TEAL extract 7 0

[achidlow]

Thanks for your suggestion.

Normally, we would recommend that this sort of function be implemented in a user or third party library package.

However the complication here is that there is a difference of behaviour between extract with stack args vs immediates - when a 0 is passed as the length via stack args, it means the result is an empty byte-slice. When it is passed as an immediate (which is only possible when it's a compile time constant and <= 255), the result is as you indicated.

This difference of behaviour is unfortunate on multiple levels - it prevents a simple user function being written here, because that code has no way of altering the resulting TEAL based on whether the value is a compile time constant or not (at least at present anyway). It also creates problems during optimisation.

We certainly intend to do something in this space. The solution needs to take into account existing code (ideally, we would go back in time and expose extract-with-immediate and extract-with-stack-args as different functions altogether in algopy.op), and ideally it would also remove the optimisation barrier around the difference in behaviour.

As a minor point, algopy.op contains a subset of allowed raw TEAL operations, so any higher-level function wouldn't belong there.