The Glorious Glasgow Haskell Compilation System User's Guide ...

More documents

Recommendations

Info

The Glorious Glasgow Haskell Compilation System User’s Guide, Version 6.10.4 19 / 224 2.4.3.3 The :main and :run commands When a program is compiled and executed, it can use the getArgs function to access the command-line arguments. However, we cannot simply pass the arguments to the main function while we are testing in ghci, as the main function doesn’t take its directly. Instead, we can use the :main command. This runs whatever main is in scope, with any arguments being treated the same as command-line arguments, e.g.: Prelude> let main = System.Environment.getArgs >>= print Prelude> :main foo bar ["foo","bar"] We can also quote arguments which contains characters like spaces, and they are treated like Haskell strings, or we can just use Haskell list syntax: Prelude> :main foo "bar baz" ["foo","bar baz"] Prelude> :main ["foo", "bar baz"] ["foo","bar baz"] Finally, other functions can be called, either with the -main-is flag or the :run command: Prelude> let foo = putStrLn "foo" >> System.Environment.getArgs >>= print Prelude> let bar = putStrLn "bar" >> System.Environment.getArgs >>= print Prelude> :set -main-is foo Prelude> :main foo "bar baz" foo ["foo","bar baz"] Prelude> :run bar ["foo", "bar baz"] bar ["foo","bar baz"] 2.4.4 The it variable Whenever an expression (or a non-binding statement, to be precise) is typed at the prompt, GHCi implicitly binds its value to the variable it. For example: Prelude> 1+2 3 Prelude> it * 2 6 What actually happens is that GHCi typechecks the expression, and if it doesn’t have an IO type, then it transforms it as follows: an expression e turns into let it = e; print it which is then run as an IO-action. Hence, the original expression must have a type which is an instance of the Show class, or GHCi will complain: Prelude> id :1:0: No instance for (Show (a -> a)) arising from use of ‘print’ at :1:0-1 Possible fix: add an instance declaration for (Show (a -> a)) In the expression: print it In a ’do’ expression: print it
The Glorious Glasgow Haskell Compilation System User’s Guide, Version 6.10.4 20 / 224 The error message contains some clues as to the transformation happening internally. If the expression was instead of type IO a for some a, then it will be bound to the result of the IO computation, which is of type a. eg.: Prelude> Time.getClockTime Wed Mar 14 12:23:13 GMT 2001 Prelude> print it Wed Mar 14 12:23:13 GMT 2001 The corresponding translation for an IO-typed e is it reverse [] What should GHCi do? Strictly speaking, the program is ambiguous. show (reverse []) (which is what GHCi computes here) has type Show a => a and how that displays depends on the type a. For example: ghci> (reverse []) :: String "" ghci> (reverse []) :: [Int] [] However, it is tiresome for the user to have to specify the type, so GHCi extends Haskell’s type-defaulting rules (Section 4.3.4 of the Haskell 98 Report (Revised)) as follows. The standard rules take each group of constraints (C1 a, C2 a, ..., Cn a) for each type variable a, and defaults the type variable if 1. The type variable a appears in no other constraints 2. All the classes Ci are standard. 3. At least one of the classes Ci is numeric. At the GHCi prompt, or with GHC if the -XExtendedDefaultRules flag is given, the following additional differences apply: • Rule 2 above is relaxed thus: All of the classes Ci are single-parameter type classes. • Rule 3 above is relaxed this: At least one of the classes Ci is numeric, or is Show, Eq, or Ord. • The unit type () is added to the start of the standard list of types which are tried when doing type defaulting. The last point means that, for example, this program: main :: IO () main = print def instance Num () def :: (Num a, Enum a) => a def = toEnum 0 prints () rather than 0 as the type is defaulted to () rather than Integer. The motivation for the change is that it means IO a actions default to IO (), which in turn means that ghci won’t try to print a result when running them. This is particularly important for printf, which has an instance that returns IO a. However, it is only able to return undefined (the reason for the instance having this type is so that printf doesn’t require extensions to the class system), so if the type defaults to Integer then ghci gives an error when running a printf.
Page 1 and 2: The Glorious Glasgow Haskell Compil
Page 33: The Glorious Glasgow Haskell Compil
Page 85 and 86:
The Glorious Glasgow Haskell Compil
Page 87 and 88:
Page 89 and 90:
Page 91 and 92:
Page 93 and 94:
Page 95 and 96:
Page 97 and 98:
Page 99 and 100:
Page 101 and 102:
Page 103 and 104:
Page 105 and 106:
Page 107 and 108:
Page 109 and 110:
Page 111 and 112:
Page 113 and 114:
Page 115 and 116:
Page 117 and 118:
Page 119 and 120:
Page 121 and 122:
Page 123 and 124:
Page 125 and 126:
Page 127 and 128:
Page 129 and 130:
Page 131 and 132:
Page 133 and 134:
Page 135 and 136:
Page 137 and 138:
Page 139 and 140:
Page 141 and 142:
Page 143 and 144:
Page 145 and 146:
Page 147 and 148:
Page 149 and 150:
Page 151 and 152:
Page 153 and 154:
Page 155 and 156:
Page 157 and 158:
Page 159 and 160:
Page 161 and 162:
Page 163 and 164:
Page 165 and 166:
Page 167 and 168:
Page 169 and 170:
Page 171 and 172:
Page 173 and 174:
Page 175 and 176:
Page 177 and 178:
Page 179 and 180:
Page 181 and 182:
Page 183 and 184:
Page 185 and 186:
Page 187 and 188:
Page 189 and 190:
Page 191 and 192:
Page 193 and 194:
Page 195 and 196:
Page 197 and 198:
Page 199 and 200:
Page 201 and 202:
Page 203 and 204:
Page 205 and 206:
Page 207 and 208:
Page 209 and 210:
Page 211 and 212:
Page 213 and 214:
Page 215 and 216:
Page 217 and 218:
Page 219 and 220:
Page 221 and 222:
Page 223 and 224:
Page 225 and 226:
Page 227 and 228:
Page 229 and 230:
Page 231 and 232:
Page 233 and 234:
Page 235 and 236:
Page 237 and 238:
Page 239:
show all

The Glorious Glasgow Haskell Compilation System User's Guide ...

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?