7.13. Deferring type errors to runtime
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## 7.13. Deferring type errors to runtime

While developing, sometimes it is desirable to allow compilation to succeed even if there are type errors in the code. Consider the following case:

    module Main where

    a :: Int
    a = 'a'

    main = print "b"

Even though `a` is ill-typed, it is not used in the end, so if all that we're interested in is `main` it can be useful to be able to ignore the problems in `a`.

For more motivation and details please refer to the HaskellWiki page or the original paper.

### 7.13.1. Enabling deferring of type errors

The flag `-fdefer-type-errors` controls whether type errors are deferred to runtime. Type errors will still be emitted as warnings, but will not prevent compilation.

At runtime, whenever a term containing a type error would need to be evaluated, the error is converted into a runtime exception. Note that type errors are deferred as much as possible during runtime, but invalid coercions are never performed, even when they would ultimately result in a value of the correct type. For example, given the following code:

    x :: Int
    x = 0

    y :: Char
    y = x

    z :: Int
    z = y

evaluating `z` will result in a runtime type error.

### 7.13.2. Deferred type errors in GHCi

The flag `-fdefer-type-errors` works in GHCi as well, with one exception: for "naked" expressions typed at the prompt, type errors don't get delayed, so for example:

    Prelude> fst (True, 1 == 'a')

    <interactive>:2:12:
        No instance for (Num Char) arising from the literal `1'
        Possible fix: add an instance declaration for (Num Char)
        In the first argument of `(==)', namely `1'
        In the expression: 1 == 'a'
        In the first argument of `fst', namely `(True, 1 == 'a')'

Otherwise, in the common case of a simple type error such as typing `reverse True` at the prompt, you would get a warning and then an immediately-following type error when the expression is evaluated.

This exception doesn't apply to statements, as the following example demonstrates:

    Prelude> let x = (True, 1 == 'a')

    <interactive>:3:16: Warning:
        No instance for (Num Char) arising from the literal `1'
        Possible fix: add an instance declaration for (Num Char)
        In the first argument of `(==)', namely `1'
        In the expression: 1 == 'a'
        In the expression: (True, 1 == 'a')
    Prelude> fst x
    True

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