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6.4.23. Deferring type errors to runtime — Glasgow Haskell Compiler 9.12.2 User's Guide

Since GHC 7.6.1.

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 Wiki page or the original paper.

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. You can use -Wno-deferred-type-errors to suppress these warnings.

This flag implies the -fdefer-typed-holes and -fdefer-out-of-scope-variables flags, which enables this behaviour for Typed Holes and variables. Should you so wish, it is possible to enable -fdefer-type-errors without enabling -fdefer-typed-holes or -fdefer-out-of-scope-variables, by explicitly specifying -fno-defer-typed-holes or -fno-defer-out-of-scope-variables on the command-line after the -fdefer-type-errors flag.

-fdefer-type-errors¶

Since:

7.6

Implies:

-fdefer-typed-holes, -fdefer-out-of-scope-variables

Defer as many type errors as possible until runtime. At compile time you get a warning (instead of an error). At runtime, if you use a value that depends on a type error, you get a runtime error; but you can run any type-correct parts of your code just fine. See also -Wdeferred-type-errors.

-fdefer-typed-holes¶

Since:

7.10

Defer typed holes errors (errors about names with a leading underscore (e.g., “_”, “_foo”, “_bar”)) until runtime. This will turn the errors produced by typed holes into warnings. Using a value that depends on a typed hole produces a runtime error, the same as -fdefer-type-errors (which implies this option). See Typed Holes.

Implied by -fdefer-type-errors. See also -Wtyped-holes.

-fdefer-out-of-scope-variables¶

Since:

8.0

Defer variable out-of-scope errors (errors about names without a leading underscore) until runtime. This will turn variable-out-of-scope errors into warnings. Using a value that depends on an out-of-scope variable produces a runtime error, the same as -fdefer-type-errors (which implies this option). See Typed Holes.

Implied by -fdefer-type-errors. See also -Wdeferred-out-of-scope-variables.

At runtime, whenever a term containing a type error would need to be evaluated, the error is converted into a runtime exception of type TypeError. 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 TypeError.

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

The errors that can be deferred are:

• Out of scope term variables

• Equality constraints; e.g. ord True gives rise to an insoluble equality constraint Char ~ Bool, which can be deferred.

• Type-class and implicit-parameter constraints

All other type errors are reported immediately, and cannot be deferred; for example, an ill-kinded type signature, an instance declaration that is non-terminating or ill-formed, a type-family instance that does not obey the declared injectivity constraints, etc etc.

In a few cases, some constraints cannot be deferred. Specifically:

• Kind errors in a type or kind signature, partial type signatures, or pattern signature. e.g.

  This type signature contains a kind error which cannot be deferred.

• Type equalities under a forall (c.f. #14605).

• Kind errors in a visible type application. e.g.

• Kind errors in a default declaration. e.g.

  default( Double, Int Int )
  

• Errors involving linear types (c.f. #20083). e.g.

• Illegal representation polymorphism, e.g.

  f :: forall rep (a :: TYPE rep). a -> a
  f a = a
  

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