While studying polyvariadic functions in Haskell I stumbled across the following SO questions:
How to create a polyvariadic haskell function?
Haskell, polyvariadic function and type inference
and thought I will give it a try by implementing a function which takes a variable number of strings and concatenates/merges them into a single string:
{-# LANGUAGE FlexibleInstances #-}
class MergeStrings r where
merge :: String -> r
instance MergeStrings String where
merge = id
instance (MergeStrings r) => MergeStrings (String -> r) where
merge acc = merge . (acc ++)
This works so far if I call merge with at least one string argument and if I provide the final type.
foo :: String
foo = merge "a" "b" "c"
Omitting the final type results in an error, i.e., compiling the following
bar = merge "a" "b" "c"
results in
test.hs:12:7: error:
• Ambiguous type variable ‘t0’ arising from a use of ‘merge’
prevents the constraint ‘(MergeStrings t0)’ from being solved.
Relevant bindings include bar :: t0 (bound at test.hs:12:1)
Probable fix: use a type annotation to specify what ‘t0’ should be.
These potential instances exist:
instance MergeStrings r => MergeStrings (String -> r)
-- Defined at test.hs:6:10
instance MergeStrings String -- Defined at test.hs:4:10
• In the expression: merge "a" "b" "c"
In an equation for ‘bar’: bar = merge "a" "b" "c"
|
12 | bar = merge "a" "b" "c"
|
The error message makes perfect sense since I could easily come up with, for example
bar :: String -> String
bar = merge "a" "b" "c"
baz = bar "d"
rendering bar not into a single string but into a function which takes and returns one string.
Is there a way to tell Haskell that the result type must be of type String? For example, Text.Printf.printf "hello world" evaluates to type String without explicitly defining.
printf works without type annotation because of type defaulting in GHCi. The same mechanism that allows you to eval show $ 1 + 2 without specifying concrete types.
GHCi tries to evaluate expressions of type IO a, so you just need to add appropriate instance for MergeStrings:
instance (a ~ ()) => MergeStrings (IO a) where
merge = putStrLn