[docs] Remove wrong migration instruction

[oliviertassinari]

A continuation of the discussion on bc45825#r42461020. From what I understand, our best option is to no support the previous case. Happy to consider alternatives.

[mui-pr-bot]

Details of bundle changes

Generated by 🚫 dangerJS against 4259c81

[mbrookes]

Should we also add a note to the customization/spacing page "a function" section that the function must return a string?

[oliviertassinari]

Why does it need to return a string? If a number is returned it will assume px. Maybe it should mention the default unit instead?

[oliviertassinari]

@mbrookes There is another case that is unclear, what's the expected behavior when you provide an array for the spacing? The issue with the typing is to get

https://github.com/mui-org/material-ui/blob/af9f26f333410c0f6e54437370ad384a3337be18/packages/material-ui/src/styles/createSpacing.d.ts#L6

be a number or a string, depending on the theme option input. I don't see how to make it happen.

[eps1lon]

be a number or a string, depending on the theme option input. I don't see how to make it happen.

Could you add a runtime test for that behavior? It's not clear to me how spacing would ever return a number.

[oliviertassinari]

@eps1lon Spacing used to return a number or a string, it's no longer possible, it can only be a string now. We have been discussing with Matt how to bring this behavior back, we couldn't see a simple approach with the types.

[eps1lon]

We have been discussing with Matt how to bring this behavior back, we couldn't see a simple approach with the types.

Be aware that any code that's hard to express with types is also hard to explain to humans. What problem would overloading solve here?

[oliviertassinari]

Be aware that any code that's hard to express with types is also hard to explain to humans.

@eps1lon Agree, this is why I'm leaning toward giving up on a way to have v4 behavior, going for all-in in the new string approach.

What problem would overloading solve here?

Being able to have:

const theme = createMuiTheme({
  spacing: factor => 8 * factor,
});

theme.spacing(2); // = 8

right now we have:

const theme = createMuiTheme({
  spacing: factor => 8 * factor,
});

theme.spacing(2); // = 8px

[eps1lon]

I understand the different return types. I'm asking what you'd do with this and why this problem should only be solved for a single argument.

[oliviertassinari]

I understand the different return types. I'm asking what you'd do with this and why this problem should only be solved for a single argument.

Oh, sorry, you were on the second order thinking. I don't know, also a reason why my proposal: so we get feedback and compeling use cases from developers 😁.