# Safely Catching A List By Its Tail

Previously we saw some examples of how refinements could be used to encode invariants about basic `Int` values. Today, let's see how refinements allow us specify and verify structural invariants about recursive data types like lists. In particular, we will learn about at a new mechanism called a `measure`, use measures to describe the length of a list, and use the resulting refinement types to obtain compile-time assurances that canonical list manipulating operations like `head`, `tail`, `foldl1` and (incomplete) pattern matches will not blow up at run-time.

```26: module ListLengths where
27:
28: import Prelude hiding (length, map, filter, head, tail, foldl1)
30: import qualified Data.HashMap.Strict as M
31: import Data.Hashable
```

## Measuring the Length of a List¶

To begin, we need some instrument by which to measure the length of a list. To this end, let's introduce a new mechanism called measures which define auxiliary (or so-called ghost) properties of data values. These properties are useful for specification and verification, but don't actually exist at run-time. That is, measures will appear in specifications but never inside code.

Let's reuse this mechanism, this time, providing a definition for the measure

```48: measure len :: forall a. [a] -> GHC.Types.Int
49: len ([])     = 0
50: len (y:ys)   = 1 + (len ys)
```

The description of `len` above should be quite easy to follow. Underneath the covers, LiquidHaskell transforms the above description into refined versions of the types for the constructors `(:)` and `[]`, Something like

```57: data [a] where
58:   []  :: forall a. {v: [a] | (len v) = 0 }
59:   (:) :: forall a. y:a -> ys:[a] -> {v: [a] | (len v) = 1 + (len ys) }
```

To appreciate this, note that we can now check that

```65: {-@ xs :: {v:[String] | (len v) = 1 } @-}
66: {VV : [[(GHC.Types.Char)]] | (len([VV]) = 1)}xs = {VV : [(GHC.Types.Char)] | (len([VV]) >= 0)}"dog" y:{VV : [(GHC.Types.Char)] | (len([VV]) >= 0)}
-> ys:[{VV : [(GHC.Types.Char)] | (len([VV]) >= 0)}]
-> {VV : [{VV : [(GHC.Types.Char)] | (len([VV]) >= 0)}] | (len([VV]) = (1 + len([ys])))}: {VV : [{VV : [{VV : (GHC.Types.Char) | false}] | false}] | (len([VV]) = 0),
(len([VV]) >= 0)}[]
67:
68: {-@ ys :: {v:[String] | (len v) = 2 } @-}
69: {VV : [[(GHC.Types.Char)]] | (len([VV]) = 2)}ys = {VV : [{VV : [(GHC.Types.Char)] | (len([VV]) >= 0)}] | (len([VV]) >= 0)}[{VV : [(GHC.Types.Char)] | (len([VV]) >= 0)}"cat", {VV : [(GHC.Types.Char)] | (len([VV]) >= 0)}"dog"]
70:
71: {-@ zs :: {v:[String] | (len v) = 3 } @-}
72: {VV : [[(GHC.Types.Char)]] | (len([VV]) = 3)}zs = {VV : [(GHC.Types.Char)] | (len([VV]) >= 0)}"hippo" y:[(GHC.Types.Char)]
-> ys:[[(GHC.Types.Char)]]
-> {VV : [[(GHC.Types.Char)]] | (len([VV]) = (1 + len([ys])))}: {VV : [[(GHC.Types.Char)]] | (VV = ys),
(len([VV]) = 2),
(len([VV]) >= 0)}ys
```

Dually, when we de-construct the lists, LiquidHaskell is able to relate the type of the outer list with its constituents. For example,

```79: {-@ zs' :: {v:[String] | (len v) = 2 } @-}
80: {VV : [[(GHC.Types.Char)]] | (len([VV]) = 2)}zs' = case {VV : [[(GHC.Types.Char)]] | (VV = zs),
(len([VV]) = 3),
(len([VV]) >= 0)}zs of
81:         h : t -> {VV : [[(GHC.Types.Char)]] | (VV = t),(len([VV]) >= 0)}t
```

Here, from the use of the `:` in the pattern, LiquidHaskell can determine that `(len zs) = 1 + (len t)`; by combining this fact with the nugget that `(len zs) = 3` LiquidHaskell concludes that `t`, and hence, `zs'` contains two elements.

Let's flex our new vocabulary by uttering types that describe the behavior of the usual list functions.

First up: a version of the standard `length` function, slightly simplified for exposition.

```99: {-@ length :: xs:[a] -> {v: Int | v = (len xs)} @-}
100: length :: [a] -> Int
101: xs:[a] -> {VV : (GHC.Types.Int) | (VV = len([xs]))}length []     = x:(GHC.Prim.Int#) -> {VV : (GHC.Types.Int) | (VV = (x  :  int))}0
102: length (x:xs) = {VV : (GHC.Types.Int) | (VV = (1  :  int))}1 x:(GHC.Types.Int)
-> y:(GHC.Types.Int) -> {VV : (GHC.Types.Int) | (VV = (x + y))}+ xs:[a] -> {VV : (GHC.Types.Int) | (VV = len([xs]))}length {VV : [a] | (VV = xs),(len([VV]) >= 0)}xs
```

Note: Recall that `measure` values don't actually exist at run-time. However, functions like `length` are useful in that they allow us to effectively pull or materialize the ghost values from the refinement world into the actual code world (since the value returned by `length` is logically equal to the `len` of the input list.)

Similarly, we can speak and have confirmed, the types for the usual list-manipulators like

```115: {-@ map      :: (a -> b) -> xs:[a] -> {v:[b] | (len v) = (len xs)} @-}
116: (a -> b)
-> x1:[a] -> {VV : [b] | (len([VV]) = len([x1])),(len([VV]) >= 0)}map _ []     = {VV : [{VV : a | false}] | (len([VV]) = 0)}[]
117: map f (x:xs) = (a -> bf {VV : a | (VV = x)}x) y:a -> ys:[a] -> {VV : [a] | (len([VV]) = (1 + len([ys])))}: ((a -> b)
-> x1:[a] -> {VV : [b] | (len([VV]) = len([x1])),(len([VV]) >= 0)}map a -> bf {VV : [a] | (VV = xs),(len([VV]) >= 0)}xs)
```

and

```123: {-@ filter :: (a -> Bool) -> xs:[a] -> {v:[a] | (len v) <= (len xs)} @-}
124: (a -> (GHC.Types.Bool))
-> x1:[a] -> {VV : [a] | (len([VV]) >= 0),(len([VV]) <= len([x1]))}filter _ []     = {VV : [{VV : a | false}] | (len([VV]) = 0)}[]
125: filter f (x:xs)
126:   | a -> (GHC.Types.Bool)f {VV : a | (VV = x)}x         = {VV : a | (VV = x)}x y:a -> ys:[a] -> {VV : [a] | (len([VV]) = (1 + len([ys])))}: (a -> (GHC.Types.Bool))
-> x1:[a] -> {VV : [a] | (len([VV]) >= 0),(len([VV]) <= len([x1]))}filter a -> (GHC.Types.Bool)f {VV : [a] | (VV = xs),(len([VV]) >= 0)}xs
127:   | otherwise   = (a -> (GHC.Types.Bool))
-> x1:[a] -> {VV : [a] | (len([VV]) >= 0),(len([VV]) <= len([x1]))}filter a -> (GHC.Types.Bool)f {VV : [a] | (VV = xs),(len([VV]) >= 0)}xs
```

and, since doubtless you are wondering,

```133: {-@ append :: xs:[a] -> ys:[a] -> {v:[a] | (len v) = (len xs) + (len ys)} @-}
134: x4:[a]
-> x3:[a]
-> {VV : [a] | (len([VV]) = (len([x4]) + len([x3]))),
(len([VV]) = (len([x3]) + len([x4]))),
(len([VV]) >= 0)}append [] [a]ys     = {VV : [a] | (VV = ys),(len([VV]) >= 0)}ys
135: append (x:xs) ys = {VV : a | (VV = x)}x y:a -> ys:[a] -> {VV : [a] | (len([VV]) = (1 + len([ys])))}: x4:[a]
-> x3:[a]
-> {VV : [a] | (len([VV]) = (len([x4]) + len([x3]))),
(len([VV]) = (len([x3]) + len([x4]))),
(len([VV]) >= 0)}append {VV : [a] | (VV = xs),(len([VV]) >= 0)}xs {VV : [a] | (VV = ys),(len([VV]) >= 0)}ys
```

We will return to the above at some later date. Right now, let's look at some interesting programs that LiquidHaskell can prove safe, by reasoning about the size of various lists.

## Example 1: Safely Catching A List by Its Tail (or Head)¶

Now, let's see how we can use these new incantations to banish, forever, certain irritating kinds of errors. Recall how we always summon functions like `head` and `tail` with a degree of trepidation, unsure whether the arguments are empty, which will awaken certain beasts

```150: Prelude> head []
151: *** Exception: Prelude.head: empty list
```

LiquidHaskell allows us to use these functions with confidence and surety! First off, let's define a predicate alias that describes non-empty lists:

```159: {-@ predicate NonNull X = ((len X) > 0) @-}
```

Now, we can type the potentially dangerous `head` as:

```165: {-@ head   :: {v:[a] | (NonNull v)} -> a @-}
166: {VV : [a] | (len([VV]) > 0)} -> ahead (x:_) = {VV : a | (VV = x)}x
167: head []    = {VV : [(GHC.Types.Char)] | false} -> {VV : a | false}liquidError {VV : [(GHC.Types.Char)] | (len([VV]) >= 0)}"Fear not! 'twill ne'er come to pass"
```

As with the case of divide-by-zero, LiquidHaskell deduces that the second equation is dead code since the precondition (input) type states that the length of the input is strictly positive, which precludes the case where the parameter is `[]`.

Similarly, we can write

```178: {-@ tail :: {v:[a] | (NonNull v)} -> [a] @-}
179: {VV : [a] | (len([VV]) > 0)} -> [a]tail (_:xs) = {VV : [a] | (VV = xs),(len([VV]) >= 0)}xs
180: tail []     = {VV : [(GHC.Types.Char)] | false}
-> {VV : [{VV : a | false}] | false}liquidError {VV : [(GHC.Types.Char)] | (len([VV]) >= 0)}"Relaxeth! this too shall ne'er be"
```

Once again, LiquidHaskell will use the precondition to verify that the `liquidError` is never invoked.

Let's use the above to write a function that eliminates stuttering, that is which converts `"ssstringssss liiiiiike thisss"` to `"strings like this"`.

```190: {-@ eliminateStutter :: (Eq a) => [a] -> [a] @-}
191: (GHC.Classes.Eq a) -> [a] -> [a]eliminateStutter [a]xs = ({VV : [a] | (len([VV]) > 0)} -> a)
-> xs:[{VV : [a] | (len([VV]) > 0)}]
-> {VV : [a] | (len([VV]) = len([xs]))}map {VV : [a] | (len([VV]) > 0)} -> ahead ({VV : [{VV : [a] | (len([VV]) > 0)}] | ((len([xs]) > 0) => (len([VV]) > 0)),
(len([VV]) >= 0)}
-> {VV : [a] | (len([VV]) >= 0)})
-> {VV : [{VV : [a] | (len([VV]) > 0)}] | ((len([xs]) > 0) => (len([VV]) > 0)),
(len([VV]) >= 0)}
-> {VV : [a] | (len([VV]) >= 0)}\$ x1:{VV : [a] | (len([VV]) >= 0)}
-> {VV : [{VV : [a] | (len([VV]) > 0)}] | ((len([x1]) > 0) => (len([VV]) > 0)),
(len([VV]) >= 0)}groupEq {VV : [a] | (VV = xs),(len([VV]) >= 0)}xs
```

The heavy lifting is done by `groupEq`

```197: x1:{VV : [a] | (len([VV]) >= 0)}
-> {VV : [{VV : [a] | (len([VV]) > 0)}] | ((len([x1]) > 0) => (len([VV]) > 0)),
(len([VV]) >= 0)}groupEq []     = {VV : [{VV : [{VV : a | false}] | false}] | (len([VV]) = 0)}[]
198: groupEq (x:xs) = ({VV : a | (VV = x)}xy:a -> ys:[a] -> {VV : [a] | (len([VV]) = (1 + len([ys])))}:{VV : [a] | (VV = ys),
(VV = ys),
(len([VV]) = len([ys])),
(len([VV]) >= 0),
(len([VV]) <= len([ys]))}ys) y:{VV : [a] | (len([VV]) > 0)}
-> ys:[{VV : [a] | (len([VV]) > 0)}]
-> {VV : [{VV : [a] | (len([VV]) > 0)}] | (len([VV]) = (1 + len([ys])))}: x1:{VV : [a] | (len([VV]) >= 0)}
-> {VV : [{VV : [a] | (len([VV]) > 0)}] | ((len([x1]) > 0) => (len([VV]) > 0)),
(len([VV]) >= 0)}groupEq {VV : [a] | (VV = zs),
(VV = zs),
(len([VV]) = len([zs])),
(len([VV]) >= 0),
(len([VV]) <= len([zs]))}zs
199:                  where ({VV : [a] | (VV = ys),
(len([VV]) = len([ys])),
(len([VV]) >= 0),
(len([VV]) <= len([ys]))}ys,{VV : [a] | (VV = zs),
(len([VV]) = len([zs])),
(len([VV]) >= 0),
(len([VV]) <= len([zs]))}zs) = (a -> (GHC.Types.Bool)) -> [a] -> ([a] , [a])span ({VV : a | (VV = x)}x x:a
-> y:a -> {VV : (GHC.Types.Bool) | ((? Prop([VV])) <=> (x = y))}==) {VV : [a] | (VV = xs),(len([VV]) >= 0)}xs
```

which gathers consecutive equal elements in the list into a single list. By using the fact that each element in the output returned by `groupEq` is in fact of the form `x:ys`, LiquidHaskell infers that `groupEq` returns a list of non-empty lists. (Hover over the `groupEq` identifier in the code above to see this.) Next, by automatically instantiating the type parameter for the `map` in `eliminateStutter` to `(len v) > 0` LiquidHaskell deduces `head` is only called on non-empty lists, thereby verifying the safety of `eliminateStutter`. (Hover your mouse over `map` above to see the instantiated type for it!)

## Example 2: Risers¶

The above examples of `head` and `tail` are simple, but non-empty lists pop up in many places, and it is rather convenient to have the type system track non-emptiness without having to make up special types. Let's look at a more interesting example, popularized by Neil Mitchell which is a key step in an efficient sorting procedure, which we may return to in the future when we discuss sorting algorithms.

```224: risers           :: (Ord a) => [a] -> [[a]]
225: (GHC.Classes.Ord a)
-> zs:[a] -> {VV : [[a]] | ((len([zs]) > 0) => (len([VV]) > 0))}risers []        = {VV : [{VV : [{VV : a | false}] | false}] | (len([VV]) = 0)}[]
226: risers [x]       = {VV : [{VV : [{VV : a | false}] | false}] | (len([VV]) = 0),
(len([VV]) >= 0)}[{VV : [{VV : a | (VV = x)}] | (len([VV]) >= 0)}[{VV : a | (VV = x)}x]]
227: risers (x:y:etc) = {VV : [{VV : a | false}] | (len([VV]) = 0),(len([VV]) >= 0)}if {VV : a | (VV = x)}x x:a
-> y:a -> {VV : (GHC.Types.Bool) | ((? Prop([VV])) <=> (x <= y))}<= {VV : a | (VV = y)}y then ({VV : a | (VV = x)}xy:a -> ys:[a] -> {VV : [a] | (len([VV]) = (1 + len([ys])))}:{VV : [a] | (VV = s),
(VV = s),
(len([VV]) = len([s])),
(len([VV]) >= 0),
(len([VV]) <= len([s]))}s)y:[a] -> ys:[[a]] -> {VV : [[a]] | (len([VV]) = (1 + len([ys])))}:{VV : [[a]] | (VV = ss),
(VV = ss),
(len([VV]) = len([ss])),
(len([VV]) >= 0),
(len([VV]) <= len([ss]))}ss else {VV : [{VV : a | (VV = x),(VV > y)}] | (len([VV]) >= 0)}[{VV : a | (VV = x)}x]y:[a] -> ys:[[a]] -> {VV : [[a]] | (len([VV]) = (1 + len([ys])))}:({VV : [a] | (VV = s),
(VV = s),
(len([VV]) = len([s])),
(len([VV]) >= 0),
(len([VV]) <= len([s]))}sy:[a] -> ys:[[a]] -> {VV : [[a]] | (len([VV]) = (1 + len([ys])))}:{VV : [[a]] | (VV = ss),
(VV = ss),
(len([VV]) = len([ss])),
(len([VV]) >= 0),
(len([VV]) <= len([ss]))}ss)
228:     where
229:       ({VV : [a] | (VV = s),
(len([VV]) = len([s])),
(len([VV]) >= 0),
(len([VV]) <= len([s]))}s, {VV : [[a]] | (VV = ss),
(len([VV]) = len([ss])),
(len([VV]) >= 0),
(len([VV]) <= len([ss]))}ss)    = {VV : [[a]] | (len([VV]) > 0)}
-> ([a] , {VV : [[a]] | (len([VV]) >= 0)})safeSplit ({VV : [[a]] | ((len([etc]) > 0) => (len([VV]) > 0)),
(len([VV]) > 0)}
-> ([a] , {VV : [[a]] | (len([VV]) >= 0)}))
-> {VV : [[a]] | ((len([etc]) > 0) => (len([VV]) > 0)),
(len([VV]) > 0)}
-> ([a] , {VV : [[a]] | (len([VV]) >= 0)})\$ zs:[a] -> {VV : [[a]] | ((len([zs]) > 0) => (len([VV]) > 0))}risers ({VV : a | (VV = y)}yy:a -> ys:[a] -> {VV : [a] | (len([VV]) = (1 + len([ys])))}:{VV : [a] | (VV = etc),(len([VV]) >= 0)}etc)
```

The bit that should cause some worry is `safeSplit` which simply returns the `head` and `tail` of its input, if they exist, and otherwise, crashes and burns

```237: {VV : [a] | (len([VV]) > 0)} -> (a , {VV : [a] | (len([VV]) >= 0)})safeSplit (x:xs)  = x1:a -> b -> (a , b)({VV : a | (VV = x)}x, {VV : [a] | (VV = xs),(len([VV]) >= 0)}xs)
238: safeSplit _       = {VV : [(GHC.Types.Char)] | false}
-> {VV : ({VV : a | false} , {VV : [{VV : a | false}] | false}) | false}liquidError {VV : [(GHC.Types.Char)] | (len([VV]) >= 0)}"don't worry, be happy"
```

How can we verify that `safeSplit` will not crash ?

The matter is complicated by the fact that since `risers` does sometimes return an empty list, we cannot blithely specify that its output type has a `NonNull` refinement.

Once again, logic rides to our rescue!

The crucial property upon which the safety of `risers` rests is that when the input list is non-empty, the output list returned by risers is also non-empty. It is quite easy to clue LiquidHaskell in on this, namely through a type specification:

```255: {-@ risers :: (Ord a)
256:            => zs:[a]
257:            -> {v: [[a]] | ((NonNull zs) => (NonNull v)) } @-}
```

Note how we relate the output's non-emptiness to the input's non-emptiness,through the (dependent) refinement type. With this specification in place, LiquidHaskell is pleased to verify `risers` (i.e. the call to `safeSplit`).

## Example 3: MapReduce¶

Here's a longer example that illustrates this: a toy map-reduce implementation.

First, let's write a function `keyMap` that expands a list of inputs into a list of key-value pairs:

```274: keyMap :: (a -> [(k, v)]) -> [a] -> [(k, v)]
275: (a -> {VV : [(b , c)] | (len([VV]) >= 0)}) -> [a] -> [(b , c)]keyMap a -> {VV : [(b , c)] | (len([VV]) >= 0)}f [a]xs = (a -> [(b , c)]) -> [a] -> [(b , c)]concatMap a -> {VV : [(b , c)] | (len([VV]) >= 0)}f {VV : [a] | (VV = xs),(len([VV]) >= 0)}xs
```

Next, let's write a function `group` that gathers the key-value pairs into a `Map` from keys to the lists of values with that same key.

```282: (GHC.Classes.Eq a)
-> (Data.Hashable.Class.Hashable a)
-> [(a , b)]
-> (Data.HashMap.Base.HashMap a {VV : [b] | (len([VV]) > 0)})group [(a , b)]kvs = ((a , b)
-> (Data.HashMap.Base.HashMap a {VV : [b] | (len([VV]) > 0)})
-> (Data.HashMap.Base.HashMap a {VV : [b] | (len([VV]) > 0)}))
-> (Data.HashMap.Base.HashMap a {VV : [b] | (len([VV]) > 0)})
-> [(a , b)]
-> (Data.HashMap.Base.HashMap a {VV : [b] | (len([VV]) > 0)})foldr ((a , b)
-> (Data.HashMap.Base.HashMap a {VV : [b] | (len([VV]) > 0)})
-> (Data.HashMap.Base.HashMap a {VV : [b] | (len([VV]) > 0)})\(k, v) (Data.HashMap.Base.HashMap a {VV : [b] | (len([VV]) > 0)})m -> a
-> b
-> (Data.HashMap.Base.HashMap a {VV : [b] | (len([VV]) > 0)})
-> (Data.HashMap.Base.HashMap a {VV : [b] | (len([VV]) > 0)})inserts {VV : a | (VV = k)}k {VV : a | (VV = v)}v {VV : (Data.HashMap.Base.HashMap a {VV : [b] | (len([VV]) > 0)}) | (VV = m)}m) (Data.HashMap.Base.HashMap {VV : a | false} {VV : [{VV : b | false}] | false})M.empty {VV : [(a , b)] | (VV = kvs),(len([VV]) >= 0)}kvs
```

The function `inserts` simply adds the new value `v` to the list of previously known values `lookupDefault [] k m` for the key `k`.

```289: (GHC.Classes.Eq a)
-> (Data.Hashable.Class.Hashable a)
-> a
-> b
-> (Data.HashMap.Base.HashMap a {VV : [b] | (len([VV]) > 0)})
-> (Data.HashMap.Base.HashMap a {VV : [b] | (len([VV]) > 0)})inserts ak av (Data.HashMap.Base.HashMap a {VV : [b] | (len([VV]) > 0)})m = a
-> {VV : [b] | (len([VV]) > 0)}
-> (Data.HashMap.Base.HashMap a {VV : [b] | (len([VV]) > 0)})
-> (Data.HashMap.Base.HashMap a {VV : [b] | (len([VV]) > 0)})M.insert {VV : a | (VV = k)}k ({VV : a | (VV = v)}v y:a -> ys:[a] -> {VV : [a] | (len([VV]) = (1 + len([ys])))}: {VV : [a] | (VV = vs),(len([VV]) >= 0)}vs) {VV : (Data.HashMap.Base.HashMap a {VV : [b] | (len([VV]) > 0)}) | (VV = m)}m
290:   where {VV : [a] | (len([VV]) >= 0)}vs    = {VV : [a] | (len([VV]) >= 0)}
-> b
-> (Data.HashMap.Base.HashMap b {VV : [a] | (len([VV]) >= 0)})
-> {VV : [a] | (len([VV]) >= 0)}M.lookupDefault {VV : [{VV : a | false}] | (len([VV]) = 0),(len([VV]) >= 0)}[] {VV : a | (VV = k)}k {VV : (Data.HashMap.Base.HashMap a {VV : [b] | (len([VV]) > 0)}) | (VV = m)}m
```

Finally, a function that reduces the list of values for a given key in the table to a single value:

```297: reduce    :: (v -> v -> v) -> M.HashMap k [v] -> M.HashMap k v
298: (x2:a -> x3:a -> {VV : a | (VV > x2),(VV > x3)})
-> (Data.HashMap.Base.HashMap b {VV : [a] | (len([VV]) > 0)})
-> (Data.HashMap.Base.HashMap b a)reduce x1:a -> x2:a -> {VV : a | (VV > x1),(VV > x2)}op = ({VV : [a] | (len([VV]) > 0)} -> a)
-> (Data.HashMap.Base.HashMap b {VV : [a] | (len([VV]) > 0)})
-> (Data.HashMap.Base.HashMap b a)M.map ((a -> a -> a) -> {VV : [a] | (len([VV]) > 0)} -> afoldl1 x1:a -> x2:a -> {VV : a | (VV > x1),(VV > x2)}op)
```

where `foldl1` is a left-fold over non-empty lists

```304: {-@ foldl1      :: (a -> a -> a) -> {v:[a] | (NonNull v)} -> a @-}
305: (a -> a -> a) -> {VV : [a] | (len([VV]) > 0)} -> afoldl1 a -> a -> af (x:xs) =  (a -> a -> a) -> a -> [a] -> afoldl a -> a -> af {VV : a | (VV = x)}x {VV : [a] | (VV = xs),(len([VV]) >= 0)}xs
306: foldl1 _ []     =  {VV : [(GHC.Types.Char)] | false} -> {VV : a | false}liquidError {VV : [(GHC.Types.Char)] | (len([VV]) >= 0)}"will. never. happen."
```

We can put the whole thing together to write a (very) simple Map-Reduce library

```312: mapReduce   :: (Eq k, Hashable k)
313:             => (a -> [(k, v)])    -- ^ key-mapper
314:             -> (v -> v -> v)      -- ^ reduction operator
315:             -> [a]                -- ^ inputs
316:             -> [(k, v)]           -- ^ output key-values
317:
318: (GHC.Classes.Eq a)
-> (Data.Hashable.Class.Hashable a)
-> (b -> {VV : [(a , c)] | (len([VV]) >= 0)})
-> (x7:c -> x8:c -> {VV : c | (VV > x7),(VV > x8)})
-> [b]
-> [(a , c)]mapReduce a -> {VV : [(b , c)] | (len([VV]) >= 0)}f x1:a -> x2:a -> {VV : a | (VV > x1),(VV > x2)}op  = (Data.HashMap.Base.HashMap a b) -> [(a , b)]M.toList
319:                 ((Data.HashMap.Base.HashMap a b) -> [(a , b)])
-> ([c] -> (Data.HashMap.Base.HashMap a b)) -> [c] -> [(a , b)]. (x2:a -> x3:a -> {VV : a | (VV > x2),(VV > x3)})
-> (Data.HashMap.Base.HashMap b {VV : [a] | (len([VV]) > 0)})
-> (Data.HashMap.Base.HashMap b a)reduce x1:a -> x2:a -> {VV : a | (VV > x1),(VV > x2)}op
320:                 ((Data.HashMap.Base.HashMap a {VV : [b] | (len([VV]) > 0)})
-> (Data.HashMap.Base.HashMap a b))
-> ([c]
-> (Data.HashMap.Base.HashMap a {VV : [b] | (len([VV]) > 0)}))
-> [c]
-> (Data.HashMap.Base.HashMap a b). [(a , b)]
-> (Data.HashMap.Base.HashMap a {VV : [b] | (len([VV]) > 0)})group
321:                 ([(a , b)]
-> (Data.HashMap.Base.HashMap a {VV : [b] | (len([VV]) > 0)}))
-> ([c] -> [(a , b)])
-> [c]
-> (Data.HashMap.Base.HashMap a {VV : [b] | (len([VV]) > 0)}). (a -> {VV : [(b , c)] | (len([VV]) >= 0)}) -> [a] -> [(b , c)]keyMap a -> {VV : [(b , c)] | (len([VV]) >= 0)}f
```

Now, if we want to compute the frequency of `Char` in a given list of words, we can write:

```327: {-@ charFrequency :: [String] -> [(Char, Int)] @-}
328: charFrequency     :: [String] -> [(Char, Int)]
329: [[(GHC.Types.Char)]] -> [((GHC.Types.Char) , (GHC.Types.Int))]charFrequency     = ([(GHC.Types.Char)]
-> {VV : [((GHC.Types.Char) , {VV : (GHC.Types.Int) | (VV > 0)})] | (len([VV]) >= 0)})
-> (x8:{VV : (GHC.Types.Int) | (VV > 0)}
-> x9:{VV : (GHC.Types.Int) | (VV > 0)}
-> {VV : (GHC.Types.Int) | (VV > 0),(VV > x8),(VV > x9)})
-> [[(GHC.Types.Char)]]
-> [((GHC.Types.Char) , {VV : (GHC.Types.Int) | (VV > 0)})]mapReduce x1:[(GHC.Types.Char)]
-> {VV : [((GHC.Types.Char) , {VV : (GHC.Types.Int) | (VV = 1),
(VV = len([xs])),
(VV > 0)})] | (len([VV]) = len([x1])),
(len([VV]) >= 0)}wordChars x:(GHC.Types.Int)
-> y:(GHC.Types.Int) -> {VV : (GHC.Types.Int) | (VV = (x + y))}(+)
330:   where x1:[a]
-> {VV : [(a , {VV : (GHC.Types.Int) | (VV = 1),
(VV = len([xs])),
(VV > 0)})] | (len([VV]) = len([x1])),(len([VV]) >= 0)}wordChars = (a
-> (a , {VV : (GHC.Types.Int) | (VV = 1),
(VV = len([xs])),
(VV > 0)}))
-> xs:[a]
-> {VV : [(a , {VV : (GHC.Types.Int) | (VV = 1),
(VV = len([xs])),
(VV > 0)})] | (len([VV]) = len([xs]))}map (c:a
-> ({VV : a | (VV = c)} , {VV : (GHC.Types.Int) | (VV = 1),
(VV = len([xs])),
(VV > 0)})\ac -> x1:a -> b -> (a , b)({VV : a | (VV = c)}c, {VV : (GHC.Types.Int) | (VV = (1  :  int))}1))
```

You can take it out for a spin like so:

```336: [((GHC.Types.Char) , (GHC.Types.Int))]f0 = [[(GHC.Types.Char)]] -> [((GHC.Types.Char) , (GHC.Types.Int))]charFrequency [ {VV : [(GHC.Types.Char)] | (len([VV]) >= 0)}"the", {VV : [(GHC.Types.Char)] | (len([VV]) >= 0)}"quick" , {VV : [(GHC.Types.Char)] | (len([VV]) >= 0)}"brown"
337:                    , {VV : [(GHC.Types.Char)] | (len([VV]) >= 0)}"fox", {VV : [(GHC.Types.Char)] | (len([VV]) >= 0)}"jumped", {VV : [(GHC.Types.Char)] | (len([VV]) >= 0)}"over"
338:                    , {VV : [(GHC.Types.Char)] | (len([VV]) >= 0)}"the", {VV : [(GHC.Types.Char)] | (len([VV]) >= 0)}"lazy"  , {VV : [(GHC.Types.Char)] | (len([VV]) >= 0)}"cow"   ]
```

Look Ma! No Types: LiquidHaskell will gobble the whole thing up, and verify that none of the undesirable `liquidError` calls are triggered. By the way, notice that we didn't write down any types for `mapReduce` and friends. The main invariant, from which safety follows is that the `Map` returned by the `group` function binds each key to a non-empty list of values. LiquidHaskell deduces this invariant by inferring suitable types for `group`, `inserts`, `foldl1` and `reduce`, thereby relieving us of that tedium. In short, by riding on the broad and high shoulders of SMT and abstract interpretation, LiquidHaskell makes a little typing go a long way.

## Conclusions¶

Well folks, thats all for now. I trust this article gave you a sense of

1. How we can describe certain structural properties of data types, such as the length of a list,

2. How we might use refinements over these properties to describe key invariants and establish, at compile-time, the safety of operations that might blow up on unexpected values at run-time, and perhaps, most importantly,

3. How we can achieve the above, whilst just working with good old lists, without having to make up new types (which have the unfortunate effect of cluttering programs with their attendant new functions) in order to enforce special invariants.

Ranjit Jhala
2013-01-31 16:12