How to fix "The compiler is unable to type-check this expression in reasonable time" error

Once in a while, you might come across this error:

The compiler is unable to type-check this expression in reasonable time; try breaking up the expression into distinct sub-expressions

There might be several reasons that cause this error. I can show you one way to fix it.

Cause of the problem

The cause of this can vary, but it boils down to quite the same thing.

That is the combination of expressions, e.g., =, +, try, await, is too complex for Swift compiler.

You might be surprised that the code that causes the error isn't that complex to the human eye.

Here is an example of code that can cause the error.

Text(String(index + 1) + " - " + recipe.directions[index])

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How to fix "the compiler is unable to type-check this expression in reasonable time" error

Since the cause can vary and the problematic code is hard to notice. I can only give you a rule of thumb to solve this problem.

First, we need to locate the code that causes the error.

Here is how you find one.

1. Find chunk of code that contain a lot of expression. If you don't know where to start, a code containing many plus operations^[1] (+) might be a good start.
2. Comment out the suspicious chunk of code and rebuild to see whether the error is gone.
3. Repeat step two until you find a code that causes the problem.

Once you find the problematic code, you have to make it simpler.

There are many ways to do it. I can give you two ways to tackle this.

1. Break a long expression into smaller expressions
2. Reduce the amount of expression

Break a long expression into smaller expressions

In this example, we break one expression into three smaller ones.

The total number of expressions is the same (three plus operations), but each expression contains less (one plus operation for each expression).

// From
var longExpression = "a" + "b" + "c" + "d"
    
// To
var temp1 = "a" + "b"
var temp2 = "c" + "d"
var smallerExpression = temp1 + temp2

Reduce the amount of expression

In this example, we reduce the number of plus operations (+) from three to one.

// From
var longExpression = "a" + "b" + "c" + "d"
    
// To
var reduceExpression = "ab" + "cd"

Here is a real-world example. The code Text(String(index + 1) + " - " + recipe.directions[index]) cause the "the compiler is unable to type-check this expression in reasonable time" error.

I can fix it by reducing the amount of expression by converting it to Text("\(index + 1) - \(recipe.directions[index])").

// From
Text(String(index + 1) + " - " + recipe.directions[index])

// To
Text("\(index + 1) - \(recipe.directions[index])")

We reduce the number of plus operations from two to zero.