How can I do string interpolation on smartpy?

Question

I'm trying to provide better error messages. I've tried several things already that don't work such as regular python fstrings, regular string concatenation. I'm tried to using the sp.string function now, but it still doesn't work.

Example:

x_i = (sp.to_int(params.x + i))
y_j = (sp.to_int(params.y + j))
err_msg = "contains pixel(" + sp.string(x_i) + ") that has been bought already"
sp.failwith(err_msg)

Error message when running it:

Error: Error in Scenario
Literal format error: (string(toInt(add(attr(params 48)x 57)(iter i 55)58)58))

Answer 1

It is not possible to cast an int to a string in Michelson.

I recommend you to have static error messages, such as:

class Errors:
  SOME_ERROR = "SOME_ERROR"

# some contract
class Contract(sp.Contract):
  ...

  x_i = (sp.to_int(params.x + I))
  y_j = (sp.to_int(params.y + j))
  sp.failwith(Errors.SOME_ERROR)

SmartPy also gives you the option to define the exception verbosity.

Answer 2

def string_of_nat(params):
    c   = sp.map({x : str(x) for x in range(0, 10)})
    x   = sp.local('x', params)
    res = sp.local('res', [])
    sp.if x.value == 0:
        res.value.push('0')
    sp.while 0 < x.value:
        res.value.push(c[x.value % 10])
        x.value //= 10
    return sp.concat(res.value)

ref: https://smartpy.io/releases/20210118-6f466eee56038df8fda8b2f77819b83398346296/ide?template=stringManipulations.py